Tracking system for video disc player

Abstract

A video disc player is described for use with a video disc having frequency modulated video information recorded thereon in the form of a plurality of concentric circles or a single spiral. The information track comprises successively positioned light reflective and light non-reflective regions. A focused light beam is caused to be positioned over the center of an information track and the light reflected from the information track is gathered by an objective lens for application to electronic circuitry for recovering the recorded frequency modulated video signals. Radial tracking means are described for maintaining the focused light spot to impinge upon the center of an information track. Lens focusing means are described for positioning the objective lens at the optimum focused position above the information track for gathering the maximum amount of reflected light from the information track. FM processing means are described for reconstructing the recovered frequency modulated video information such that the ratio between the amplitude of the signals as recorded is essentially the same in the signals as recovered from the video disc member. Further servo means are described for handling the selective change of the intersection of the reading beam with the video disc member in a predetermined preferred mode of operation.

Description

RELATED PATENT APPLICATIONS

This is a division of application Ser. No. 298,405, filed Sept. 1, 1981, now U.S. Pat. No. 4,439,848, which is a continuation of application Ser. No. 131,513, filed Mar. 18, 1980, now abandoned, which is a continuation of application Ser. No. 890,670, filed Mar. 27, 1978, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the method and means for reading a frequency modulated video signal stored in the form of successively positioned reflective and non-reflective regions on a plurality of information tracks carried by a video disc. More specifically, an optical system is employed for directing a reading beam to impinge upon the information track and for gathering the reflected signals modulated by the reflective and non-reflective regions of the information track. A frequency modulated electrical signal is recovered from the reflected light modulated signal. The recovered frequency modulated electrical signal is applied to a signal processing section wherein the recovered frequency modulated signal is prepared for application to a standard television receiver and/or monitor. The recovered light modulated signals are applied to a plurality of servo systems for providing control signals which are employed for keeping the lens at the optimum focus position with relation to the information bearing surface of the video disc and to maintain the focused light beam in a position such that the focused light spot impinges at the center of the information track.

SUMMARY OF THE INVENTION

The present invention is directed to a video disc player operating to recover frequency modulated video signals from an information bearing surface of a video disc. The frequency modulated video information is stored in a plurality of concentric circles or a single spiral extending over an information bearing portion of the video disc surface. The frequency modulated video signal is represented by indicia arranged in track-like fashion on the information bearing surface portion of the video disc. The indicia comprise successively positioned reflective and non-reflective regions in the information track.

A laser is used as the source of a coherent light beam and an optical system is employed for focusing the light beam to a spot having a diameter approximately the same as the width of the indicia positioned in the information track. A microscopic objective lens is used for focusing the read beam to a spot and for gathering up the reflected light caused by the spot impinging upon successively positioned light reflective and light non-reflective regions. The use of the microscopically small indicia typically 0.5 microns in width and ranging between one micron and 1.5 microns in length taxes the resolving power of the lens to its fullest. In this relationship, the lens acts as a low pass filter. In the gathering of the reflected light and passing the reflected light through the lens when operating at the maximum resolution of the lens, the gathered light assumes a sinusoidal-shaped like modulated beam representing the frequency modulated video signals contained on the video disc member.

The output from the microscopic lens is applied to a signal recovery system wherein the reflected light beam is employed first as an information bearing light member and second as a control signal source for generating radial tracking errors and focus errors. The information bearing portion of the recovered frequency modulated video signal is applied to an FM processing system for preparation prior to transmission to a standard TV receiver and/or a TV monitor.

The control portion of the recovered frequency modulated video signal is applied to a plurality of servo subsystems for controlling the position of the reading beam on the center of the information track and for controlling the placing of the lens for gathering the maximum reflected light when the lens is positioned at its optimum focused position. A tangential servo subsystem is employed for determining the time base error introduced into the reading process due to the mechanics of the reading system. This time base error appears as a phase error in the recovered frequency modulated video signal.

The phase error is detected by comparing a selected portion of the recovered frequency modulated signal with an internally generated signal having the correct phase relationship with the predetermined portion of the recovered frequency modulated video signal. The predetermined relationship is established during the original recording on the video disc. In the preferred embodiment, the predetermined portion of the recovered frequency modulated video signal is the color burst signal. The internally generated reference frequency is the color subcarrier frequency. The color burst signal was originally recorded on the video disc under control of an identical color subcarrier frequency. The phase error detected in this comparison process is applied to a mirror moving in the tangential direction which adjusts the location at which the focused spot impinges upon the information track. The tangential mirror causes the spot to move along the information track either in the forward or reverse direction for providing an adjustment equal to the phase error detected in the comparison process. The tangential mirror in its broadest sense is a means for adjusting the time base of the signal read from the video disc member to adjust for time base errors injected by the mechanics of the reading system.

In an alternative form of the invention, the predetermined portion of the recovered frequency modulated video signal is added to the total recorded frequency modulated video signal at the time of recording and the same frequency is employed as the operating point for the highly controlled crystal oscillator used in the comparison process.

In the preferred embodiment when the video disc player is recovering frequency modulated video signals representing television pictures, the phase error comparison procedure is performed for each line of television information. The phase error is used for the entire line of television information for correcting the time base error for one full line of television information. In this manner, incremental changes are applied to correct for the time base error. These are constantly being recomputed for each line of television information.

A radial tracking servo subsystem is employed for maintaining radial tracking of the focused light spot on one information track. The radial tracking servo subsystem responds to the control signal portion of the recovered frequency modulated signal to develop an error signal indicating the offset from the preferred center of track position to the actual position. This tracking error is employed for controlling the movement of a radial tracking mirror to bring the light spot back into the center of track position.

The radial tracking servo subsystem operates in a closed loop mode of operation and in an open loop mode of operation. In the closed loop mode of operation, the differential tracking error derived from the recovered frequency modulated video signal is continuously applied through the radial tracking mirror to bring the focus spot back to the center of track position. In the open loop mode of operation, the differential tracking error is temporarily removed from controlling the operation of radial tracking mirror. In the open loop mode of operation, various combinations of signals take over control of the movement of the radial tracking mirror for directing the point of impingement of the focused spot from the preferred center of track position on a first track to a center of track position on an adjacent track. A first control pulse causes the tracking mirror to move the focused spot of light from the center of track position on a first track and move towards a next adjacent track. This first control pulse terminates at a point prior to the focused spot reaching the center of track position in the next adjacent track. After the termination of the first control pulse, a second control pulse is applied to the radial tracking mirror to compensate for the additional energy added to the tracking mirror by the first control pulse. the second control pulse is employed for bringing the focused spot into the preferred center of track focus position as soon as possible. The second control pulse is also employed for preventing oscillation of the read spot about the second information track. A residual portion of the differential tracking error is also applied to the radial tracking mirror at a point calculated to assist the second control pulse in bringing the focused spot to rest at the center of track focus position in the next adjacent track.

A stop motion subsystem is employed as a means for generating a plurality of control signals for application to the tracking servo subsystem to achieve the movement of a focused spot tracking the center of a first information track to a separate and spaced location in which the spot begins tracking the center of the next adjacent information track. The stop motion subsystem performs its function by detecting a predetermined signal recovered from the frequency modulated video signal which indicates the proper position within the recovered frequency modulated video signal at which time the jumping operation should be initiated. This detection function is achieved, in part, by internally generating a gating circuit indicating that portion of the recovered frequency modulated video signal within which the predetermined signal should be located.

In response to the predetermined signal, which is called in the referredany for the radial tracking servo subsystem to reacquire proper tracking of the next adjacent information track. In this embodiment of operation where the differential tracking error is removed from the tracking mirror drivers, a substitute pulse is generated for giving a clean unambiguous signal to the tracking mirror drivers to direct the tracking mirror to move to its next assigned location. This signal in the preferred embodiment is identified as the stop motion pulse and comprises regions of pre-emphasis at the beginning and end of the stop motion pulse which are tailored to direct the tracking mirror drivers to move the focused spot to the predetermined next track location and to help maintain the focused spot in the proper tracking position. In review, one mode of operation of the video disc player removes the differential tracking error signal from application to the tracking mirror drivers and no additional signal is substituted therefor. In a further embodiment of operation of the video disc player, the differential tracking error signal is replaced by a particularly shaped stop motion pulse.

In a still further mode of operation of the tracking mirror servo subsystem 40, the stop motion pulse which is employed for directing the focused beam to leave a first information track and depart for a second adjacent information track is used in combination with a compensation signal applied directly to the radial tracking mirrors to direct the mirrors to maintain focus on the next adjacent track. In the preferred embodiment, the compensation pulse is applied to the tracking mirror drivers after the termination of the stop motion pulse.

In a still further embodiment of the tracking servo subsystem 40, the differential tracking error signal is interrupted for a period less than the time necessary to perform the stop motion mode of operation and the portion of the differential tracking error allowed to pass into the tracking mirror drivers is calculated to assist the radial tracking mirrors to achieve proper radial tracking.

Referring to FIG. 11, there is shown a block diagram of the tangential servo subsystem 80. A first input signal to the tangential servo subsystem 80 is applied from the FM processing system 32 over the line 82. The signal present on the line 82 is the video signal available from the video distribution amplifiers as contained in the FM processing system 32. The video signal on the line 82 is applied to a sync pulse separator circuit 520 over a line 522 and to a chroma separator filter 523 over a line 524 The video signal on the line 82 is also applied to a burst gate separator circuit 525 over a line 525a.

The function of the vertical sync pulse separator circuit 520 is to separate the vertical sync signal from the video signal. The vertical sync signal is applied to the stop motion subsystem 44 over the line 92. The function of the chroma separator filter 523 is to separate the chroma portion from the total video signal received from the FM processing circuit 32. The output from the chroma separator filter 523 is applied to the FM corrector portion of the FM processing circuit 32 over the line 142. The output signal from the chroma separator filter 523 is also applied to a burst phase detector circuit 526 over a line 528. The burst phase detector circuit 526 has a second input signal from a color subcarrier oscillator circuit 530 over a line 532. The purpose of the burst phase detector circuit 526 is to compare the instantaneous phase of the color burst signal with a very accurately generated color subcarrier oscillator signal generated in the oscillator 530. The phase difference detected in the burst phase detector circuit 526 is applied to a sample and hold circuit 534 over a line 536. The function of the sample and hold circuit is to store a voltage equivalent of the phase difference detected in the burst phase detector circuit 526 for the time during which the full line of video information containing that color burst signal, used in generating the phase difference, is read from the disc 5.

The purpose of the burst gate separator 525 is to generate an enabling signal indicating the time during which the color burst portion of the video waveform is received from the FM processing unit 32. The output signal from the burst gate separator 525 is applied to the FM corrector portion of the FM processing system 32 over a line 144. The same burst gate timing signal is applied to the sample and hold circuit 534 over a line 538. The enabling signal on the line 538 gates the input from the burst phase detector 526 into the sample and hold circuit 534 during the color burst portion of the video signal.

The color subcarrier oscillator circuit 530 applies the color subcarrier frequency to the audio processing circuit 114 over a line 140. The color subcarrier oscillator circuit 530 supplies the color subcarrier frequency to a divide circuit 540 over a line 541 which divides the color subcarrier frequency by three hundred and eighty-four for generating the motor reference frequency. The motor reference frequency signal is applied to the spindle servo subsystem 50 over the line 94.

The output from the sample and hold circuit 534 is applied to an automatic gain controlled amplifier circuit 542 over a line 544. The automatic gain controlled amplifier 542 has a second input signal from the carriage position potentiometer as applied thereto over the line 84. The function of the signal on the line 84 is to change the gain of the amplifier 542 as the reading beam 4 radially moves from the inside track to the outside track and/or conversely when the reading beam moves from the outside track to the inside track. The need for this adjustment to change with a change in the radial position is caused by the formation of the reflective regions 10 and non-reflective regions 11 with different dimensions from the outside track to the inside track. The purpose of the constant rotational speed from the spindle motor 48 is to turn the disc 5 through nearly thirty revolutions per second to provide thirty frames of information to the television receiver 96. The length of a track at the outermost circumference is much longer than the length of a track at the innermost circumference. Since the same amount of information is stored in one revolution at both the inner and outer circumference, the size of the reflective and non-reflective regions 10 and 11 respectively are adjusted from the inner radius to the outer radius. Accordingly, this change in size requires that certain adjustment in the processing of the detected signal read from the video disc 5 are made for optimum operation. One of the required adjustments is to adjust the gain of the amplifier 542 which adjusts for the time base error as the reading point radially changes from an inside to an outside circumference. The carriage position potentiometer (not shown) generates a sufficiently accurate reference voltage indicating the radial position of the point of impingement of the reading beam 4 onto the video disc 5. The output from the amplifier 542 is applied to a compensation circuit 545 over a line 546. The compensation network 545 is employed for preventing any system oscillations and instability. The output from the compensation network 545 is applied to a tangential mirror driver circuit 500 over a line 550. The tangential mirror driver circuit 500 was described with reference to FIG. 9. The circuit 500 comprises a pair of push/pull amplifiers. The output from one of the push/pull amplifiers (not shown) is applied to the tangential mirror 26 over a line 88. The output from the second push/pull amplifier (not shown) is applied to the tangential mirror 26 over a line 90.

TIME BASE ERROR CORRECTION MODE OF OPERATION

The recovered FM video signal, from the surface of the video disc 5 is corrected, for time base errors introduced by the mechanics of the reading process, in the tangential servo subsystem 80. Time base errors are introduced into the reading process due to the minor imperfections in the video disc 5. A time base error introduces a slight phase change into the recovered FM video signal. A typical time base error base correction system includes a highly accurate oscillator for generating a source of signals used as a phase standard for comparison purposes. In the preferred embodiment, the accurate oscillator is conveniently chosen to oscillate at the color subcarrier frequency. The color subcarrier frequency is also used during the writing process for controlling the speed of revolution of the writing disc during the writing process. In this manner, the reading process is phase controlled by the same highly accurate oscillator as was used in the writing process. The output from the highly controlled oscillator is compared with the color burst signal of a FM color video signal. An alternative system records a highly accurate frequency at any selected frequency during the writing process. During the reading process, this frequency would be compared with a highly accurate oscillator in the player and the phase difference between the two signals is sensed and is employed for the same purpose.

The color burst signal forms a small portion of the recovered FM video signal. A color burst signal is repeated in each line of color T.V. video information in the recovered FM video signal. In the preferred embodiment, each portion of the color burst signal is compared with the highly accurate subcarrier oscillator signal for detecting the presence of any phase error. In a different embodiment, the comparison may not occur during each availability of the color burst signal or its equivalent, but may be sampled at randomly or predetermined locations in the recoverd signal containing the recorded equivalent of the color burst signal. When the recorded information is not so highly sensitive to phase error, the comparison may occur at greater spaced locations. In general, the phase difference between the recorded signal and the locally generated signal is repetitively sensed at spaced locations on the recording surface for adjusting for phase errors in the recovered signal. In the preferred embodiment this repetitive sensing for phase error occurs on each line of the FM video signal.

The detected phase error is stored for a period of time extending to the next sampling process. This phase error is used to adjust the reading position of the reading beam so as to impinge upon the video disc at a location such as to correct for the phase error.

Repetitive comparison of the recorded signal with the locally generated, highly accurate frequency, continuously adjusts for an incremental portion of the recovered video signal recovered during the sampling periods.

In the preferred embodiment, the phase error changes as the reading beam radially tracks across the information bearing surface portion of the video disc 5. In this embodiment, a further signal is required for adjusting the phase error according to the instantaneous location of the reading beam to adjust the phase error according to its instantaneous location on the information bearing portion of the video disc 5. This additional signal is caused by the change in physical size of the indicia contained on the video disc surface as the radial tracking position changes from the inner location to the outer location. The same amount of information is contained at an inner radius as at an outer radius and hence the indicia must be smaller at the inner radius when compared to the indicia at the outer radius.

In an alternative embodiment, when the size of the indicia is the same at the inner radius and at the outer radius, this additional signal for adjusting for instantaneous radial position is not required. Such an embodiment would be operable with video disc members which are in strip form rather than in disc form and when the information is recorded using indicia of the same size on a video disc member.

In the preferred embodiment, a tangential mirror 26 is the mechanism selected correcting for the time base errors introduced by the mechanics of the reading system. Such a mirror is electronically controlled and is a means for changing the phase of the recovered video signal read from the disc by changing the time base on which the signals are read from the disc. This is achieved by directing the mirror to read the information from the disc at an incremental point earlier or later in time when compared to the time and spacial location during which the phase error was detected. The amount of phase error determines the degree of change in location and hence time in which the information is read.

When no phase error is detected in the time base correcting system the point of impingement of the read beam with the video disc surface 5 is not moved. When a phase error is detected during the comparison period, electronics signals are generated for changing the point of impingement so that the recovered information from the video disc is available for processing at a point in time earlier or later when compared to the comparison period. In the preferred embodiment, this is achieved by changing the spacial location of the point of intersection of the read beam with the video disc surface 5.

Referring to FIG. 12, there is shown a block diagram of the stop motion subsystem 44 employed in the video disc player 1. The waveform shown with reference to FIGS. 13A, 13B and 13C are used in conjunction with the block diagram shown in FIG. 12 to explain the operation of the stop motion system. The video signal from the FM processing unit 32 is applied to an input buffer stage 551 over the line 134. The output signal from the buffer 551 is applied to a DC restorer 552 over a line 554. The function of the DC restorer 552 is to set the blanking voltage level at a constant uniform level. Variations in signal recording and recovery oftentimes result in video signals available on the line 134 with different blanking levels. The output from the DC restorer 552 is applied to a white flag detector circuit 556 over a line 558. The function of the white flag detector 556 is to identify the presence of an all white level video signal existing during an entire line of one or both fields contained in a frame of television information. While the white flag detector has been described as being detecting an all white video signal during a complete line interval of a frame of television information, the white flag may take other forms. One such form would be a special number stored in a line. Alternatively, the white flag detector can respond to the address indicia found in each video frame for the same purpose. Other indicia can also be employed. However, the use of an all white level signal during an entire line interval in the television frame of information has been found to be the most reliable.

The vertical sync signal from the tangential servo 80 is applied to a delay circuit 560 over a line 92. The output from the delay circuit 560 is supplied to a vertical window generator 562 over a line 564. The function of the window generator 562 is to generate an enabling signal for application to the white flag detector 556 over the line 566 to coincide with that line interval in which the white flag signal has been stored. The output signal from the generator 562 gates the predetermined portion of the video signal from the FM detector and generates an output white flag pulse whenever the white flag is contained in the portion of the video signal under surveillance. The output from the white flag detector 556 is applied to a stop motion pulse generator 567 over a line 568 a gate 569 and a further line 570. The gate 569 has as a second input signal, over the line 132, the STOP MOTION MODE enabling signal from the function generator 47.

The differential tracking error from the signal recovery subsystem 30 is applied to a zero crossing detector and delay circuit 571 over the lines 42 and 46. The function of the zero crossing detector circuit 571 is to identify when the lens crosses the mid points 425 and/or 426 between two adjacent tracks 422 and 423 as shown with reference to line A of FIG. 8. This mid point is the point at which the differential tracking error shown in line C of FIG. 8 at point 441b corresponds to the mid point 426 between adjacent tracks 424 and 423. It is important to note that the differential tracking signal output also indicates the same level signal at point 440c which identifies the optimum focusing point at which the tracking servo system 40 seeks to position the lens in perfect tracking alignment on the mid point 429 of the track 423 when the tracking suddenly jumps from track 424 to track 423. Accordingly, a means must be provided for recognizing the difference between points 441b and 440c on the differential error signal shown in line C of FIG. 8.

The output of the zero crossing detector and delay circuit 571 is applied to the stop motion pulse generator 567 over a line 572. The stop motion pulse generated in the generator 567 is applied to a plurality of locations, the first of which is as a loop interrupt pulse to the tracking servo 40 over the line 108. A second output signal from the stop motion pulse generator 567 is applied to a stop motion compensation sequence generator 573 over a line 574a. The function of the stop motion compensation sequence generator 573 is to generate a compensation pulse waveform for application to the radial tracking mirror to cooperate with the actual stop motion pulse sent directly to the tracking mirror over the line 104. The stop motion compensation pulse is sent to the tracking servo over the line 106.

With reference to line A of FIG. 8, the center to center distance, indicated by the line 420, between adjacent tracks is presently fixed at 1.6 microns. The tracking servo mirror gains sufficient inertia upon receiving a stop motion pulse that the focused spot from the mirror jumps from one track to the next adjacent track. The inertia of the tracking mirror under normal operating conditions causes the mirror to swing past the one track to be jumped. Briefly, the stop motion pulse on the line 104 causes the radial tracking mirror 28 to leave the track on which it is cracking and jump to the next sequential track. A short time later, the radial tracking mirror receives a stop motion compensation pulse to remove the added inertia and direct the tracking mirror into tracking the next adjacent track without skipping one or more tracks before selecting a track for tracking.

In order to insure the optimum cooperation between the stop motion pulse from the generator 567 and the stop motion compensation pulse from the generator 573, the loop interrupt pulse on line 108 is sent to the tracking servo to remove the differential tracking error signal from being applied to the tracking error amplifiers 500 during the period of time that the mirror is purposely caused to leave one track under direction of the stop motion compensation pulse from the generator 567 and to settle upon a next adjacent track under the direction of the stop motion compensation pulse from the generator 573.

As an introduction to the detail detailed understanding of the interaction between the step motion system 44 and the tracking servo system 40, the waveform shown in FIGS. 13A, 13B and 13C are described.

Referring to line A of FIG. 13A, there is shown the normal tracking mirror drive signals to the radial tracking mirror 28. As previously discussed, there are two driving signals applied to the tracking mirror 28. The radial tracking A signal represented by a line 574 and a radial tracking B signal represented by a line 575. Since the information track is normally in the shape of a spiral, there is a continuous tracking control signal being applied to the radial tracking mirror for following the spiral shaped configuration of the information track. The time frame of the information shown in the waveform shown in line A represents more than a complete revolution of the disc. A typical normal tracking mirror drive signal waveform for a single revolution of the disc is represented by the length of the line indicated at 576. The two discontinuities shown at 578 and 580 on waveforms 574 and 575, respectively, indicate the portion of the normal tracking period at which a stop motion pulse is given. The stop motion pulse is also referred to as a jump back signal and these two terms are used to describe the output from the generator 567. The stop motion pulse is represented by the small vertically disposed discontinuity present in the lines 574 and 575 at points 578 and 580, respectively. The remaining waveforms contained in FIGS. 13A, 13B and 13C are on an expanded time base and represent those electrical signals which occur just before the beginning of this jump back period, through the jump back period and continuing a short duration beyond the jump back period.

The stop motion pulse generated by the stop motion pulse generator 567 and applied to the tracking servo system 40 over the line 104 is represented on line C of FIG. 13A. The stop motion pulse is ideally not a squarewave but has areas of pre-emphasis located generally at 582 and 584. These areas of pre-emphasis insure optimum reliability in the stop motion system 44. The stop motion pulse can be described as rising to a first higher voltage level during the initial period of the stop motion pulse period. Next, the stop motion pulse gradually falls of to a second voltage level, as at 583. The level at 583 is maintained during the duration of the stop motion pulse period. At the termination of the stop motion pulse, the waveform falls to a negative voltage level at 585 below the zero voltage level at 586 and rises gradually to the zero voltage level at 586.

Line D of FIG. 13 represents the differential tracking error signal received from the recovery system 30 over the lines 42 and 46. The waveform shown on line D of FIG. 13A is a compensated differential tracking error achieved through the use of the combination of a stop motion pulse and a stop motion compensation pulse applied to the radial tracking mirror 28 according to the teaching of the present invention.

Line G of FIG. 13A represents the loop interrupt pulse generated by the stop motion pulse generator 567 and applied to the tracking servo subsystem 40 over the line 108. As previously mentioned, it is best to remove the differential tracking error signal as represented by the waveform on line D from application to the radial tracking mirror 28 during the stop motion interval period. The loop interrupt pulse shown on line G achieves this gating function. However, by inspection, it can be seen that the differential tracking error signal lasts for a period longer than the loop interrupt pulse shown on line G. The waveform shown on line E is the portion of the differential tracking error signal shown on line D which survives the gating by the loop interrupt pulse shown on line G. The waveform shown on line E is the compensated tracking error as interrupted by the loop interrupt pulse which is applied to the tracking mirror 28. Referring to line F, the high frequency signal represented generally under the bracket 590 indicates the output waveform of the zero crossing detector circuit 571 in the stop motion system 44. A zero crossing pulse is generated each time the differential error tracking signal shown in line D of FIG. 13A crosses through a zero bias level. Which While the information shown under the bracket 590 is helpful in maintaining a radial tracking mirror 28 in tracking a single information track, such information must be gated off at the beginning of the stop motion interval as indicated by the dashed lines 592 connecting the start of the stop motion pulse in line C of FIG. 13A and the absence of zero crossing detector pulses shown on line F of FIG. 13A. By referring again to line D), the differential tracking error signal rises to a first maximum at 594 and falls to a second opposite but equal maximum at 596. At point 598, the tracking mirror is passing over the zero crossing point 426 between two adjacent tracks 424 and 423 as shown with reference to line A of FIG. 8. This means that the mirror has traveled half way from the first track 424 to the second track 423. At this point indicated by the number 598, the zero crossing detector generates an output pulse indicated at 600. The output pulse 600 terminates the stop motion pulse shown on line C as represented by the vertical line segment 602. This termination of the stop motion pulse starts the negative pre-emphasis period 584 as previously described. The loop interrupt pulse is not affected by the output 600 of the zero crossing detector 571. In the preferred embodiment, improved peformance is achieved by presenting the differential tracking error signal from being applied to the radial tracking mirror 28 too early in the jump back sequence before the radial tracking mirror 28 has settled down and acquired firm radial tracking of the desired track. As shown by reference to the waveform shown in line F, the zero crossing detector again begins to generate zero crossing pulses when the differential tracking error signal reappears as indicated at point 604. Referring to line H of FIG. 13A, there is shown a waveform representing the stop motion compensation sequence which begins coincidental with the end of the loop interrupt pulse shown on line G.

Referring to FIG. 13B, there is shown a plurality of waveforms explaining the relationship between the stop motion pulse as shown on line C of FIG. 13A, and the stop motion compensation pulse waveform as shown on the line H of FIG. 13A and repeated for convenience on line E of FIG. 13B. The compensation pulse waveform is used for generating a differential compensated tracking error as shown with reference to line D of FIG. 13B.

Line A of FIG. 13B shows the differential uncompensated tracking error signal as developed in the signal recovery subsystem 30. The waveform shown in FIG. A represents the radial tracking error signal as the read beam makes an abrupt departure from an information track on which is was tracking and moves towards one of the adjacent tracks positioned on either side of the track being read. The normal tracking error signal, as the beam oscillates slightly down the information track, is shown at the region 610 of Line A. The tracking error represents the slight side to side (radial) motion of the read beam 4 to the successively positioned reflective and non-reflective regions on the disc 5 as previously described. A point 612 represents the start of a stop motion pulse. The uncompensated tracking error is increasing to a first maximum indicated at 614. The region between 612 and 614 shows an increase in tracking error indicating the departure of the read beam from the track being read. From point 614, the differential tracking error signal drops to a point indicated at 616 which represents the mid point of an information track as shown at point 426 in line A of FIG. 8. However, the distance traveled by the read beam between points 612 and 616 on curve A in FIG. 13B is a movement of 0.8 microns and is equal to length of line 617. The uncompensated radial tracking error rises to a second maximum at point 618 as the read beam begins to approach the next adjacent track 423. The tracking error reaches zero at point 622 but is unable to stop and continues to a new maximum at 624. The radial tracking mirror 28 possesses sufficient inertia so that it is not able to instantaneously stop in response to the differential tracking error signal detecting a zero error at point 622 as the read beam crosses the next adjacent information track. Accordingly, the raw tracking error increases to a point indicated at 624 wherein the closed loop servoing effect of the tracking servo subsystem slows the mirror down and brings the read beam back towards the information track represented by the zero crossing differential tracking error as indicated at point 625. Additional peaks are identified at 626 and 628. These show a gradual damping of the differential tracking error as the radial tracking mirror becomes gradually positioned in its proper location to generate a zero tracking error, such as at points 612, 622, 625. Additional zero crossing locations are indicated at 630 and 632. The portion of the waveform shown in line A existing after point 632 shows a gradual return of the raw tracking error to its zero position as the read spot gradually comes to rest on the next adjacent track 423.

Point 616 represents a false indication of zero tracking error as the read beam passes over the center 426 of the region between adjacent tracks 424 and 423.

For optimum operation in a stop motion situation wherein the read beam jumps to the next adjacent track, the time allowed for radial tracking mirror 28 to reacquire proper radial tracking is 300 microseconds. This is indicated by the length of the line 634 shown on line B. By observation, it can be seen that the radial tracking mirror 28 has not yet acquired zero radial error position at the expiration of the 300 microsecond time period. Obviously, if more time were available to achieve this result, the waveform shown with reference to FIG. A 13A would be suitable for those systems having more time for the radial tracking mirror to reacquire zero differential tracking error on the center of the next adjacent track.

Referring briefly to line D of FIG. 13 13B, line 634 is redrawn to indicate that the compensated radial tracking error signal shown in line D does not include those large peaks shown with reference to line A. The compensated differential tracking error shown in line D is capable of achieving proper radial tracking by the tracking servo subsystem within the time frame allowed for proper operation of the video disc player 1. Referring briefly to line E of FIG. 13A, the remaining tracking error signal available after interruption by the loop interrupt pulse is of the proper direction to cooperate with the stop motion compensation pulses described hereinafter to bring the radial tracking mirror to its optimum radial process position as soon as possible.

The stop motion compensation generator 573 shown with reference to FIG. 12, applies the waveform shown in line E of FIG. 13B to the radial tracking mirror 28 by way of the line 106 and the amplifier 500 shown in FIG. 9. The stop motion pulse directs the radial tracking mirror 28 to leave the tracking of one information track and begin to seek the tracking of the next adjacent track. In response to the pulse from the zero crossing detector 571 shown in FIG. 12, the stop motion pulse generator 567 is caused to generate the stop motion compensation pulse shown in line E.

Referring to line E of FIG. 13B, the stop motion compensation pulse waveform comprises a plurality of individual and separate regions indicated at 640, 642 and 644, respectively. The first region 640 of the stop motion compensation pulse begins as the differential uncompensated radial tracking error at point 616 cross the zero reference level indicating that the mirror is in a mid track crossing situation. At this time, the stop motion pulse generator 567 generates the first portion 640 of the compensation pulse which is applied directly to the tracking mirror 28. The generation of the first portion 640 of the stop motion compensation pulse has the effect of reducing the peak 624 to a lower radial tracking displacement as represented by the new peak 624' as shown in line B. It should be kept in mind that the waveforms shown in FIG. 13B are schematic only to show the overall interrelationship of the various pulses used in the tracking servo subsystem and the stop motion subsystem to cause a read beam to jump from one track to next adjacent track. Since the peak error 624' is not as high as the error at peak 624, this has the effect of reducing the error at peak error point 626 and generally shifting the remaining portion of the waveform to the left such that the zero crossings at 625', 630' and 632' all occur sooner than they would have occurred without the presence of the stop motion compensation pulse.

Referring back to line E of FIG. 13B, the second portion 642 of the stop motion compensation pulse is of a second polarity when compared to the first region 640. The second portion 642 of the stop motion compensation pulse occurs at a point in time to compensate for the tracking error shown at 626' of line B. This results in an even smaller radial tracking error being generated at that time and this smaller radial tracking error is represented as point 626" on line C. Since the degree of the radial tracking error represented by the point 626" of line C is significantly smaller than that shown with reference to point 626' of line B, the maximum error in the opposite direction shown at point 626" is again significantly smaller than that represented at point 626 of line A. This counteracting of the natural tendency of the radial tracking mirror 28 to oscillate back and forth over the information track is further dampened as indicated by the further movement to the left of points 628" and 626" with reference to their relative locations shown in lines B and A.

Referring again to line E of FIG. 13B and the third region 644 of the stop motion compensation pulse, this region 644 occurs at the time calculated to dampen the remaining long term tracking error as represented that portion of the error signal to the right of the zero crossing point 632" shown in line C. Region 644 is shown to be approximately equal and opposite to this error signal which would exist if the portion 644 of compensation pulse did not exist. Referring to line D of FIG. 13B, there is shown the differential and compensated radial tracking error representative of the motion of the light beam as it is caused to depart from one information track being read to the next adjacent track under the control of a stop motion pulse and a stop motion compensation pulse. It should be noted that the waveform shown in line D of FIG. 13B can represent the movement in either direction although the polarity of various signals would be changed to represent the different direction of movement.

The cooperation between the stop motion subsystem 44 and the tracking servo subsystem 40 during a stop motion period will what now be described with reference to FIGS. 9 and 12 and their related waveforms. Referring to FIG. 9, the tracking servo subsystem 40 is in operation just prior to the initiation of a stop motion mode to maintain the radial tracking mirror 28 in its position centered directly atop of information track. In order to maintain this position, the differential tracking error is detected in the signal recovery subsystem 30 and applied to the tracking servo subsystem 40 by the line 42. In this present operating mode, the differential tracking error passes directly through the tracking servo loop switch 480, the amplifier 510 and the push/pull amplifiers 500. That portion of the waveform shown at 591 on line D of FIG. 13A as being traversed.

The function generator 47 generates a stop motion mode signal for application to the stop motion mode gate 569 over a line 132. The function of the stop motion mode gate 569 is to generate a pulse in response to the proper location in a television frame for the stop motion mode to occur. This point is detected by the combined operation of the total video signal from the FM processing board 32 being applied to the white flag detector 556 over a line 134 in combination with the vertical sync pulse developed in the tangential servo system 80 and applied over a line 92. The window generator 562 provides an enabling signal which corresponds with a predetermined portion of the video signal containing the white flag indicator. The white flag pulse applied to the stop motion mode gate 569 is gated to the stop motion pulse generator 567 in response to the enabling signal received from the function generator 47 over the line 132. The enabling signal from the stop motion mode gate 569 initiates the stop motion pulse as shown with reference to line C of FIG. 13A. The output from the zero crossing detector 571 indicates the end of the stop motion pulse period by application of a signal to the stop motion pulse generator 567 over the line 572. The stop motion pulse from the generator 567 is applied to the tracking servo loop interrupt switch 480 by way of the gate 482 and the line 108. The function of the tracking servo loop interrupt switch 480 is to remove the differential tracking error currently being generated in the signal recovery subsystem 30 from the push/pull amplifiers 500 driving the radial tracking mirror 28. Accordingly, the switch 480 opens and the differential tracking error is no longer applied to the amplifiers 500 for driving the radial tracking mirror 28. Simultaneously, the stop motion pulse from the generator 567 is applied to the amplifiers 500 over the line 104. The stop motion pulse, in essence, is substituted for the differential tracking error and provides a driving signal to the push/pull amplifiers 500 for starting the road spot to move to the next adjacent information track to be read.

The stop motion pulse from the generator 567 is also applied to the stop motion compensation sequence generator 573 wherein the waveform shown with reference to line H of FIG. 13A and line E of FIG. 13B is generated. By inspection of line H, it is to be noted that the compensation pulse shown on line H occurs at the termination of the loop interrupt pulse on line G, which loop interrupt pulse is triggered by the start of the stop motion pulse shown on line C. The compensation pulse is applied to the push/pull amplifiers 500, over the line 106, shown in FIGS. 9 and 12, for damping out any oscillation in the operation of radial tracking mirror 28 caused by the application of the stop motion pulse.

As previously mentioned, the compensation pulse is initiated at the termination of the loop interrupt signal. Occurring simultaneously with the generation of the compensation pulse, the tracking servo loop interrupt switch 480 closes and allows the differential tracking error to be reapplied to the push/pull amplifiers 500. The typical waveform available at this point is shown in line E of FIG. 13A which cooperates with the stop motion compensation pulse to quickly bring the radial tracking mirror 28 into suitable radial tracking alignment.

Referring briefly to line A of FIG. 13C, two frames of television video information being read from the video disc 5 are shown. Line A represents the differential tracking error signal having abrupt discontinuities located at 650 and 652 representing the stop motion mode of operation. Discontinuities of smaller amplitude are shown at 654 and 656 to show the effect of errors on the surface of the video disc surface in the differential tracking error signal. Line B of FIG. 13C shows the FM envelope as it is read from the video disc surface. The stop motion periods at 658 and 660 show that the FM envelope is temporarily interrupted as the reading spot jumps tracks. Changes in the FM envelope at 662 and 664 show the temporary loss of FM as tracking errors cause the tracking beam to temporarily leave the information track.

In review of the stop motion mode of operation, the following combinations occur in the preferred embodiment. In a first embodiment, the different tracking error signal is removed from the tracking mirror 28 and a stop motion pulse is substituted therefor to cause the radial tracking mirror to jump one track from that track being tracked. In this embodiment, the stop motion pulse has areas of pre-emphasis such as to help the radial tracking mirror to regain tracking of the new track to which it has been positioned. The differential tracking error is reapplied into the tracking servo subsystem and cooperate with the stop motion pulse applied to the radial tracking mirror to reacquire radial tracking. The differential tracking error can be re-entered into the tracking servo system for optimum results. In this embodiment, the duration of the loop interrupt pulse is varied for gating off the application of the differential tracking error into the push/pull amplifiers 500. The stop motion pulse is of fixed length in this embodiment. An alternative to this fixed length of the stop motion pulse is to initiate the end of the stop motion pulse at the first zero crossing detected after the beginning of the stop motion pulse was initiated. Suitable delays can be entered into this loop to remove any extraneous signals that may slip through due to misalignment of the beginning of the stop motion pulse and the detection of zero crossings in the detector 571.

A further embodiment includes any one of the above combinations and further includes the generation of a stop motion compensation sequence. In the preferred embodiment, the stop motion compensation sequence is initiated with the termination of the loop interrupt period. Coincidental with the termination of the loop interrupt period, the differential tracking error is reapplied into the tracking servo subsystem 40. In a further embodiment, the stop motion compensation pulse can be entered into the tracking servo subsystem over the line 106 at a period fixed in time from the beginning of the stop motion pulse as opposed to the ending of the loop interrupt pulse. The stop motion compensation sequence comprises a plurality of separate and distinct regions. In the preferred embodiment, the first region opposes the tendency of the tracking mirror to overshoot the next adjacent track and directs the mirror back into radial tracking of that next adjacent particular track. A second region is of lower amplitude than the first region and of opposite polarity to further compensate the motion of the radial tracking mirror as the spot again overshoots the center portion of the next adjacent track but in the opposite direction. The third region of the stop motion compensation sequence is of the same polarity as the first region, but of significantly lower amplitude to further compensate any tendency of the radial tracking mirror having the focus spot again leave the information track.

While in the preferred embodiment, the various regions of the stop motion sequence are shown to consist of separate individual regions. It is possible for these regions to be themselves broken down into individual pulses. It has been found by experiment that the various regions can provide enhanced operation when separated by zero level signals. More specifically, a zero level condition exists between region one and region two allowing the radial tracking mirror to move under its own inertia without the constant application of a portion of the compensation pulse. It has also been found by experiment that this quiescent period of the compensation sequence can coincide with the reapplication of the differential tracking error to the radial tracking mirrors. In this sense, region one, shiown at 640, of the compensation sequence cooperates with the portion 604 shown in line E of FIG. 13A from the differential tracking error input into the tracking loop.

By observation of the compensation waveform shown in line E of FIG. 13B, it can be observed that the various regions tend to begin at a high amplitude and fall off to very low compensation signals. It can also be observed that the period of the various regions begin at a first relatively short time period and gradually become longer in duration. This coincides with the energy contained in the tracking mirror as it seeks to regain radial tracking. Initially in the track jumping sequence, the energy is high and the early portions of the compensation pulse are appropriately high to counteract this energy. Thereafter, as energy is removed from the tracking mirror, the corrections become less so as to bring the radial tracking mirror back into radial alignment as soon as possible.

Referring to FIG. 14, there is shown a block diagram of the FM processing system 32 employed in the video disc player 1. The frequently modulated video signal recovered from the disc 5 forms the input to the FM processing unit 32 over the line 34. The frequency modulated video signal is applied to a distribution amplifier 670. The distribution amplifier provides three equal unloaded representations of the received signal. The first output signal from the distribution amplifier is applied to a FM corrector circuit 672 over a line 673. The FM corrector circuit 672 operates to provide variable gain amplification to the received frequency modulated video signal to compensate for the mean transfer function of the lens 17 as it reads the frequency modulated video signal from the disc. The lens 17 is operating close to its absolute resolving power and as a result, recovers the frequency modulated video signal with different amplitudes corresponding to different frequencies.

The output from the FM corrector 672 is applied to an FM detector 674 over a line 675. The FM detector generates discriminated video for application to the remaining circuits requiring such discriminated video in the video disc player. A second output signal from the distribution amplifier 670 is applied to the tangential servo subsystem 80 over a line 82. A further output signal from the distribution amplifier 670 is applied to the stop motion subsystem 44 over the line 134.

Referring to FIG. 15, there is shown a more detailed block diagram of the FM corrector 672 shown in FIG. 14. The FM video signal from the amplifier 670 is applied to an audio subcarrier trap circuit 676 over the line 673. The function of the subcarrier trap circuit 676 is to remove all audio components from the frequency modulated video signal prior to application to a frequency selective variable gain amplifier 678 over a line 680.

The control signals for operating the amplifier 678 include a first burst gate detector 682 having a plurality of input signals. A first input signal is the chroma portion of the FM video signal as applied over a line 142. The second input signal to the burst gate 682 is the burst gate enable signal from the tangential servo system 80 over the line 144. The function of the burst gate 682 is to gate into an amplitude detector 684 over a line 686 that portion of the chroma signal corresponding to the color burst signal. The output from the amplitude detector 684 is applied to a summation circuit 688 over a line 690. A second input to the summation circuit 688 is from a variable burst level adjust potentiometer 692 over a line 694. The function of the amplitude detector 684 is to determine the first order lower chroma side band vector and apply it as a current representation to the summation circuit 688. The burst level adjust signal on the line 694 from the potentiometer 692 operates in conjunction with this vector to develop a control signal to an amplifier 696. The output from the summation circuit is applied to the amplifier 696 over the line 698. The output from the amplifier 696 is a control voltage for application to the amplifier 678 over a line 700.

Referring to FIG. 16, there is shown a number of waveforms helpful in understanding the operation of the FM corrector shown in FIG. 15. The waveform represented by the line 701 represents the FM corrector transfer function in generating control voltages for application to the amplifier 678 over the line 700. The line 702 includes four sections of the curve indicated generally at 702, 704, 706 and 708. These components 702, 704, 706 and 708 represent the various control voltages generated in response to the comparison with the instantaneous color burst signal amplitude and the pre set level.

Line 710 represents the mean transfer function of the objective lens 17 employed for reading the successive light reflective regions 10 and light nonreflective regions 11. It can be seen upon inspection that the gain versus frequency response of the lens falls off as the lens reads the frequency modulated representations of the video signal. Referring to the remaining portion of FIG. 16, there is shown the frequency spectrum of the frequency modulated signals as read from the video disc. This indicates that the video signals are located principally between the 7.5 and 9.2 megahertz region at which the frequency response of the lens shown on line 710 is showing a significant decrease. Accordingly, the control voltage from the amplifier 696 is variable in nature to compensate for the frequency response of the lens. In this manner the effective frequency response of the lens is brought into a normalized or uniform region.

FM CORRECTOR SUBSYSTEM--NORMAL MODE OF OPERATION

The FM corrector subsystem functions to adjust the FM video signal received from the disc such that all recovered FM signals over the entire frequency spectra of the recovered FM signals are all amplified to a level, relative one to the other to reacquire their substantially identical relationships one to the other as they existed during the recording process.

The microscopic lens 17 employed in the video disc player 1 has a mean transfer characteristic such that it attenuates the higher frequencies more than it attenuates the lower frequencies. In this sense, the lens 17 acts similar to a low pass filter. The function of the FM corrector is to process the received FM video signal such that the ratio of the luminance signal to the chrominance signal is maintained regardless of the position on the disc from which the FM video signal is recovered. This is achieved by measuring the color burst signal in the lower chroma side band and storing a representation of its amplitude. This lower chroma side band signal functions as a reference amplitude.

The FM video signal is recovered from the video disc as previousdly described. The chrominance signal is removed from the FM video signal and the burst gate enable signal gates the color burst signal present on each line of FM video information into a comparison operation. The comparison operation effectively operates for sensing the difference between the actual amplitude of the color burst signal recovered from the video disc surface with a reference amplitude. The reference amplitude has been adjusted to the correct level and the comparison process indicates an error signal between the recovered amplitude of the color burst signal and the reference color burst signal indicating the difference in amplitude between the two signals. The error signal generated in this comparison operation can be identified as the color burst error amplitude signal. This color burst error amplitude signal is employed for adjusting the gain of a variable gain amplifier to amplify the signal presently being recovered from the video disc 5 to amplify the chrominance signal more than the luminance signal. This variable amplification provides a variable gain over the frequency spectrum. The higher frequencies are amplified more than the lower frequencies. Since the chrominance signals are at the higher frequencies, they are amplified more than the luminance signals. This variable amplification of signals results in effectively maintaining the correct ratio of the luminance signal to the chrominance signal as the reading process radially moves from the outer periphery to the inner periphery. As previously mentioned, the indicia representing the FM video signal on the video disc 5 change in size from the outer periphery to the inner periphery. At the inner periphery they are smaller than at the outer periphery. The smallest size indicia are at the absolute resolution power of the lens and the lens recovers the FM signal represented by this smallest size indicia at a lower amplitude value than the lower frequency members which are larger in size and spaced farther apart.

In a preferred mode of operation, the audio signals contained in the FM video signal are removed from the FM video signal before application to the variable gain amplifier. The audio information is contained around a number of FM subcarrier signals and it has been found by experience that the removal of these FM subcarrier audio signals provides enhanced correction of the remaining video FM signal in the variable gain amplifier.

In an alternative mode of operation the frequency band width applied to the variable gain amplifier is that band width which is affected by the mean transfer function of the objective lens 17. More specifically, when a portion of the total FM recovered from the video disc lies in a range not affected by the mean transfer function, then this portion of the total waveform can be removed from that portion of the FM signal applied to the variable gain amplifier. In this manner, the operation of the variable gain amplifier is not complicated by signals having a frequency which need not be corrected because of the resolution characteristics of the objective lens 17.

The FM corrector functions to sense the absolute value of a signal recovered from the video disc, which signal is known to suffer an amplitude change due to the resolution power of the objective lens 17 used in the video disc signal. This known signal is then compared against a reference signal indicating the amplitude that the known signal should have. The output from the comparison is an indication of the additional amplification required for all of the signals lying in the frequency spectra affected by the resolving power of the lens. The amplifier is designed to provide a variable gain over the frequency spectra. Furthermore, the variable gain is further selective based on the amplitude of the error signal. Stated another way for a first error signal detected between the signal recovered from the disc and the reference frequency, the variable gain amplifier is operated at a first level of variable amplification over the entire frequency range of the affected signal. For a second level of error signal, the gain across the frequency spectra is adjusted a different amount when compared for the first color burst error amplitude signal.

Referring to FIG. 17, there is shown a block diagram of the FM detector circuit 674 shown with reference to FIG. 14. The corrected frequency modulated signal from the FM corrector 672 is applied to a limiter 720 over the line 675. The output from the limiter is applied to a drop-out detector and compensation circuit 722 over a line 724. A suitable drop-out detector circuit is described in a co-pending application Ser. No. 299,891 filed Oct. 24, 1972 entitled "Drop-Out Compensator" and filed in the name of Wayne Ray Dakin. It is the function of the limiter to change the corrected FM video signal into a discriminated video signal. The output from the drop-out detector 722 is applied to a low pass filter 726 over a line 728. The output from the low pass filter 726 is applied to a wide band video distribution amplifier 730 whose function is to provide a plurality of output signals on the line 66, 82, 134, 154, 156, 164 and 166, as previously described. The function of the FM detector is to change the frequency modulated video signal into a discriminated video signal as shown with reference to lines A and B of FIG. 18. The frequency modulated video signal is represented by a carrier frequency having carrier variations in time changing about the carrier frequency. The discriminated video signal is a voltage varying in time signal generally lying within the zero to one volt range suitable for display on the television monitor 98 over the line 166.

Referring to FIG. 19, there is shown a block diagram of the audio processing circuit 114. The frequency modulated video signal from the distribution amplifier 670 of the FM processing unit 32, as shown with reference to FIG. 14, applies one of its inputs to an audio demodulator circuit 740. The audio demodulator circuit provides a plurality of output signals, one of which is applied to an audio variable controlled oscillator circuit 742 over a line 744. A first audio output is available on a line 746 for application to the audio accessory unit 120 and a second audio output signal is available on a line 747 for application to the audio accessory unit 120 and/or the audio jacks 117 and 118. The output from the audio voltage controlled oscillator is a 4.5 megahertz signal for application to the RF modulator 162 over the line 172.

Referring to FIG. 20, there is shown a block diagram of the audio demodulator circuit 740 shown with reference to FIG. 19. The frequency modulated video signal is applied to a first band pass filter 750 having a central band pass frequency of 2.3 megahertz, over the line 160 and a second line 751. The frequency modulated video signal is applied to a second band pass filter 752 over the line 160 and a second line 754. The first band pass filter 750 strips the first audio channel from the FM video signal, applies it to an audio FM discriminator 756 over a line 758. The audio FM discriminator 756 provides an audio signal in the audio range to a switching circuit 760 over a line 762.

The second band pass filter 752 having a central frequency of 2.8 megahertz operates to strip the second audio channel from the FM video input signal and applies this frequency spectra of the total FM signal to a second video FM discriminator 764 over a line 766. The second audio channel in the audio frequency range applied to the switching circuit 760 over a line 768.

The switching circuit 760 is provided with a plurality of additional input signals. A first of which is the audio squelch signal from the tracking servo subsystem as applied thereto over the line 116. The second input signal is a selection command signal from the function generator 47 as applied thereto over the line 170. The output from the switching circuit is applied to a first amplifier circuit 770 over a line 771 and to a second amplifier circuit 772 over a line 773. The lines 771 and 773 are also connected to a summation circuit indicated at 774. The output from the summation circuit 774 is applied to a third amplifier circuit 776. The output from the first amplifier 770 is the channel one audio signal for application to the audio jack 117. The output from the second amplifier 772 is the second channel audio signal for application to the audio jack 118. The output from the third amplifier 776 is the audio signal to the audio VCO 742 over the line 744. Referring briefly to FIG. 21, there is shown on line A the frequency modulated envelope as received from the distribution amplifier in the FM processing unit 32. The output of the audio FM discriminator for one channel is shown on line B. In this manner, the FM signal is changed an audio frequency signal for application to the switching circuits 760, as previously described.

Referring to FIG. 22, there is shown a block diagram of the audio voltage controlled oscillator 742 as shown with reference to FIG. 19. The audio signal from the audio demodulator is applied to a band pass filter 780 over the line 744. The band pass filter passes the audio frequency signals to a summation circuit 782 by way of a pre-emphasis circuit 784 and a first line 786 and a second line 788.

The 3.58 megahertz color subcarrier frequency from the tangential servo system 80 is applied to a divide circuit 790 over the line 140. The divide circuit 790 divides the color subcarrier frequency by 2048 and applies the output signal to a phase detector 792 over a line 794. The phase detector has a second input signal from the 4.5 megahertz voltage controlled oscillator circuit as applied to a second divide circuit 798 and a first line 800 and 802. The divide circuit 798 divides the output of the VCO 796 by 1144. The output from the phase detector is applied to an amplitude and phase compensation circuit 804. The output from the circuit 804 is applied as a third input to the summation circuit 782. The output from the voltage controlled oscillator 796 is also applied to a low pass filter 806 by the line 800 and a second line 808. The output from the filter 806 is the 4.5 megahertz frequency modulated signal for application to the RF modulator 182 by the line 172. The function of the audio voltage controlled oscillator circuit is to prepare the audio signal received from the audio demodulator 740 to a frequency which can be applied to the RF modulator 162 so as to be processed by a standard television receiver 96.

Referring briefly to FIG. 23, there can be seen on line A a waveform representing the audio signal received from the audio demodulators and available on the line 744. Line B of FIG. 23 represents the 4.5 megahertz carrier frequency. Line C of FIG. 23 represents the 4.5 megahertz modulated audio carrier which is generated in the VCO circuit 796 for application to the RF modulator 162.

Referring to FIG. 24, there is shown a block diagram of the RF modulator 162 employed in the video disc player. The video information signal from the FM processing circuit 32 is applied to a DC restorer 810 over the line 164. The DC restorer 810 readjusts the blanking level of the received video signal. The output from the restorer 810 is applied to a first balanced modulator 812 over a line 814.

The 4.5 megahertz modulated signal from the audio VCO is applied to a second balanced modulator 816 over the line 172. An oscillator circuit 818 functions to generate a suitable carrier frequency corresponding to one of the channels of a standard television receiver 96. In the preferred embodiment, the Channel 3 frequency is selected. The output from the oscillator 818 is applied to the first balanced modulator 812 over a line 820. The output from the oscillator 818 is applied to the second balanced modulator 816 over the line 822. The output from the modulator 812 is applied to a summation circuit 824 over a line 826. The output from the second balanced modulator 816 is applied to the summation circuit 824 over the line 828. Referring briefly to the waveform shown in FIG. 25, line A shows the 4.5 megahertz frequency modulated signal received from the audio VCO over the line 172. Line B of FIG. 25 shows the video signal received from the FM processing circuit 32 over the line 164. The output from the summation circuit 824 is shown on line C. The signal shown on line C is suitable for processing by a standard television receiver. The signal shown on line C is such as to cause the standard television receiver 96 to display the sequential frame information as applied thereto.

Referring briefly to FIG. 26, there is shown a video disc 5 having contained thereon a schematic representation of an information track at an outside radius as represented by the numeral 830. An information track schematically shown at the inside radius is shown by the numeral 832. The uneven form of the information track at the outside radius demonstrates an extreme degree of eccentricity arising from the effect of uneven cooling of the video disc 5.

Referring briefly to FIG. 27, there is shown a schematic view of a video disc 5 having contained thereon an information track at an outside radius represented by the numeral 834. An information track at an inside radius is represented by the numeral 836. This FIG. 27 shows the eccentricity effect of an off-center relationship of the tracks to a central aperture indicated generally at 838. More specifically, the off-center aperture effectively causes the distance represented by a line 840 to be effectively different from the length of the line 842. Obviously, one can be larger than the other. This represents the off-centered position of the center aperture hole 838.

Referring to FIG. 28, there is shown a logic diagram representing the first mode of operation of the focus servo 36.

The logic diagram shown with reference to FIG. 28 comprises a plurality of AND function gates shown at 850, 852, 854 and 856. The AND function gate 850 has a plurality of input signals, the first of which is the LENS ENABLE applied over a line 858. The second input signal to the AND gate 850 is the FOCUS SIGNAL applied over a line 860. The AND gate 852 has a plurality of input signals, the first of which is the FOCUS SIGNAL applied thereto for the line 860 and a second line 862. The second input signal to the AND function gate 852 is the lens enable signal on a line 864. The output from the AND function gate 852 is the ramp enable signal which is available for the entire period the ramp signal is being generated. The output from the AND function gate 852 is also applied as an input signal to the AND function gate 854 over a line 866. The AND function gate 854 has a second input signal applied over the line 868. The signal on the line 868 is the FM detected signal. The output from the AND function gate 854 is the focus acquire signal. This focus acquire signal is also applied to the ramp generator 278 for disabling the ramping waveform at that point. The AND function gate 856 is equipped with a plurality of input signals, the first of which is the FOCUS SIGNAL applied thereto over the line 860 and an additional line 870. The second input signal to the AND function gate 856 is a ramp end signal applied over a line 872. The output signal from the AND function gate 856 is the withdraw lens enabling signal. Briefly, the logic circuitry shown with reference to FIG. 28 generates the basic mode of operation of the lens servo. Prior to the function generator 47 generating a lens enable signal, the LENS ENABLE signal is applied to the AND function gate 850 along with the FOCUS SIGNAL. This indicates that the player is in a inactivated condition and the output signal from the AND function gate indicates that the lens is in the fully withdrawn position.

When the function generator generates a lens enable signal for application to the AND gate 852, the second input signal to the AND gate 852 indicates that the video disc player 1 is not in the focus mode. Accordingly, the output signal from the AND gate 852 is the ramp enable signal which initiates the ramping waveform shown with reference to line B of FIG. 6A. The ramp enable signal also indicates that the focus servo is in the acquire focus mode of operation and this enabling signal forms a first input to the AND function gate 854. The second input signal to the AND function gate 854 indicates that FM has been successfully detected and the output from the AND function gate 854 is the focused acquire signal indicating that the normal play mode has been successfully entered and frequency modulated video signals are being recovered from the surface of the video disc. The output from the AND function gate 856 indicates that a successful acquisition of focus was not achieved in the first focus attempt. The ramp end signal on the line 872 indicates that the lens has been fully extended towards the video disc surface. The FOCUS SIGNAL on the line 870 indicates that focus was not successfully acquired. Accordingly, the output from the AND function gate 856 withdraws the lens to its upper position at which time a focus acquire operation can be reattempted.

Referring to FIG. 29, there is shown a logic diagram illustrating the additional modes of operation of the lens servo. A first AND gate 880 is equipped with a plurality of input signals, the first of which is the focus signal generated by the AND gate 854 and applied to the AND gate 880 over a line 869. The FM DETECT SIGNAL is applied to the AND gate 880 over a line 882. The output from the AND gate 880 is applied to an OR gate 84 over a line 886. A second input signal is applied to the OR gate 884 over a line 888. The output from the OR function gate 884 is applied to a first one-shot circuit shown at 890 over a line 892 to drive the one-shot into its state for generating an output signal on the line 894. The output signal on the line 894 is applied to a delay circuit 896 over a second line 898 and to a second AND function gate 900 over a line 902. The AND function gate 900 is equipped with a second input signal on which the FM detect signal is applied over a line 904. The output form the AND function gate 900 is applied to reset the first one-shot 890 over a line 906.

The output from the delay circuit 896 is applied as a first input signal to a third AND function gate 908 over a line 910. The AND function gate 908 is equipped with a second input signal which is the RAMP RESET SIGNAL applied to the AND function gate 908 over a line 912. The output from the AND function gate 908 is applied as a first input signal to an OR circuit 914 over a line 916.

The output from the OR function gate 914 is the ramp reset enabling signal which is applied at least a fourth AND function gate 918 over a line 920. The second input signal to the AND function gate 918 is the output signal from the first one-shot 890 over the line 894 and a second line 922. The output from the AND function gate 918 is applied to a second one-shot circuit 924 over a line 926. The output from the second one-shot indicates the timing period of the focus ramp voltage shown on line B of FIG. 6A. The input signal on line 926 activates the one-shot 924 to generate its output signal on a line 928 for application to a delay circuit 930. The output from the delay circuit 930 forms one input to a sixth AND function gate 932 over a line 932. The AND function gate 934 has as its second input signal the FOCUS SIGNAL available on a line 936. The output from the AND function gate 932 is applied as the second input signal to the OR function gate 914 over a line 938. The output from the AND function gate 932 is also applied to a third one-shot circuit 940 over a line 942. The output from the third one-shot is applied to a delay circuit 942 over a line 944. As previously mentioned, the output from the delay circuit 942 is applied to the OR function gate 884 over the line 888.

The one-shot 890 is the circuit employed for generating the timing waveform shown on line D of FIG. 6A. The second one-shot 924 is employed for generating a waveform shown on line E of FIG. 6A. The third one-shot 940 is employed for generating the waveform shown on line F of FIG. 6A.

In one form of operation, the logic circuitry shown in FIG. 29 operates to delay the attempt to reacquire focus due to momentary losses of FM caused by imperfections on the video disc. This is achieved in the following manner. The AND function gate 880 generates an output signal on the line 886 only when the video disc player is in the focus mode and there is a temporary loss of FM as indicated by the FM DETECT SIGNAL on line 882. The output signal on the line 886 triggers the first one-shot to generate a timing period of predetermined short length during which the video disc player will be momentarily stopped from reattempting to acquire lost focus superficially indicated by the availability of the FM DETECT SIGNAL on the line 882. The output from the first one-shot forms one input to the AND function gate 900. If the FM detect signal available on 984 reappears prior to the timing out of the time period of the first one-shot, the output from the AND circuit 900 resets the first one-shot 890 and the video disc player continues reading the reacquired FM signal. Assuming that the first one-shot is not reset, then the following sequence of operation occurs. The output from the delay circuit 896 is gated through the AND function gate 908 by the RAMP RESET SIGNAL available on line 912. The RAMP RESET SIGNAL is available in the normal focus play mode. The output from the AND gate 908 is applied to the OR gate 914 for generating the reset signal causing the lens to retrack and begin its focus operation. The output from the OR gate 914 is also applied to a turn on the second one-shot which establishes the shape of the ramping waveform shown in FIG. B. The output from the second one-shot 924 is essential coextensive in time with the ramping period. Accordingly, when the output from the second one-shot is generated, the machine is caused to return to the attempt to acquire focus. When focus is successfully acquired, the FOCUS SIGNAL on line 936 does not gate the output from the delay circuit 930 through to the OR function gate 914 to restart the automatic focus procedure. However, when the video disc player does not acquire focus the FOCUS SIGNAL on line 936 gates the output from the delay circuit 930 to restart automatically the focus acquire mode. When focus is successfully acquired, the output from the delay line is not gated through and the player continues in its focus mode.

While the invention has been particularly shown and described with reference to a preferred embodiment and alterations thereto, it would be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims

1. A method of tracking for use in a player for deriving information from spaced information tracks on an information bearing surface, the player including a source beam of radiation for impinging upon the information bearing surface and following an information track thereon, beam position control means having a first portion responsive to the position of the beam of radiation relative to an information track being followed to produce a tracking error signal and a second portion coupled to the first portion in a closed loop mode and responsive to the tracking error signal for controlling the position of the beam impingement point on the information carrier relative to the information track being followed, the method comprising the steps of:

enabling the beam position control means to change from the closed loop mode to an open loop mode;

uncoupling the second portion from the first portion to establish the open loop mode;

driving the second portion in the open loop mode to move the source beam from a first one of the tracks towards a second one of the tracks;

searching for a selected location of the source beam intermediate the first track and moving the source beam towards the second track; and

determining when the beam is located at a prescribed point intermediate the first and second tracks; and,

recoupling the second portion to the first portion to re-establish the closed loop mode and control the movement of the beam, in response to completing the search for the selected location, as the beam approaches the second track, whereby the beam then follows a path formed by the second track a prescribed time after it is determined that said beam is located at said prescribed point.

2. In a player for recovering information from a selected one of a plurality of spaced information tracks on an information-bearing surface, the player including means for providing a beam of radiation to follow an information track, means for imparting relative movement between the surface and the beam along a selected track, a tracking system comprising:

(a) beam position control means having a first portion and a second portion, said first portion responsive to the position of the beam of radiation relative to an information track being followed to produce a tracking error signal indicative of the position of the beam of radiation relative to the information track, said second portion including means for controlling the latitudinal position of the beam of radiation relative to the track on the information-bearing surface;

(b) control means for coupling the tracking error signal from the first portion to the second portion in a first mode of operation, to controllably position the beam of radiation in alignment with a first selected track on the surface; and

(c) means for producing a control pulse signal, the control means further operating, in a second mode of operation, to uncouple the first portion from the second portion, and to couple said control pulse signal to the second portion to controllably move the beam of radiation toward a second selected track on the surface;

(d) means for producing a mode change enabling signal coupled to said control means for selectively causing said control means to change its operating mode from said first mode of operation to said second mode of operation;

(e) the control means including detector means responsive to the tracking error detector means being arranged for determining when the beam of radiation has been moved to a prescribed position intermediate the first track and the second track, and for terminating the control pulse signal at that time; and

(f) the control means further operating to recouple the first portion to the second portion a prescribed time after terminating the control pulse signal.

3. In a player for recovering information from a selected one of plurality of spaced information tracks on an information-bearing surface, the player including means for providing a beam of radiation to follow an information track, means for imparting relative movement between the surface and the beam along a selected track, and tracking error detector means for producing a tracking error signal indicative of the position of the beam of radiation relative to an information track, a tracking system comprising:

(a) beam positioning means for controlling the latitudinal position of the beam of radiation relative to the track on the information-bearing surface;

(b) control means for coupling the tracking error signal from the tracking error detector means to the beam positioning means, in a first mode of operation, to controllably position the beam of radiation in alignment with a first selected track on the surface; and

(c) means for producing a control pulse signal, the control means further operating, in a second mode of operation, to uncouple the tracking error signal from the beam positioning means, and to couple said control pulse signal to the beam positioning means, to controllably move the beam of radiation toward a second selected track on the surface;

(d) means for producing a mode change enabling signal coupled to said control means for selectively causing said control means to change its operating mode efrom from said first mode of operation to said second mode of operation;

(e) the control means including detector means responsive to the tracking error detector means being arranged for determining when the beam of radiation has been moved to a prescribed position intermediate the first track and the second track, and for terminating the control pulse signal at that time; and

(f) the control means further operating to recouple the tracking error signal to the beam positioning means a prescribed time after terminating the control pulse signal.

4. A tracking system as defined in claim 3, wherein further including detector means responsive to the tracking error detector means, the detector means terminates terminating the control pulse signal when the beam of radiation is located substantially midway between the first track and the second track.

5. A tracking system as defined in claim 3, wherein the control means recouples the tracking error signal to the beam positioning means before the beam of radiation reaches the second selected track.

6. In a player for recovering information from a selected one of a plurality of spaced information tracks on an information-bearing surface, the player including means for providing a beam of radiation to follow an information track, and means for imparting relative movement between the surface and the beam along a selected track, a tracking system comprising:

a tracking error detector means responsive to the position of the beam of radiation relative to an information track being followed to produce a tracking error signal indicative of the position of the beam of radiation relative to the information track;

beam positioning means responsive to an input control signal for controllably moving the beam of radiation laterally relative to the track on the information-bearing surface;

control means for selectively coupling the tracking error signal from said error detector means to said beam positioning means in a first mode of operation, to controllably position the beam of radiation in a direction laterally of a first selected track on the surface to produce minimum tracking error and thereby maintain the position of the beam of radiation in alignment with the first selected track; and

pulse generating means for producing a control pulse signal, the control means further operating, in a second mode of operation, to uncouple said tracking error detector means from said beam positioning means, and to couple said control pulse signal from said pulse generating means to said beam positioning means to controllably move the beam of radiation toward a second selected track on the surface;

means for producing a mode change enabling signal coupled to said control means for selectively causing said control means to change its operating mode from said first mode of operation to said second mode of operation;

said control means including analyzing means responsive to the tracking error signal from said tracking error detector means for determining when the beam of radiation has been moved to a prescribed position intermediate the first track and the second track, and further including means for terminating the control pulse signal when the beam has moved to said prescribed position in response to said determining means having determined that said beam of radiation has been moved to said prescribed position; and,

said control means further operating to recouple said tracking error detector means to said beam positioning means a prescribed time after terminating the control pulse signal.

7. A method for employing a player apparatus to repetitively follow at least one track of a plurality of substantially concentric information tracks recorded on an information-bearing surface of an optical disc, said player apparatus including means for rotating said disc; means for impinging an optical beam upon said information-bearing surface; means for radially translating said beam relative to said plurality of tracks; means for generating a tracking error signal indicative of the deviation of the radial position of said beam from a preferred radial position; tracking means, operatively associated with said radial translating means and responsive to said tracking error signal, during a closed loop mode of operation, to cause said beam to follow successive ones of said plurality of tracks; said method including the steps of:

employing said tracking means, in said closed loop mode of operation, to cause said beam to follow a first one of said plurality of tracks;

uncoupling said tracking error signal from said tracking means, to thereby change the mode of operation of said tracking means from said closed loop of operation to an open loop mode of operation, whereby said tracking means is not responsive to said tracking error signal;

generating a control pulse signal and coupling said control pulse signal to said tracking means, in said open loop of mode of operation, said tracking means being responsive to said control pulse signal to move said beam from said first track towards a second one of said plurality of tracks;

terminating said control pulse signal and recoupling said tracking error signal to said tracking means before said beam reaches said second track, whereby said tracking means is responsive to said tracking error signal to cause said beam to follow at least said second track; and,

thereafter repeating, in a consecutive manner, said uncoupling, said generating, and said terminating and recoupling steps, a plurality of times, to thereby cause said beam to repetitively follow at least said second track. 8. The method as set forth in claim 7, wherein said terminating and recoupling step includes the steps of:

determining when said beam has been moved to a prescribed position between said first track and said second track;

terminating said control pulse signal in response to said determining step; and,

recoupling said tracking error signal to said tracking means a prescribed time after terminating said control pulse signal. 9. The method as set forth in claim 7, wherein said first track and said second track are immediately adjacent to each other. 10. In a player apparatus of the type including a beam steering means operable in a closed loop of mode of operation, under the influence of a tracking error signal coupled thereto, for causing an optical beam to follow successive ones of a plurality of substantially concentric information tracks recorded on an information-bearing surface of an optical disc, a tracking system operable to repetitively follow at least one of said plurality of tracks, said tracking system including:

control means for uncoupling said tracking error signal from said beam steering means to thereby change the mode of operation of said beam steering means from said closed loop mode of operation to an open loop mode of operation, whereby said beam steering means is not responsive to said tracking error signal;

means for generating a control pulse signal, said control means functioning to couple said control pulse signal to said beam steering means, in said open loop mode of operation, said beam steering means being responsive to said control pulse signal to move said beam from a track being followed towards an adjacent track;

means for actuating said control means to uncouple said control pulse signal from and then recouple said tracking error signal to said beam steering means, before said beam reaches said adjacent track, to thereby return the mode of operation of said beam steering means from said open loop mode of operation back to said closed loop mode of operation, whereby said beam steering means is responsive to said tracking error signal to cause said beam to follow at least said adjacent track; and,

means for causing repetitive operation of said control means, said control pulse signal generating means, and said actuating means, in a consecutive manner, to thereby cause said beam to repetitively follow at least said second track. 11. The tracking system as set forth in claim 10, wherein said control means is configured to recouple said tracking error signal to said beam steering means a prescribed time after uncoupling said control pulse signal from said beam steering means.