An audio bitstream is read from a digital video disc DVD for transfer, subsequent to parsing thereof, via an IEC 958 protocolled interface, for use in a multi-channel audio reproduction apparatus. For each respective audio channel MPEG audio samples are packaged recurrently in burst payloads, and these burst payloads are packaged as user data in iec958 format frames. pause bursts are used for signalling absence of audio for all associated channels with, each pause burst representing such audio absence during a perceptibly acceptable time interval only.
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0. 25. A device for transferring a non-PCM encoded multichannel audio bitstream, the device comprising:
a processor configured to
recurrently package audio data words of the non-PCM encoded multichannel bitstream into burst payloads of audio data-bursts that have a predetermined format,
generate pause data-bursts indicating an absence of audio samples in the bitstream for a predetermined duration; and
output successive data-bursts including the audio data-bursts and the pause data-bursts.
0. 29. A device for receiving a non-PCM encoded bitstream and outputting multichannel audio data for subsequent reproduction, the device comprising:
a receiver configured to receive a non-PCM encoded bitstream for a plurality of audio channels as a sequence of data bits;
a decoder configured to detect a presence or absence of audio data in the sequence of data bits and configured to decode audio information and output decoded audio information depending on the detection of audio data and to conceal the absence of audio data depending on the detection of the absence of audio data.
0. 28. A device for outputting audio samples of a non-PCM encoded multichannel audio bitstream which have audio data discontinuities, the device comprising:
a processor configured to
detect the presence or absence of audio data in an audio channel of a non-PCM encoded bitstream;
recurrently package data words of the audio channel of the non-PCM encoded audio bitstream into payloads of audio data-bursts when audio data is present and into pause data-bursts in response to the audio data discontinuities in the audio channel;
output the sequence of audio data-bursts and pause data-bursts.
0. 26. A device for transmitting a non-PCM encoded multichannel bitstream of audio samples with soft mute blocks to conceal the effect of audio data discontinuities when the audio samples are played, the device comprising:
a processor configured to:
detect the presence of an audio data discontinuity in a particular audio channel in a bitstream;
determine the length of the audio data discontinuity;
generate one or more soft mute blocks depending on the length of the audio data discontinuity;
insert the soft mute blocks into the audio bitstream to conceal the audio discontinuity when the audio is played;
transmitting the audio bitstream containing the soft mute blocks to user audio equipment.
0. 10. A device for packaging audio samples of a non-PCM encoded multichannel audio bitstream into a sequence of iec958 format frames, the device comprising:
a processor configured to
detect the presence or absence of audio data in an audio channel of a non-PCM encoded bitstream;
generate pause data-bursts in response to the absence of audio data in the audio channel,
recurrently package data words of the audio channel of the non-PCM encoded audio bitstream into payloads of audio data-bursts when audio data is present
recurrently package the audio data-bursts and the pause data-bursts in a sequence of iec958 format frames;
output the sequence of iec958 format frames including the inserted pause data-bursts.
0. 27. A device for receiving a non-PCM encoded multichannel bitstream of audio samples which have audio data discontinuities and for providing soft mute blocks to conceal the audio data discontinuities when the audio samples are played, the device comprising:
a processor configured to:
receive a non-PCM encoded multichannel bitstream of audio samples from user audio equipment;
detect the presence of an audio data discontinuity in a particular audio channel in a bitstream;
determine the length of the audio data discontinuity;
generate one or more soft mute blocks based on a length of the audio data discontinuity;
insert the soft mute blocks into the audio bitstream to conceal the audio discontinuity when the audio is played;
output the audio bitstream including the soft mute blocks.
0. 1. A method of transferring a non-PCM encoded multichannel bitstream to digital audio reproduction apparatus via an interface which is adapted for transfer of PCM encoded bitstreams, the non-PCM encoded multichannel bitstream resulting from parsing a multichannel bitstream received from a digital video disc (DVD), comprising the steps of:
(i) recurrently packaging audio samples of the non-PCM encoded multichannel bitstream in data burst payloads;
(ii) packing the data burst payloads as user data in successive frames having a predetermined format including pause bursts signifying the absence of audio in all channels, each pause burst having a duration in the range below 10 milliseconds; and
(iii) decoding and supplying the packaged data burst payloads of the respective channels to respective channel inputs of said audio reproduction apparatus.
0. 2. A method as claimed in
0. 3. A method as claimed in
0. 4. A method as claimed in
0. 5. A method as claimed in
0. 6. A method as claimed in
0. 7. A coupling device for transferring a non-PCM encoded multichannel bitstream to digital audio reproduction apparatus via an interface which is adapted for transfer of PCM encoded bitstreams, the non-PCM encoded multichannel bitstream resulting from parsing a multichannel bitstream read from a digital video disc (DVD); comprising:
means for recurrently packaging audio samples of the non-PCM encoded multichannel bitstream in data burst payloads;
means for packing the data burst payloads as user data in successive frames having a predetermined format including pause bursts signifying the absence of audio in all channels, each pause burst having a duration in the millisecond range below 10 milliseconds; and
means for decoding and supplying the packaged data burst payloads of the respective channels to respective channel inputs of said audio reproduction apparatus.
0. 8. A method of transferring a non-PCM encoded multichannel bitstream to digital audio reproduction apparatus via an interface which is adapted for transfer of PCM encoded bitstreams, the non-PCM encoded multichannel bitstream resulting from parsing a multichannel bitstream read from a digital video disc (DVD), the parsed bitstream being received as a sequence of frames which accumulates, for each channel, a uniform number of data bits, comprising the steps of:
storing each frame in an intermediate frame buffer;
detecting the presence of data pertaining to a particular output channel or the absence of such data as represented by one or more sequential pause bursts;
upon detecting the presence of said data in said detecting step, decoding and outputting decoded information for the particular channel; and
upon detecting one or more sequential pause bursts in the detecting step, controlling a soft mute block to conceal the absence of said data.
0. 9. A coupling device for transferring a non-PCM encoded multichannel bitstream to digital audio reproduction apparatus via an interface which is adapted for transfer of PCM encoded bitstreams, the non-PCM encoded multichannel bitstream resulting from parsing a multichannel bitstream read from a digital video disc (DVD), comprising:
means for receiving said parsed bitstream as a sequence of frames which each accommodate, for each channel, a uniform number of data bits;
an intermediate frame buffer for storing each frame;
detection means for detecting the presence of data pertaining to a particular output channel or the absence of such data as represented by one or more sequential pause bursts;
decoding means controlled by said detection means for decoding and outputting from a stored frame decoded information for said particular channel; and
means responsive to detection by said detection means of said one or more sequential pause bursts to control an audio soft mute block of said one or more sequential pause bursts to conceal the absence of data in said mute block.
0. 11. The device of claim 10, wherein the processor is configured to generate the data-bursts with a burst preamble and a payload, a size of the payload depending on the type of the data-burst.
0. 12. The device of claim 11, wherein the burst preamble consists of four 16-bit control words.
0. 13. The device of claim 12, wherein information in a third control word of the four control words indicates the type of data-burst.
0. 14. The device of claim 10, wherein the audio samples are MPEG audio samples, and the processor is configured to generate the pause data-bursts with a repetition time interval that is equal to at most one MPEG LFE sample recurrency time interval, the pause data-burst repetition time being 96 audio sample periods.
0. 15. The device of claim 14, wherein the processor is configured to generate the pause data-bursts with a repetition time interval equal to an integer multiple of 32 audio sample periods.
0. 16. The device of claim 10, wherein the processor is configured to indicate a presence of the pause data-bursts in a third control word in a header of the pause data-bursts and to insert a 16 bit gap-length parameter in a payload of a first pause data-burst of a series of pause data-bursts, and a 32-bit payload of each pause burst being stuffed with zeros except the 16-bit gap length parameter.
0. 17. The device of claim 10, wherein the pause data-bursts have a repetition time that is at least five times shorter than a repetition time of any non-pause data-burst.
0. 18. The device of claim 10, wherein the processor is configured to generate MPEG-1 layer data-bursts with a repetition time interval of 384 audio sample periods per channel.
0. 19. The device of claim 10, wherein the processor is configured to generate MPEG-1 layer 2 or 3, or MPEG-2 without extension type data-bursts with a repetition time interval of 1152 audio sample periods per channel.
0. 20. The device of claim 10, wherein the processor is configured to generate MPEG-2 with extension type data-bursts with a repetition time interval of 1152 audio sample periods per channel.
0. 21. The device of claim 10, wherein the processor is configured to generate MPEG-2 layer 1 low sample rate type data-bursts with a repetition time interval of 384 audio sample periods per channel.
0. 22. The device of claim 10, wherein the processor is configured to generate MPEG2 layer 2 or 3 low sample rate type data-bursts with a repetition time interval of 1152 audio sample periods per channel.
0. 23. The device of claim 10, wherein the processor is configured to add stuffing zeroes after the date-bursts to fill a space between data-bursts, the minimum number of stuffing zeros being 32, the amount of stuffing depending on a predetermined recurrency period for the date-bursts that depends on the type of data-burst.
0. 24. The device of claim 10, wherein the audio channel is one of a plurality of audio channels and the processor configured to recurrently multiplex the data-bursts among the plurality of channels.
0. 30. The device of claim 29, wherein the absence of audio is detected by detecting presence of one or more pause frames.
0. 31. The device of claim 29, wherein the decoder mutes the audio output based on the presence of one or more pause frames.
0. 32. The device of claim 31, wherein the mute is a soft mute.
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This application claims the benefit or priority of and describes the relationships between the following applications: wherein this application is an application for reissue of U.S. Pat. No. 6,070,062, which issued Jun. 13, 2000, and is also a continuation of U.S. patent application Ser. No. 10/170,822, filed Jun. 13, 2002, which is also an application for reissue of U.S. Pat. No. 6,076,062, from U.S. patent application Ser. No. 08/798,198, filed Dec. 9, 1996, which is the National Stage of International Application No. PCT/IB1996/001267, filed Nov. 21, 1996, which claims the priority of foreign application EPO 95203376 filed Dec. 7, 1995.
The invention relates to a method of transferring a non-PCM encoded audio bitstream read from a digital medium, subsequent to parsing thereof, via an IEC 958 protocolled interface to a multi-channel audio reproduction apparatus.
Digital video disc standardizing is proceeding at an accelerated pace. Commercially available MPEG1 decoder circuit SAA2502 is able to decode compressed digital audio received as a continuous bit stream. Present-day MPEG2 technology has standardized 5 channels, to wit: Left, Right, Center, Left Surround, Right Surround, and furthermore a low frequency enhancement (LFE) channel. The MPEG2 bit stream is distributed into frames of 1152 samples for each of the actual channels, and player operation is controllable in a non-uniform manner on a frame-to-frame basis. For example, the number of actual channels may vary, and certain ones or all of them may be outputting silence.
General background to the invention is given by the following earlier documents, all being at least co-assigned to the present assignee and being herein incorporated by reference:
EP Patent 402 973, EP Patent Application 660 540, corresponding U.S. Pat. No. 5,323,396, issued Jun. 21, 1994, which is a continuation of U.S. application Ser. No. 07/532,462, abandoned; U.S. Pat. No. 5,606,618, issued Feb. 25, 1997; U.S. Pat. No. 5,530,655, issued Jun. 25, 1996; U.S. Pat. No. 5,539,829, issued Jul. 23, 1996; U.S. Pat. No. 5,777,992, issued Jul. 7, 1998; and pending U.S. application Ser. No. 08/488,536, describing a Musicam Layer 1 encoder and decoder for L and R signals;
EP 678 226, corresponding to U.S. Pat. No. 5,544,247, issued Aug. 6, 1996, describing encoding and decoding of L, R and C channels;
U.S. patent application Ser. Nos. 08/032,915, 08/180,004, 08/427,046, describing the matrixing of bitrate-reduced L, R, C, SL and SR signals.
Now, in a consumer application (SPDIF) of the above specified digital video disc, two subframes are specified that each can simultaneously carry 32 bit data words. This allows to transfer via the IEC 958 bitstream either 2-channel linear PCM audio, or a set of alternating bitstreams, but not those configurations simultaneously. The IEC 958 standard specifies a widely used method for interconnecting digital audio equipment with 2-channel linear PCM audio. A need has been encountered to allow transferring non-PcM encoded audio bitstreams for consumer applications in the same protocolled environment, and in particular pause bursts, in case one or more of the audio channels would represent silence. In particular, the granularity of such pause representation at the receiver side should be sufficiently brief from a perceptive standpoint.
Accordingly, amongst other things, it is an object of the present invention to extend present protocols to allow transfer of non-PCM encoded audio bitstreams for consumer applications in the same protocolled environment.
This and other objects, features and advantages according to the present invention are provided by a method for decoding a non-PCM encoded audio bit stream, which can be read from a digital medium. Preferably, the method includes the steps, executed for each respective audio channel, of recurrently packaging MPEG audio samples in burst payloads, and packaging the burst payloads as user data in IEC958 format frames, including pause bursts which signal the absence of audio for all associated channels. According to one aspect of the present invention, each pause burst represents such audio absence during a perceptively acceptable time interval only.
According to the state of the art, the above granularity could reach several tens of milliseconds, which the present inventor has found unacceptably long. According to the invention, the granularity is in the millisecond range which is acceptable in all circumstances encountered at present.
These and other objects, features and advantages according to the present invention are proved by a method for receiving a non-PCM encoded audio bitstream emanating from a parsed bitstream read from a digital versatile disc DVD via an IEC 958 protocolled interface for use in a multi-channel reproduction apparatus. Advantageously, the method includes steps for receiving the parsed bitstream as a sequence of frames each accommodating, for each applicable bitstream, a uniform number of data bits, storing each frame in an intermediate frame buffer; detecting presence or absence of data pertaining to a particular output channel. Moreover, in response to the detecting step, executing decoding and outputting decoded information for the particular channel. However, under control of one or more pause bursts received, as representing a sequential multiplicity of absence of the detecting, controlling a soft mute block. It will be appreciated that this feature provides for straightforward decoding of the pause bursts for subsequent representation by a soft mute block.
These and other objects, features and advantages according to the present invention are provided by a device for implementing the method described immediately above, either on the encoding or the decoding side. Further advantageous aspects of the invention are set forth hereinafter.
These and further aspects and advantages of the invention will be discussed more in detail hereinafter with reference to the disclosure of preferred embodiments, and more in particular with reference to the following drawings, in which:
1. Data Formats
For better detailing the invention, first various applicable information formats are described.
According to the present invention, for consumer applications, the channel status word as built from a sequence of C bits has the following meaning: bit bO with value 0 indicates consumer PCM audio, bit bi with value 1 indicates Non-linear PCM samples, bits 8 through 15 contain a category code. Furthermore, the MPEG header indicates audio sample rate and sample size in bits.
Now, audio bit streams as read from the DVD disc may contain gaps, that may be due to pauses in the audio, or to a trick mode of the related video source, such as the transition to a freeze picture produced in track mode. Now, during transfer in bursts on the IEC 958, these gaps in the bit stream may remain unused or rather, filled with bursts of the data_type ‘pause’ to be described hereinafter. If the gap occurs in MPEG1 layer 1, or in MPEG 1 layer 2 or 3 data, or in MPEG2 without extension, or in MPEG2 data with extension audio bitstream, the gap will be filled with a sequence of bursts of data_type ‘pause’. These bursts may have the minimum allowable length therefor, corresponding to 32 sample periods. Preferably, this number is three times as long, so corresponding to 96 sample periods, which is the recurrency of LFE samples.
It is possible on this interface to simultaneously convey multiple multi-channel non_PCM encoded data streams, for example relating to both a main audio service and to an associated audio service. In that case, the burst of the associated service occurs before the burst of the main service to which it is associated.
Pc
repetition of burst in
Bits
value
content
number of sample periods
0-4
0
Null data
@4096
1
AC-3 stream
1536
2
SMPTE time stamp
3
MPEG1 layer 1 data
384
4
MPEG1 layer ⅔ or MPEG 2
1152
without extension
5
MPEG2 with extension
1152
6
PAUSE
32 or 96
7
ACX data
1024
8
MPEG 2 layer 1 low sample rate
384
9
MPEG2 layer ⅔ low rate
1152
10-31
Reserved
The content of further bits of Pc is irrelevant to the present invention. The provision of the relatively brief ‘pause’ burst allows a low granularity size of ‘soft mute’ intervals controlled thereby. The indication of the various burst type specifications by Pc bit values 3, 4, 5, 8, 9, allows an extremely flexible control policy.
Now, an MPEG2 frame comprises 1152 samples for each encoded channel. The burst as shown in the uppermost row, is headed by a burst_preamble, followed by the payload, and stuffed with stuffing zero bits. The payload numbers up to 36768=1152×32 bits. Furthermore, there are at least 32 stuffing zeroes and 64 bits for the Pa..Pd header. Bitstreams matching the MPEG layer 2 data type are:
A burst with an Audio frame consists of a synchronized and concatenated Base frame (MPEG1 compatible) and an extension frame.
Now, by using units of 32 sample periods per subband filter, synchronization is maintained. In this respect,
2. Hardware Embodiments
For attaining full functionality of MPEG2, an external multichannel MC_box 46 has been provided. To this effect, first in player 30, the MPEG data is configured according to the burst format described with respect to the earlier Figures. Next, this requires an output channel 33 for data according to the standardized IEC 958 protocol, and which is used to convey a non-PCM bitstream inclusive of various commands for the MC_box. The channel may be based on galvanic interconnection or optical fibre. Optionally, interconnection is by a uni- or bidirectional channel 48, in particular for transmitting commands to the DVD player. The channel may be protocolled according to D2B described in U.S. Pat. No. 4,429,384 to the present assignee. Moreover as shown, a FIFO 28 is provided that by way of example accommodates 8 k Bytes as generally required for intermediate storage of MPEG data, a bus interface circuit 32 of commercially available type TDA1315, and a control interface circuit 34 of type MSM6307, organized according to the D2B protocol. Alternatively, block 32 receives commands from the microprocessor 26 on the data path, rather than on its control path.
Like the DVD player, MC_box 46 has an internal control path 41, interface circuit 38 of type MSM6307, and control processing in microprocessor 40. In correspondence to FIFO 28, the MC_box 46 has a relatively small FIFO 44. This stores the data of one bitstream while the previous one is decoded locally. The decoding pertains first to the burst level, and next to the sample level. The output from FIFO 44 feeds MC_decoder 42 that may output up to seven audio channels as indicated: Left, Right, LFE/C, Left center surround, Right center surround, Left surround and Right surround. As shown, these are grouped on four I2S interfaces, according to a protocol described in U.S. Pat. No. 4,755,817 to the present assignee. Alternatively FIFO 44 plus decoder 42 are combined into a single hardware block and controlled directly by the commands contained in the IEC 958 data. Moreover, the MC Box attaches to the secondary control channel 48 by means of circuit 38.
Block 58 is an intermediate buffer that can hold n blocks as specified supra, optimized as regards to cost versus allowable occurrence of over/underflow; expected value of n is about four. Line 70 transmits a stop/go signal to DSP shell 54 that functions as source; line 68 transmits a request signal from the data destination block 60.
Block 60 executes the demultiplexing function with respect to the maximum of seven channels received; it is based on a similar Motorola 56000 DSP processor. In particular, block 62 symbolizes the subband filtering, whereas block 64 symbolizes an LFE ups ample filter. Again, the processor shell has been indicated by block 66. During each execution cycle, 32 subchannels per channel are filtered, and unloaded by means of a dual port RAM: the length of a cycle is thus for a sample frequency of 44.1 kHz:32/44, lk=0.725 millisec. The delay length of the RAM for each channel is advantageously equal to 3*32 subsamples. Filtering takes place when 3*32 subsamples have been received, otherwise the subband filter will output all zeroes signalling an audio pause, which thus has a reduced granularity with respect to prior art. The processor will contain a ‘free running’ function, and will continually output audio samples at uniform intervals. The first DSP 56 will continually produce audio samples in bursts of 1152 samples per channel, 12 groups of 3*32 samples each for each and every channel. The real-time demand is on subband filter 62. If applicable, decoder 56/54 is put on “hold” to avoid an overflow of buffer 58.
The MC Box does not have a user control interface, but the data received on the IEC unidirectional interconnect 33 are used for effecting control, inclusive of the soft-mute and concealing feature according to the invention. If required, the D2B interconnect allows for sending control signals in the reverse direction. The multichannel decoder 60 can be controlled by decoder 54, such as by means of an I2C interface as disclosed in U.S. Pat. No. 4,689,740. This will be robust enough to recover from error conditions. However, no status output to a user is deemed necessary. If underflow occurs in buffer 58, the soft mute feature is controlled subsequently.
The hand-shake between the sub-sample buffer and decoder DSP 56 is implemented by a token that indicates the current owner of the block in question; this token is transferred when synchronization has been effected (77, 88). In the flowchart, block 78 detects either audio data, or a pause. Unless third pause, data type detecting is continued (78). Upon meeting a non-pause, decoding is continued in block 80, and the decoding result is outputted on line 81, subject to the reception of a blocking token to put the processor on hold via block 82. The handshake is between blocks 80 and 82. Bidirectional connection 83 allows reacting to the filling degree of buffer 58. When the third pause is received (84), block 86 prepares zero output blocks for outputting on line 81 as an alternative to the decoding results from block 80, to function as ‘soft mute’ information.
In the implementation, block 50 detects whether the buffer 58 is not empty. If empty, zeroes are output in such a way as to maintain synchronization. If not empty and a token has been passed, the block output is at right, and 32 samples are outputted for each actual channel, plus a single LFE sample. If not empty and no token has been passed, the block output is at left, and 32 zero subband samples are outputted to emulate a pause. Both outputs from block 50 lead to the input of subb and filter 62 and LFE upsample filter 64 in
The token indicates which processor is currently the owner of the block. An owner has read/write access to a block, non-owners can only read, such as read the token. Block ownership is only passed along by the owner of the block, render the actual owner to non-owner. After power-on, all tokens will be handed to the decoder DSP. Absent a token, the subband filter will clear all registers and will filter exclusively zeroes. When synchronizing on the Burst_Preamble, the first token shall be passed to the subband filter DSP after an expected ‘worst case’ decoding time.
PAUSE data-bursts are intended to fill small discontinuities in the bitstream, the gaps which may occur between two data-bursts of a non-PCM encoded audio data type. PAUSE data-bursts convey information of the audio decoder that a gap exists. The PAUSE data-bursts may also indicate the actual length of the audio gap, or that the non-PCM audio data stream has stopped. This information may be used by the audio decoder to minimise (or conceal) the existence of the audio gap, or in the case that the bitstream stops, to trigger a fade-out of the audio. A sequence of PAUSE data-bursts can also assist decoder synchronization prior to the beginning of a non-PCM audio bitstream. It is recommended to send a short sequence of PAUSE data-bursts immediately preceding the transmission of the first audio data-burst.
Data
stuff-
P
stuff-
P
Stuff-
P
stuff-
Data
stuff-
Data
burst
ing
ing
ing
ing
burst
ing
burst
R
R
R
R
R
R
In this example, P indicates a PAUSE burst, P+subsequent stuffing represents the repetition time of PAUSE, and the total gap in between the data bursts is three times as long. The length of Data burst+stuffing is the repetition time of the burst. The PAUSE burst is transferred with the same bit stream number as the bit stream number of the audio data stream which contains the gap that the PAUSE data-bursts are filling, or for which synchronization is being assisted.
The PAUSE data-burst contains the burst_preamble and a 32-bit payload. The first 16 bits of the payload contain the audio gap_length parameter. The remaining bis are reserved, and should all be set to ‘0’. The audio gap_length parameter is an optional indication of the actual audio gap length. This is the length, measured in IEC958 frames, between the first bit of Pa of the first PAUSE data-burst and the first bit of Pa of the next Audio data-burst. The detailed use of the PAUSE data-burst depends on the data-type of the Audio data-burst. For example, gaps between AC-3 data bursts are filled with a sequence of very short PAUSE bursts, and the repetition time of PAUSE data-bursts between data-bursts of an MPEG type is related to the algorithm. The gap_length parameter of the first PAUSE data-burst of the sequence may be used to indicate the length of the audio gap which is being bridged by the sequence of PAUSE data-bursts. The PAUSE data-bursts in the sequence which follows the initial PAUSE data-burst do not have a gap_length specified (gap_length=0). A gap may be filled with one single sequence of PAUSE data-bursts with a single indication of audio gap_length. For example, a gap corresponding to an audio gap of 768 samples may be filled with one sequence of PAUSE data-bursts with an indication of gap-length=768 in the initial PAUSE data-burst. Or this gap could be filled with a number of smaller sequences of PAUSE data-bursts, with the initial PAUSE data-burst in each sequence indicating the gap_length bridged by that sequence (E.g. one sequence with a gap-length of 200 samples, followed by a sequence with gap-length of 568, together bridging a gap of 768 sample periods).
The information about the full length of the audio gap in the first PAUSE data-burst will allow the decoder to perform the best concealment. However, if the data source does not have the information about the full audio gap length at the time the gap begins, then it may signal an initial value for gap_length. If the data source then determines that the audio gap will be longer than the initial indication, then another sequence of PAUSE data-bursts may be initiated (following the first sequence by the repetition time) with another gap_length value to signal the decoder that the audio gap is being extended. If the gap is further extended, additional sequences may be initiated.
Audio decoders may use the gap_length information to optimise their concealment of the audio gap. The inclusion of non-zero values of gap_length is optional, data sources are not required to indicate the length of the audio gap.
The data type PAUSE has a sequence of four control words Pa, Pb, Pc, Pd followed by the Payload and the Stuffing.
“Gaps” are discontinuities in a bitstreams, and may be due to switching between bitstreams. The length of the gaps depend on the timing of switching from one to the other bitstream, and may have any value. The length of a gap however, depends on the decoder which must be able to conceal the gaps. Therefore the transmitter shall adjust the length of the gap to a multiple of the repetition time of PAUSE data-bursts. The PAUSE data-burst has its repetition time, which gives the time of transmission of Pa of the next data-burst.
Some AC-3 decoders may be capable of “concealing” audio gaps. The indication of the audio gap length (gap_length) which may be included in the payload of the PAUSE data-burst allows the decoder to know how long an audio gap will need to be concealed, and thus allow the decoder to optimise the concealment process for the actual audio gap length. AC-3 decoders will most easily conceal audio gaps which have a length equal to an integral multiple of 256 samples. Thus audio gaps of length 256, ?68, etc. IC958 frames are preferred, as follows:
repetition time of PAUSE data-burst
data-type of Audio data-burst
mandatoty
recommended
AC-3 data
3 IEC958 frames
MPEG-1 Layer 1 data
32 IEC958 frames
MPEG-1 Layer 2 or 3 data or
32 IEC958 frames
MPEG-2 without extension
MPEG-2 data with extension
32 IEC958 frames
MPEG-2, layer-1 Low sample rate
64 IEC958 frames
MPEG-2, layer-2 or 3 Low sample
64 IEC958 frames
rate
The AC-3 bitstream consists of a sequence of AC-3-frames. The data-type of a AC 3 data-burst is 01 h. An AC-3 frame contains 1536 samples for each encoded channel. The data-burst is headed with a burst_preamble, followed by the burst_payload. The burst-payload of each data-burst of AC-3 data shall contain one complete AC-3-frame. The length of the AC-3-data-burst will depend on the encoded bit rate (which determines the AC-3-frame length). An AC-3 data burst with reference instant R comprises again four control words Pa, Pb, Pc, Pd an AC-3 burst_payload, and stuffing.
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