Television – Synchronization
Patent
1995-09-14
1998-06-09
Kostak, Victor R.
Television
Synchronization
348512, 348518, H04N 504
Patent
active
057642982
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to transcoders and in particular, but not exclusively, to video transcoders for real-time conversion between a first and a second coding scheme.
RELATED ART
There are many occasions when it is necessary to transmit moving picture television or other real-time generated data, or example speech, over long distances via a transmission link. Broadcast quality television requires around 6 MHz of analogue bandwidth or in excess of 100 Mbit/s when transmitted in digital form and this is expensive to transmit and requires links of high bandwidth. In order to reduce the data rate needed to transmit the signal, various compression techniques are used. Correlation techniques, for example, reduce the amount of information needed to code a particular frame by reference to previously coded video frames. Video compression can also be obtained by allowing an acceptable degree of degradation in the picture quality in order to reduce the information content being transmitted, for example by reducing the frame resolution. So, for example, for video conferencing applications compression down to a bit rate of a few hundred kbit/s is used whereas videophone-quality pictures including sound can be compressed down to only 64 kbit/s, equivalent to a single telephone circuit, and achieve acceptable quality.
Compression techniques may, as indicated above, also be used in areas of technology other than video, for example speech transmission, in order to obtain a higher quality of received transmission for a given allowable bandwidth down the transmission link.
In the case of video coding, to take a particular example, signals (which are typically provided by a video camera) are fed to an encoder which provides an output coded data stream according to some algorithm, for example CCITT Recommendation H. 261 which provides a high degree of compression using spatial and temporal redundancies in the video signals being encoded. The coded data stream can be reconstituted into a series of video signals by a compatible decoder which decompresses the coded data stream.
The video coding technique employed will be chosen to suit the particular application. If a higher broadcast quality is needed or a higher bandwidth transmission link is available other coding algorithms taking advantage of the higher bit rate can be employed.
A characteristic of most encoders which produce highly compressed video data streams is that the rate of generation of bits by the video encoding process is not constant but will depend on the degree of correlation between the current frame being coded and the previous frame or the amount of detail in the region of the frame being coded, or both.
It is possible to provide a constant bit rate in the coded data stream by coding a series of video frames such that the average number of bits per frame is maintained at some given average level. Within this data stream fewer bits are used to code some of the video frames, with more bits being used to code frames which have more information to be coded. That is, the frame rate as determined by the coded data stream on a constant rate transmission path has a long term average which is equal to the rate at which the video frames are received but if inspected over a short time interval the rate at which the frames are transmitted in the coded data stream can vary by a large amount. This variation of the occurence of data relating to the frames within the coded data stream s hereinafter known as jitter.
This is illustrated in FIG. 1 in which it is assumed that the average frame rate of the coded data stream over a six picture frame series is the same as the picture frame rate in the original video signal but with a varying amount of jitter associated with each picture. Thus pictures 1, 2 and 6 have zero jitter as picture frames 1, 2 and 6 of the coded data stream occur at the same rate as the original video signal. The coded data associated with pictures 3 to 5, on the other hand, have varying amounts of jitter j, (as a phase lag) sin
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British Telecommunications public limited company
Kostak Victor R.
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