Television – Bandwidth reduction system – Plural video programs in single channel
Patent
1996-10-22
1998-09-08
Britton, Howard W.
Television
Bandwidth reduction system
Plural video programs in single channel
348382, 348388, 370468, H04N 708
Patent
active
058052208
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a system for transmitting a plurality of video programs simultaneously through a transmission channel having a predetermined total bitrate, comprising: assembly consisting of an identical plurality of decoding means provided in parallel for decoding n coded input signals corresponding to said programs and each comprising in cascade a demultiplexing circuit and a decoder, and said encoding assembly consisting of an identical plurality of encoding means being in cascade with said decoding means and each comprising in series a quantizer having a controllable step size and a variable length encoder output channel bitstream; of encoding means in dependence upon the respective output bitrates of said encoding means.
Television programs are often composed by means of a combination of video material from different sources, and particularly it may occur that they are available in an encoded digital form. If the bitstreams corresponding to each of these digital signals are merely switched, there will be no guarantee that the buffer states are correct. Moreover, when different programs are combined, the bitrates can be very different per program. In practice, such situations lead to bitrate peaks, and the picture quality of the encoded television signals will consequently often vary, being noticeably poor for complicated picture scenes.
The patent of the U.S. Pat. No. 5,115,309 describes a system for transmitting simultaneously a plurality of digital video data within a transmission channel having a predetermined bitrate. This system indeed comprises a plurality of parallel encoders, a circuit for controlling in dependency upon the output bitrates of these encoders the step size of a quantization step implemented in each encoder before the encoding step, and a device for combining the output bitstreams of said encoders to a single channel bitstream. However, when the video data to be transmitted are already available in the form of digital programs with very different output bitrates and the achievement of the combining step without impairing the final picture quality is consequently more difficult, such a system does not indicate any practical technical solution which could be implemented in that situation.
SUMMARY OF THE INVENTION
It is an object of the invention to propose a system for transmitting a plurality of video programs simultaneously through a transmission channel with which a higher picture quality is achieved whatever the discrepancy between the bitrates of the programs to be combined and the complexity of the picture scenes.
To this end the system according to the invention is characterized in that said control means are also provided for controlling the output bitrates of said encoding means in dependance upon complexity values associated to the coded input signals of each decoding means with respect to the joint complexity of the plurality of said coded signals.
The invention is based on the recognition that, in spite of the possible very different input bitrates of the programs, bitrate peaks can be avoided and the sum of the output bitrates can be kept constant if transcoders with an appropriate accurate bitrate control, including an improved complexity estimation, are implemented.
In a first advantageous embodiment, in which each picture of said video programs is, according to the MPEG standard, an intraframe coded or a predictively coded or a bidirectionally predictive coded picture, said system is characterized in that it comprises a common bitrate adjusting circuit for applying a respective bitrate target value to each of the control means in dependence upon the complexities of the corresponding video programs, said adjusting circuit comprising computing means for gathering bitstreams and quantization parameters available in said input bitstreams and determining said complexities, a summing device for adding these complexities, dividers of each complexity by their sum, and multipliers of the obtained ratios by the channel bitrate computed
REFERENCES:
patent: 5115309 (1992-05-01), Hang
patent: 5600365 (1997-02-01), Kondo
patent: 5606369 (1997-02-01), Keesman
patent: 5621463 (1997-04-01), Lyons
patent: 5625416 (1997-04-01), Tseng
patent: 5627825 (1997-05-01), Barraclough
Keesman Gerrit J.
Van Otterloo Petrus J.
Britton Howard W.
Gathman Laurie E.
U.S. Philips Corporation
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