Television – Bandwidth reduction system – Data rate reduction
Patent
1998-03-02
1999-12-14
Britton, Howard
Television
Bandwidth reduction system
Data rate reduction
386110, H04N 726, H04N 5783
Patent
active
060024402
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the encoding of video signals, in particular for transmission over low bandwidth transmission systems e.g. those that require less than 128 kbit/s.
Around 6 MHz of analogue bandwidth or a data rate in excess of 100 Mbit/s for digital information is required to transmit broadcast quality television signals uncompressed. Such high bit rate signals are impracticable to transmit by limited bandwidth systems. Therefore it is necessary to reduce the amount of information by compressing the video signal. This can be done by taking advantage of the correlation between neighbouring elements (pixels) of a picture and compromising between the reduction in data and the quality of the picture.
Video compression techniques rely on the correlation between neighbouring pixels, either in space and/or in time. For instance, in an area of a scene which is relatively uniform (for instance a wall of a room), the similarity of a pixel with neighbouring pixels within the area is likely to be fairly close. Similarly, in a fairly static scene, the pixels of one frame will correspond closely with the equivalent pixels of a subsequent frame. Hence pixels of a single frame can be coded with the respect of their relationship to each other (intra-frame coding) and/or with respect to their relationship with pixels of neighbouring frames (inter-frame coding). Intra-frame coded frames (intra-frames) can clearly be decoded without reference to any other frame whilst inter-frame coded frames (inter-frames) require information in relation to the frames used in the prediction. Differential techniques may also be used to compress video signals further. Differential coding techniques code only the differences between frames so meaning that less data is required than that required to encode the whole frame.
Various video compression standards have developed. One such is the International standard ISO-IEC11172 "Coding of Moving Pictures and Audio for Digital Storage Media at up to About 1.5 Mbit/s", known as MPEG1. This standard was developed for the storage of video and associated audio on digital storage media such as CD-ROM, digital audio tape (DAT), tape drives, writable optical drives or for transmission over telecommunications channels. According to the MPEG1 standard intra-frames are included in the data stream at regular intervals to allow random access to the video signal and editing thereof. The regular intra-frames also allow interaction with the video signal i.e. a user can request fast forward or fast reverse of a video signal and this may be accomplished by presenting the I frames of the video signal to the user until normal play is requested. However this increases the average bit rate per picture as compared to the normal play mode of the video signal which includes both intra- and inter-frames.
Standards relating to lower data rate systems have also evolved. For instance ITU standard H.261 relates to the coding of moving pictures at rates of n.times.64 kbit/s (where n is an integer from 1 up to 30) (for communication over an ISDN network) and ITU standard H.263 for coding moving pictures at data rates less than 64 kbit/s (for communication over a PSTN network). These encoding schemes do not require the regular inclusion of intra-frames. During the course of a video call, intra-frames are typically only transmitted if error conditions are detected by the receiver or possibly on scene changes. Whilst this means that the average data rate is reduced compared to MPEG1 (the data produced for intra-frames representing a significant proportion of the total data), it is not possible to provide interactive facilities such as fast forward or reverse since there are no clear access points to the data other than at the start. Thus H.261 and H.263 coding schemes are not suitable for providing video streams with which a user can interact.
In the article entitled "Robust H.263 Compatible Transmission for Mobile Video Server Access" by N Farber, E Steinbach and B Griod presented at the IEEE First International Workshop
REFERENCES:
patent: 5712946 (1998-01-01), Yanagihara
patent: 5717816 (1998-02-01), Boyce
patent: 5751893 (1998-05-01), Shimoda
Niko Farber et al, "Robust H 263 Compatible Transmission for Mobile Video Server Access", IEEE COMSOC & EURASIP (Sep. 4-5, 1996).
Dalby David
Leaning Anthony Richard
O'Donnell John Martin
Whiting Richard James
British Telcommunications public limited company
Britton Howard
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