Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-07-11
2001-02-20
Le, Vu (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240260
Reexamination Certificate
active
06192083
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to digital video encoding and more particularly to video encoding in which statistical multiplexing is used to combine several encoded video bit streams into a single multiplexed bit stream, e.g., for transmission on a single channel.
BACKGROUND OF THE INVENTION
Many video encoding applications utilize statistical multiplexing techniques to combine several compressed video bit streams into a single multiplexed bit stream, e.g., for transmission on a single channel. The bit rate of a given compressed stream generally varies with time based on the complexity of the corresponding video signals. A statistical multiplexer attempts to estimate the complexity of the various video frame sequences of a video signal and allocates channel bits among the corresponding compressed video bit streams so as to provide an approximately constant level of video quality across all of the multiplexed streams. For example, a given video frame sequence with a relatively large amount of spatial activity or motion may be more complex than other sequences and therefore allocated more bits than the other sequences.
An example of a statistical multiplexing encoding system is described in M. Perkins & D. Arnstein,
Statistical Multiplexing of Multiple MPEG
-2
Video Programs in a Single Channel
, SMPTE J., vol. 104, no. 9, p. 596-599, September, 1995. As described in this reference, multiple encoders each receive a respective program, encode the program, and place their compressed picture data of a video signal of the program in a corresponding buffer of fixed size pending submission to a multiplexer. (As per MPEG-2 parlance, a “program” is a collection of one or more related signals. Herein, a program is presumed to include a video signal but may also include one or more associated audio signals, a close caption text signal, etc.) A multiplexer receives the encoded programs from the multiple encoders, in the form of a bit stream. A different bit rate may be assigned to each bit stream depending on a respective estimate of the number of bits needed by the video bit stream of the corresponding program to achieve the same level of quality as the other programs with which it is multiplexed. (Generally speaking, the bit rate of the video bit stream of a program is variable whereas the bit rates of the audio and other associated bit streams of a program are constant. This invention is illustrated in the context of adjusting only the bit rate of the video bit stream portion of a program.) On the receiving end of the communication, a decoder receives the multiplexed multiple program bit stream and discards the data it does not need (e.g., if the decoder is a set top box, only the “tuned” or selected program is retained, whereas the data of each non-selected program is discarded). The retained data is inputted to the decoder's input buffer of a fixed size pending decoding. The removal of data from the decoder buffer for decoding is controlled in a strict fashion to effect a constant end-to-end delay for any selected program. In the statistical multiplexing encoding scheme, the relative timing of each to-be-multiplexed program is independent. A first encoder for a first program may have many pictures of compressed data in its buffer pending submission to the multiplexer while a second encoder for a second bit stream may have only a few pictures in its buffer pending submission to the multiplexer. A decoder that selects the first program for decoding will be receiving “earlier” pictures than a decoder that selects the second program for decoding. Such a variable delay is eliminated by each decoder lengthening or shortening the amount of time the received pictures spend in the decoder buffer pending decoding to effect the above-noted constant end-to-end delay.
FIG. 1
shows a conventional statistical multiplexer
10
. The statistical multiplexer
10
includes a number n of video sources
12
-i, i=1, 2, . . . n. Each of the n video sources
12
-i supplies a video signal (e.g., a video bit stream) to a corresponding encoder
14
-i. The encoders
14
-i generate compressed video bit streams which are supplied to a multiplexer
16
. The multiplexer
16
combines all of the compressed video bit streams into a single multiplexed bit stream. Each of the encoders
14
-i sends statistics about the video bit stream that it is encoding to a statistics computer
18
. The statistics computer
18
uses the statistics received from the encoders
14
-i to determine a suitable allocation of available channel bits among the n video bit streams. The statistics computer
18
sends information regarding the allocated bit rate of each video bit stream to the corresponding encoders
14
-i.
A significant problem with the conventional statistical multiplexer
10
is that bit allocation decisions are generally made using only a posteriori statistics. The statistics computer
18
therefore only receives information about pictures of the n video bit streams that have already been encoded, and must use this information to allocate bits among those pictures that have yet to be encoded. This reliance on a posteriori statistics can result in periods of poor video quality when the video bit streams are eventually demultiplexed, decoded and displayed. The degradation in video quality is particularly severe if one or more of the n to-be-encoded video signals includes a relatively large number of transitions between very complex pictures and very simple pictures. Such transitions can lead the statistics computer
18
to allocate an excessive number of bits to simple pictures of a given video bit stream, or to allocate an insufficient number of bits to complex pictures of the video bit stream, resulting in substantial degradation in perceived video quality for one or more of the multiplexed video bit streams.
The Perkins and Arnstein reference suggests an alternative to the a posteriori statistical multiplexing scheme which uses a priori statistics. An a priori statistical multiplexer is shown in FIG.
2
. In the system
40
, each of n video sources
42
-i for i=1, 2, . . . , n outputs a to-be-encoded program to a corresponding statistics gatherer circuit or processor
43
-i for i=1, 2, . . . , n, respectively. The statistics gatherers
43
-i measure various statistics on the video signal of the program in order to determine the level of difficulty, at that particular instant, in compressing the uncompressed program. A statistics computer
48
receives the statistics gathered by each statistics gatherer
43
-i, and allocates a bit rate to each encoder
44
-i for i=1, 2, . . . , n based on the gathered statistics. A multiplexer
46
multiplexes, i.e., combines, the compressed program bit stream to form a single multiplexed bit stream for transmission on a single channel.
As noted by the Perkins and Arnstein reference, it is difficult to find statistic metrics which lead to an appropriate allocation of bit rates. For instance, if the statistic metric employed by a statistics gatherer significantly underestimates the needs of the program for which it gathers statistics, then the video bit stream of that particular program will be encoded at a significantly lower quality level relative to the other programs. This disparity in quality level will only be corrected when the nature of the video bit stream of that program changes such that the statistics gathered for that program more accurately reflect its encoding needs.
L. Wang & A. Vincent,
Joint Rate Control for Multi-Program Video Coding
, IEEE T
RANS. ON
C
ONS
. E
LECS
., vol. 42, no. 3, August, 1996, proposes an alternative to statistical multiplexing. According to the Wang and Vincent reference, multiple programs are encoded as a single program. A super frame is defined including one frame from the video signal of each to-be-multiplexed program. A bit target is set for each super frame and the bit target may be nearly met by determining a quantization level for the super frame that will generate approximate
Linzer Elliot N.
Wells Aaron
C-Cube Semiconductor II
Le Vu
Proskauer Rose LLP
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