Television – Bandwidth reduction system – Data rate reduction
Patent
1993-04-30
1994-09-20
Chin, Tommy P.
Television
Bandwidth reduction system
Data rate reduction
348401, 348405, 348419, H04N 7137
Patent
active
053493834
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the coding of video signals.
Techniques are well known for coding digitised video signals to achieve data compression and thereby reduce the bit rate required to transmit the coded video signals. An example of such a technique is the CCITT recommendation H. 261 Video Coding Standard which employs spatial and temporal redundancies in a video coding process to achieve data compression. Such redundancies vary with picture content and hence the level of data compression and the resultant required bit rate also vary. To facilitate operation with fixed or constant bit rate transmission channels buffering of the coded video data takes place. However this buffering is insufficient to cope with large and rapid variations in data rate as experienced, for example, with a scene change or as a result of motion in the picture. In such circumstances parameters of the coding process are adjusted to reduce the coded data rate. There is, however, a resultant reduction in picture quality. One form of parameter control involves adjusting the step size of the quantisation stage of the coding process in relation to the fullness of the buffer. The overall result is that for fixed rate transmission channels the picture quality is variable, with coding distortions being particularly visible at some times where at other times channel capacity may be wasted because there are few changes to be transmitted.
The prospect of asynchronous transfer mode (ATM) networks such as broadband ISDN, CCITT recommendation I121, offers the possibility of variable bit rate transmission channels with potential benefits for the type of video coding just described. A first approach might be to dispense with the buffering of the coded video data and to exploit the variable bit rate channel of an ATM network to cope with the variable coded data rate of the video transmission. However, ATM networks, which will commonly be packet or cell based, are potentially liable to packet or cell loss, and highly predictive video coding techniques would not respond well to intermittent data loss. To overcome this problem and take advantage of the variable bit rate transmission channels, it has been proposed (N Ghanbrai, IEEE Journal of Selected Areas of Communication, vol 7, no 5, June 1989 pp771-781) to employ two layer video coding with a first, base layer coding containing essential video data and a second, enhancement layer coding containing the difference between input data and the result of the first layer coding, see FIG. 1. The coded data from the base layer coding can be sent via a constant bit rate (CBR) transmission channel with "guaranteed" packets and coded data from the enhancement layer can be transmitted over a variable bit rate (VBR) transmission channel. In the event that packets or cells are lost from the enhancement data on the VBR channel, a minimum picture quality will be maintained by the base layer data sent over the CBR channel. It is an object of the present invention to provide an improved method of coding video signals.
According to the present invention a method of coding a video signal for transmission, comprises:
coding data representing the video signal by a base layer coding operation which includes a base layer quantization step size to provide coded video data for transmission;
deriving inverse-coded video data by carrying out an inverse base layer coding operation on the coded video data;
deriving difference data from the data representing the video signal and the inverse coded video data; and
coding the difference data by an enhancement layer coding operation;
characterised in that the difference data is coded only when the energy of the difference data exceeds a variable threshold, the threshold being inversely proportional to the base layer quantization step size.
According to a further aspect of the present invention apparatus for encoding a video signal for transmission, comprises:
means for coding data representing the video signal by a base layer coding operation which includes a base layer qua
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IEEE Journal on Selected Areas in Communications, vol. 7, No. 5, Jun. 1989, IEEE, M. Ghanbari: "Two-Layer Coding of Video Signals for VBR Networks", pp. 771-781.
IEEE Trans on Communications, vol. 37, No. 4, Apr. 1989, NG Shoau-Bau et al.: "Two-Tier DPCM Codec for Video Conferencing", pp. 380-386.
Beaumont David O.
Morrison David G.
Nilsson Michael E.
Parke Ian
British Telecommunications public limited company
Chin Tommy P.
Tung Bryan S.
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