A modem that operates reliably at a symbol rate that corresponds to twice its bandwidth even when it is coupled to a receiving A/D converter that operates under control of a clock is realized by synchronizing the modem's operation to the A/D's clock. The superior operation of this modem advantageously extends to A/D clock frequencies beyond the frequency of twice the modem's bandwidth. To minimize quantization noise, the modem's output is conditioned to minimize intersymbol interference by adjusting the modem's output to the A/D converter's sampling times and slicing levels. When the A/D's clock is higher than twice the bandwidth of the modem's output signal, some intersymbol interference cannot be avoided. In accordance with this invention, the position and value of this interference is computed at the receiver and subtracted from the received signal.
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0. 34. A method of adapting a modem comprising a receiver having an A/D converter that receives an analog signal from a network via a subscriber line, steps comprising:
transmitting a predetermined digital signal from an adaptation resource to a network D/A converter for conversion to an analog signal, and transmitting said analog signal via said subscriber line to said modem during a training period.
0. 26. A modem comprising a receiver having an A/D converter capable of receiving a voiceband analog signal via a subscriber line from a network D/A converter coupled to an adaptation resource that transmits a predetermined digital signal to said network D/A converter during a training period, and with said modem capable of being adapted by the voiceband analog signal resulting from said predetermined digital signal.
11. Apparatus comprising:
an D/A converter operating under control of a first clock, for communicating signals to a A/D converter having a second clock, control means for receiving information from the A/D converter, means, responsive to said control means, for synchronizing the first clock to the second clock, and means, responsive to said control means, for adjusting levels of the output signal of the D/A converter.
12. Apparatus comprising:
an A/D converter operating under control of a first clock, for receiving signals from a D/A converter having a second clock, control means for deriving information from the signal received by the A/D converter, means, responsive to said control means, for synchronizing the first clock to the second clock, and means, responsive to said control means, for adjusting the slicing levels of the A/D converter.
0. 17. An adaptation resource for connection to a digital network and which assists in adapting a modem receiver having an A/D converter that receives a voiceband analog signal from said network via a subscriber line; wherein said adaptation resource transmits a predetermined digital signal to a network D/A converter that converts the digital signal to a voiceband analog signal for transmission via said subscriber line to said modem receiver.
13. An arrangement for communicating digital information comprising a digital-to-analog conversion means, a analog-to-digital conversion means operating with a clock, and an analog medium coupling the output of the digital-to-analog conversion means with the input of the analog-to-digital conversion means, CHARACTERIZED IN THAT:
the digital-to-analog conversion means develops an analog output in synchronism with said clock at levels that are cognizant of slicing levels of the analog-to-digital conversion means, and of said analog medium.
1. Apparatus comprising:
first means for developing a clock signal of period T seconds; and second means, responsive to the first means, for developing an output signal of the apparatus, which output signal is band-limited to frequencies below fc and contains information-bearing analog pulses, at least some of which are spaced T seconds apart; wherein T is less than ½ fc, wherein each of the information-bearing analog pulses represents a magnitude that corresponds to an applied symbol from a constellation of symbols, wherein in each set of M consecutive clock periods T there is a grouping of N of said analog samples, where M and N are integers and N is less than M, and wherein said second means comprises N analog pulse generation means with their outputs combined, and each one of the analog samples in the grouping of N of said analog samples is generated by a different one of the N analog pulse generation means.
16. A method for modifying operational parameters of a modem connected to a digital channel bank via analog lines, where the modem has a receiving portion and a transmitting portion, the method comprising the steps of:
connecting the modem, through the channel bank, to a predefined destination, which connecting is effected over digital facilities following the channel bank, having communication and signal processing capabilities, the modem receiving a known signal, through the digital facilities though the channel bank, from the predefined destination, the modem processing the received signal to develop first control signals for the receiving portion and second control signals for the transmitting portion, the modem equalizing the receiving portion in response to the first control signals developed by the step of processing, and the modem equalizing the transmitting portion in response to the second control signals developed by the step of processing.
15. An modem including a transmitting section for developing an analog signal in response to an applied digital stream, which analog signal is destined to a remote clocked conversion means, a receiving section for developing a digital string in response to an analog signal from the remote clocked conversion means, means for developing a clock in synchronism with the clock in the remote conversion means, and means for employing the developed clock in the transmitting section, the receiving section comprising:
a switched filter, a transmission-medium-emulating filter responsive the output of the switched filter, a codec connected to the output of the transmission-medium-emulating filter, a combiner for subtracting the output of the codec from the output of the transmission-medium-emulating filter, a band limiting filter responsive to an output signal of the combiner, and an input subtracter, for subtracting the output of the band limiting filter from incoming signals to form a symbol signal, the symbol signal being applied to the switched filter and to A/D means for developing said digital string.
3. The apparatus of
N analog pulse generation means, responsive to applied symbols, that contribute to the output signal of the apparatus, and switch means for applying symbols to the N analog pulse generation means to cause each one of the analog samples in the grouping of N of said analog samples to be generated by a different one of the analog pulse generation means, resulting thereby in each one of the analog pulse generation means to be associated with a particular clock period in the set of M consecutive clock periods T.
4. The apparatus of
5. The apparatus of
6. The apparatus of
7. The apparatus of
where p is an index integer, Kp is a constant (and there are p such constants), tp is the pth sampling interval, Ts is the sampling period, tq is the qth sampling interval where q is another integer index ranging from one to seven, and yp is the pth signal sample.
8. The apparatus of
9. The apparatus
10. The apparatus
14. The arrangement of
0. 18. The resource of
0. 19. The resource of
0. 20. The resource of
0. 21. The resource of
0. 22. The resource of
0. 23. The adaption resource of
0. 24. The adaptation resource of
0. 25. The resource of
0. 27. The modem of
0. 28. The modem of
0. 29. The modem of
0. 30. The modem of
0. 31. The adaptation resource of
0. 32. The modem of
0. 33. The modem of
0. 35. The method of
0. 36. The method of
0. 37. The method of
0. 38. The method of
0. 39. The method of
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This application is a re-issue of U.S. Ser. No. 07/963,539, filed on Oct. 20, 1992, now U.S. Pat. No. 5,394,437.
This invention relates to transmission of digital information over an analog medium in tandem with a digital network and, more particularly, this invention relates to high-speed modems.
U.S. Pat No. 4,924,492, issued May 8, 1990, describes a number of approaches for transmitting digital information over an analog transmission medium. To summarize, it describes a number of techniques for combining bits in a signal stream into digital words, converting those words into symbols, associating an analog signal with each symbol, and combining the analog signals to form a contiguous signal that is transmitted over an analog transmission medium. For example, with 3-bit coding, which corresponds to 8 levels and a symbol rate of 160,000 symbols per second, a PAM baseband signal of 80 kHz bandwidth can provide passage to a digital signal of 480 kb/s rate.
In connection with the T1 carrier system and an associated D channel bank (such as the D4 channel bank system offered by AT&T), voiceband channels are digitized and multiplexed to form a single 1.544 Mb/s stream. Specifically, each 3.5 kHz channel is sampled with an 8 kHz clock, and each sample is resolved into one of 256 levels that are represented by 8 bits, resulting in a 64 kb/s rate. Twenty four such channels of 64 kb/s each are combined with framing and other control bits to form a 1.544 Mb/s stream.
In the modem art where signals are restricted to voiceband channels, it has generally been accepted that the channel capacity is in the range of 30 kb/s, assuming a nominal bandwidth of between 3 to 3.5 kHz and a signal-to-noise ratio of about 30 dB. This is based on Shannon's classical work.
In the past 40 years, much progress has been made toward achieving modem data rates that approach this capacity. For example, 19.2 kb/s transmission is now possible over such voiceband channels.
On first blush it may appear that 19.2 kb/s is already close enough to the Shannon limit that sizeable improvement over this number would be very difficult to achieve. Doubling this amount would be practicably out of the question. Yet, the D channel bank provides a capacity that nominally is 64 kb/s. (Actually the capacity is not quite 64 kb/s because some capacity is allocated to signaling and line coding restrictions.)
To state the obvious, 64 kb/s communication over a subscriber line that has not been limited to 3.5 kHz band is doable, as demonstrated, for example, in the aforementioned U.S. patent. The need, however, is not merely to transmit at or near 64 kb/s but, to do so in a voiceband channel in a manner that allows coupling of the developed signal to the D channel bank. Stated in other words, there is a need for a modem whose analog output can be fashioned so well that the D/A converter in the modem and the A/D converter in the channel bank contribute no degradation.
Disclosed is a modem that operates reliably at a symbol rate (e.g., 7 kHz) that corresponds to twice the channel bandwidth even when it is coupled to a receiving A/D converter that operates under control of a clock that operates at a faster rate than this symbol rate (e.g., at 8 kHz). This modem is realized by synchronizing the modem's operation to the A/D's clock. The modem's output is conditioned by adjusting it to the A/D converter's sampling times and slicing levels so that the quantization noise problem existent in conventional arrangements is avoided. To do this properly, the intersymbol interference should also be minimized.
The intersymbol interference is minimized in one embodiment by forming the modem's output from a collection of analog samples that have a zero value at all but predefined sampling instances of the A/D's sampling clock. In another embodiment the signal created at the modem is preconditioned to take account of the intersymbol interference.
When the A/D's clock is higher than twice the bandwidth of the modem's output signal, some intersymbol interference cannot be avoided. However, in accordance with the principles of this invention, the position and value of this interference is computed at the receiver and subtracted from the received signal.
The synchronizing of the modem's operation to the receiving A/D's clock is accomplished in a number of steps. First, the receiving A/D converter's clock is detected, then its phase is ascertained, thereafter the A/D converter's slicing levels are ascertained, and finally the transmission medium's transfer characteristics are equalized.
There are three major obstacles to the coupling of the A/D converter (in the channel bank) to the analog signal that is developed in a modem's D/A converter (in response to an applied digital signal). The first is rooted in the fact that the modem has no knowledge of the timing of the sampling instants or the slicing levels at the channel bank. The second lies in the fact that there is a bandpass filter bandlimited to approximately 250 Hz-3500 Hz between the modem's D/A converter and the channel bank's A/D converter. The third lies in the unknown loss and distortion of the subscriber line between the modem and the channel bank.
In accordance with the principles of this invention an approach is taken which assumes that it is possible (e.g., through a start-up sequence) to learn the exact slicing levels of the channel bank's A/D converter, the channel bank's sampling clock instants, and also the loss and distortion properties of the subscriber line, and the channel bank circuitry. If it were not for the bandpass filter, following the principles of this invention by coupling of the modem's D/A converter to the channel bank's clock, and combining this action with appropriate selection of signals to be sent by the modem (as described in detail below), yields a data rate of up to 56 kb/s, even possibly 64 kb/s if signaling and line conditioning bits in PCM transmission are not used.
Stated in other words, when the effects of the bandpass filter are ignored, the D/A in the modem and the A/D in the channel bank can be made to appear transparent with respect to the communication of digital information when the D/A converts the digital signal to an analog signal that is sure to be reconverted to the same digital signal by the A/D converter. That means that the A/D converter is not allowed to create any quantination noise, and whatever analog sample is created must be captured by the A/D and converted back to digital form. As indicated above, that requires the A/D to be locked to the D/A. (It may be noted in passing that for the A/D to create no quantization noise, the arriving signal must be such that at the time of sampling it is close enough to the correct slicing level to result in the desired digital signal; i.e., the analog level must be s±δ, where s is the analog slicing level and δ]is less than ½ the step size, where δ may depend on s.)
Alas, the A/D converter in the D channel bank is, in fact, preceded by a bandpass filter bandlimited to 250 Hz-3.5 kHz. For analysis purposes this filter can be thought to be at the output of the modem's D/A, and indeed, one can install such a filter at the modem's output in addition to having such a filter at the input of the channel bank. With a filter at the modem's output, the modem can be thought of as a circuit that develops an analog pulse of the appropriate amplitude (corresponding to the digital signal's value) and that pulse is filtered with a filter bandlimited to 250 Hz-3500 Hz. The filtered analog pulses are combined to form the contiguous analog signal at the modem's output. In the following, the simpler case of a 3.5 kHz lowpass filter to approximate the bandpass filter is described first. Then, the bandpass case is presented.
With a flat baseband of 3.5 kHz, the best analog pulses that the modem can generate are of the form
(sometimes referred to as sinc x) with nulls spaced by 1/7000 seconds. This means that analog pulses that are generated at a 7 kHz clock can be sampled at that clock rate with no inter-symbol interference, because at the sampling instances the analog signal of previous and subsequent analog pulses are null and their contribution, therefore, is zero.
On the other hand, no analog pulses can be created that are band-limited to 3.5 kHz and which also exhibit nulls in time at an 8 kHz rate (i.e., spaced 125 μsec apart). (Exhibiting nulls at an 8 kHz rate suggests the presence of signals up to 4 kHz and such signals do not exist in the 3.5 kHz band-limited signal.) That means that creating an analog signal from a collection of analog samples that are spaced at an interval corresponding to the period of an 8 kHz clock cannot be accomplished without introducing inter-symbol interference in the created analog signal.
Stated in other words, it is not possible to choose 8000 independent samples per second and pass them through the modem when the modem's output is band-limited to 3.5 kHz.
In accordance with the principles of this invention, however, non-uniform sampling is employed to transmit 7000 independent samples per second--which is possible with a 3.5 kHz low-pass output filter--with a timing that corresponds to an 8 kHz clock. This is accomplished by sending the 7000 independent samples per second in groups of 7 samples in each interval of 8 periods of the 8 kHz clock. Of course, each of the analog samples is of a magnitude that corresponds to the symbol that created the analog sample.
To generalize, in accordance with the principles of this invention, the modem sends analog samples in groups of N analog samples in each interval of M clock periods, where N and M are integers such that
where fc is the total available bandwidth in the channel and T is the period of the channel bank's A/D converter's clock.
In "On Nonuniform Sampling of Bandwidth-Limited Signals", IRE Transactions on Circuit Theory, December 1956, pp. 251-257, J. L. Yen, demonstrated that Nyquist requirements are not violated merely by sampling a signal more often, as long as the average sampling frequency is within the Nyquist bounds. Indeed, he demonstrated that nonuniform sampling can be a recurrent nonuniform sampling, and inter-symbol interference can be avoided by employing a collection of particularly constructed analog samples. This collection of band limited signals, yp, can take the form
where p=1,2, . . . 7 and Kp is a constant. The product of sine functions in the above equation is composed of sine functions that have a period 16 T, which means that at every 8th sampling instant the function has the value zero. Since each of the functions in the product is offset by tq, it follows that the product has the value zero seven times in an 8 T interval, corresponding to the seven sine functions. This 6-out-of-8-zeros product function modulates the "sinc" function to yield a collection of six different analog pulses yp that have a nonzero value at the same one clock instant.
The above-defined yp signals form the set of analog samples that are combined (after proper modification of their amplitudes in accordance with the incoming symbols) to form the analog output of the modem, as depicted in FIG. 3.
In
Returning to
The eighth output of switch 50 is not connected. However, counter 60 is cycled through all of its 8 states, resulting in the condition that during one out of every eight clock periods the input on line 51 is not coupled to terminal 68. Buffer 40 is interposed ahead of line 51 to account for the uneven transmission of signals to filters 61-67. Thus, the input to buffer 40 supplies samples at a 7000 Hz rate, while the output of buffer 40 is at an 8000 Hz rate, with one out of every 8 clocks not delivering any data out of the buffer. Such data rate converters are fairly conventional so their design is not elaborated herein.
The above describes one realization of a transmitting modem where the set of analog samples yp is designed for a particular band of a baseband signal. It is also possible to design a collection of analog samples for a bandpass signal. For example, if the bandpass signal is symmetrically situated in the 0 to 4000 Hz band, such as between 500 Hz and 3500 Hz, there would be a collection of six analog samples yp, with each crossing zero at six instances of an 8 clock period interval, corresponding to the passage of 6000 independent samples with each collection of 8000 clocks. These analog samples may take the form:
When a pulse of unit value is applied to a time discrete transversal filter, the output is a collection of pulses that define the filter's impulse response, hk, where k is the sample instant index. In response to an applied sequence of pulses x, the filter's output r at sample instant k is
The value contributed by hj when j is not equal to 0 is, in effect, the intersymbol interference.
In accordance with this invention, it is desired to send a signal that, at the output of the channel bank's low-pass filter creates levels that are exactly at the slicing levels of the channel bank's A/D converter. In accordance with the above and because of the intersymbol interference, signals xk should be sent to the channel bank rather than the slicing level signals sk, where xk is related to sk by
This relationship can be realized by applying the output of a circuit to a filter hn-δn, where δn-1 for n=0 and δn=0 otherwise, and subtracting the output of the filter from the input to form the circuit's output. Such an arrangement is depicted in FIG. 4.
In the frequency domain, the
That means that if signal s has a non-zero signal at a frequency where H is zero, the above becomes unbounded. Stated in other words, the circuit of
This potential instability is eliminated in accordance with the principles of this invention by including a switch in the feedback loop as illustrated, for example, in FIG. 5. Specifically, the input signal is applied in
The output of switch 110 is a sequence of samples such that when passed through filter hn results in exactly the correct levels, sans the inter-symbol interference. As indicated earlier, this filter corresponds to the waveshape in the sample-hold followed by the output filter of the modem followed by the filtering action of the transmission medium and followed still by the input filter the channel bank. In
It can be shown that the
In accordance with the above teachings, the
In accordance with this invention as taught above for the lowpass case described above, modem 200 develops an analog output signal such that, for 7 sampling periods in each set of 8 sampling periods, the A/D converter in element 220 receives analog levels which are precisely at the A/D's slicing levels at the time of sampling. For the eighth sampling period, the analog value is not necessarily at the slicing level. For the bandpass case, the switching is different.
Alas, that presents a problem because the signal developed by the D/A converter in element 240 does not faithfully reproduce the analog signal at the input of element 220. This results from of the quantization noise that is introduced by the M-N samples xk corresponding to the unsent samples sk in every set of M samples which are not precisely at a slicing level.
If it is assumed that the output of subtractor 300 is a symbol stream that is identical to the symbol stream applied to the transmitter modem's subtractor 100, then the output of filter block 303 corresponds to the samples that hit the A/D converter in element 220.
Codec block 305 is an element formed from a concatenation of an A/D converter such as the one in block 220 and a D/A converter such as the one in block 240. Applying the output of filter block 304 to block 305 yields a signal out of block 305 that corresponds to a replica of the signal applied to block 305 as corrupted by the quantization noise. Subtracting the output of filter block 304 from the output of codec 305 in subtractor 306, duplicates the quantization noise developed at the output of block 240. Since the quantization noise developed in block 240 is filtered as it passes through medium 250 and is further filtered at the input of receiving modem 260, the output of subtractor 306 is applied in
One basic requirement of this invention is for the communicating modems to be locked in both frequency and phase to the clock of the network's A/D converter with which they interact. Another is for the modems' transmitted levels to be locked to the receiving A/Ds' slicing levels. There is no requirement, however, that the receiving A/D must be part of a channel bank, although it is there where a substantial commercial applicability exists for the present invention. In viewing the arrangement of
One advantage of this approach is that a single modem, i.e., the originating modem, initiates control of the entire synchronization process. One disadvantage of this approach is that, under control of the originating modem, a separate synchronization process must be carried out for modem 260. Another, and perhaps a more troubling disadvantage, is the need to equalize the connection between modem 200 and element 220 while passing signals through the unequalized connection between element 240 and modem 260 and between modem 260 and elements 240 and 220 back to modem 200.
An alternative approach causes modem 260 to enter a training mode upon connection (rather than a loop-back mode), whereupon modem 260 sends a known training sequence to modem 200; while modem 200 sends a known training sequence to modem 260. In accordance with this approach, the two signal paths (modem 200 transmitting to modem 260 and modem 260 transmitting to modem 200) are trained simultaneously. Simultaneous training of the two paths has the advantage that each path contains fewer sources of error.
An even further advantage is derived from a training approach that subdivides the problem into four segments: separately training the path from modem 200 to element 220, the path from element 240 to modem 260, the path from modem 260 to element 240, and the path from element 220 to modem 200. This training approach can best be employed at first hook-up of the modems to the network. Thereafter, the conventional adaptive equalization approach can be undertaken.
In accordance with this four segment approach, which is a network-assisted approach, when a modem is connected to the network, a connection is first established between the modem and a predetermined adaptation resource; e.g., the number 1-900-EQUALIZE. This is depicted in
In addition, if needed, it is possible to follow this procedure for each call without dialing a special number as above as the network 230 can be arranged to recognize that a modem call is being established by virtue of the calling party's ID and the call can automatically be routed to the equalization source.
When a modem such as modem 200 is connected to block 270, a predetermined digital signal is first sent to the modem by block 270. In accordance with
The signal applied by block 270 contains all of the possible levels that the A/D converter 403 must be responsive to. Hence, after equalization of element 405, the incoming signal is used to adapt A/D converter 403 (also through an internal control block) to match the incoming analog levels.
Simultaneously, the output of A/D converter 403 is applied to clock recovery circuit 406, which recovers the clock of the incoming signal (i.e. the clock of the D/A converter in element 220). This recovered clock is applied to A/D converter 403, delay element 408, and equalizer 405. Elements 405, 403, and 406 are conventional elements, well known in the art of modem technology.
Once the receiving portion of modem 200 has been adapted to the network, the transmitting portion is adapted. The need is to synchronize the modem's transmitting portion to the network clock--both in frequency and phase--, to generate the correct amplitudes so that the quantization error will not appear at the output of the A/D converter within element 220, and to predistort the transmitted output signals so that it reaches element 220 without intersymbol interference.
The general approach for training of the modem's transmitting portion is to transmit a signal to block 270, to have block 270 analyze the received signal, and in turn, to have block 270 send control information to the modem, to instruct the modem on how to change its operating parameters.
In accordance with this approach, element 407 detects a training initiation signal from block 270, and in response thereto, instructs it to cause signal generator 410 to deliver a particular signal to D/A 404. That signal is converted in block 404, conditioned in block 409, and is transmitted to block 270. Based on the signal received at block 270, another control signal is sent to element 407, directing it to adjust delay element 408. Delay element 408 adjusts the phase of the clock within element 200 to bring it in proper phase with the network clock in element 220. Once the delay is set at the proper level, adjustments are made within D/A converter 404, also under control of signals delivered by element 407, to match the output levels (as they arrive at the A/D converter in element 220) to minimize quantization error. Lastly, again under control of element 407, equalization circuit 409 is adapted to predistort the output signal so that after transmission over line 210 to element 220, the intersymbol interference as seen at element 220 is minimized.
As with elements 403, 405, and 406, elements 408, 404, 409 and 410 are well known and described in numerous publications; for example, "The Theory and Practice of Modem Design", John A. C. Bingham, John Wiley & Sons, New York, 1988. Element 407 is a logic device that merely generates the control signal necessary for controlling elements 408, 404, 410 and 409. The exact nature of the control signals is, of course, a function of the specific design of those elements. Whatever requirements are placed by those elements, control element 407 can generate them in response to input signals received from block 270 via line 210 and elements 405 and 403. Control element 407 can be implemented with a conventional microprocessor.
Finally, when modem 200 is fully equalized, and modem 260 is fully equalized (simultaneously with modem 200), block 270 may be disconnected and modems 200 and 260 connected to each other.
Ayanoglu, Ender, Dagdeviren, Nuri Ruhi, Mazo, James Emery, Saltzberg, Burton Reuben, Kalet, Irving
Patent | Priority | Assignee | Title |
6556638, | Feb 22 1999 | MIND FUSION, LLC | Method and apparatus for providing increased data speed using synchronization and bit robbing techniques |
6633582, | May 20 1999 | Cisco Technology Inc. | Symmetrical codec selection in an asymmetrical codec environment |
6633604, | Aug 11 1997 | Vocal Technologies, Ltd | Method of improving echo canceller training in modems and of discovering characteristics of a PCM channel |
8237595, | Sep 30 2009 | ENTROPIC COMMUNICATIONS, INC ; Entropic Communications, LLC | Method and apparatus for bandpass digital to analog converter |
8981980, | Sep 30 2009 | ENTROPIC COMMUNICATIONS, INC ; Entropic Communications, LLC | Method and apparatus for bandpass digital to analog converter |
RE39165, | Jun 13 1997 | PCTEL, Inc | Method and apparatus for training linear equalizers in a PCM modem |
Patent | Priority | Assignee | Title |
4597089, | Jul 11 1983 | Hycom Incorporated | Single pulse fast learn modem |
4796004, | Feb 14 1986 | Microchip Technology Incorporated; MICROCHIP ACQUISITION CORPORATION; GENERAL INSTRUMENT OF ARIZONA, INC | Integrated analog-to-digital converter |
4815109, | Jun 25 1987 | NEXTIRAONE, LLC | Sampling clock synchronization |
4868850, | Jun 04 1985 | Fujitsu Limited | MODEM communication system having training means and method for training same |
4890303, | Mar 20 1984 | U.S. Philips Corporation | Method and circuit for synchronizing a digital data transmission system with a network-external clock |
4969162, | May 09 1989 | Telerate Systems Incorporated | Polled data network auto-equalizer system and method |
4995030, | Feb 01 1988 | Memotec Datacom, Inc. | Far end echo cancellation method and apparatus |
5184347, | Jul 09 1991 | WINDWARD CORP ; HIGH POINT GUERNSEY LTD ; HIGH POINT SARL | Adaptive synchronization arrangement |
5214637, | Apr 15 1991 | Motorola, Inc | High speed two wire modem |
5237561, | Oct 05 1989 | SPYDER NAVIGATIONS L L C | Digital data transmission system |
5261118, | Oct 04 1991 | Motorola, Inc | Simulcast synchronization and equalization system and method therefor |
5268929, | Sep 28 1990 | NEC Corporation | Method of effectively normalizing an automatic equalizer in multipoint data communications system and arrangement thereof |
5285474, | Jun 12 1992 | Silicon Valley Bank | Method for equalizing a multicarrier signal in a multicarrier communication system |
5365545, | Apr 24 1992 | Motorola, Inc | MODEM-channel bank converter |
EP1313706, | |||
EP20180969, | |||
EP3249931, | |||
JP57093756, | |||
JP5793756, | |||
WO8907370, | |||
WO9322847, |
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