Abstract: A decoder for video signals, such as MPEG, which uses motion-compensated bidirectional predictive coding, performs concealment of lost or corrupted portions of a picture. For this purpose, it estimates missing motion vectors by combining the two vectors which accompany a bidirectionally coded frame to create a substitute vector. An encoder can be modified to enhance this decoder operation, including forcing at least one frame per group of frames to be coded using bidirectional prediction, and constraining the two vectors so that the substitute vector is closer to the wanted value.

Abstract: A measure of quality of compressed video signals is obtained without reference to the original uncompressed version, but generated directly from the coded image parameters, thereby avoiding the need to decode the compressed signal. A first measure is generated from the quantizer step size and a second measure is generated as a function of the number of blocks in the picture that have only one transform coefficient. The two measures are combined. Adjustments may be made to the step-size based measure to compensate for spatial or temporal masking effects.

Abstract: Compressed recorded video is to be transmitted over a variable bit-rate link, using bandwidth reservation. In order to determining the transmission rate that is to be used (and reserved) at any given time, in such a way as to make the two match so as to minimize wasted bandwidth, the data stream is split into chunks chosen such that the average bit rate for the chunk as a whole is no less that the average bit rate for any shorter chunk starting at the same point. The chunk can then be transmitted using this average rate without any buffering problems. Preferably one chooses chunks so that the average bit rate for the chunk is no less than that for any shorter or longer chunk starting at the same point. This has the benefit that a request for bandwidth never has to ask for an allocation that is higher than that specified in any previous such request. In systems that switch between streams of different degrees compression.

Abstract: A decoder for video signals, such as MPEG which use motion-compensated bidirectional predictive coding, performs concealment of lost or corrupted portions of a picture. For this purpose, it estimates missing motion vectors by combining the two vectors which accompany a bidirectionally coded frame to create a substitute vector. An encoder can be modified to enhance this decoder operation, including forcing at least one frame per group of frames to be coded using bidirectional prediction, and constraining the two vectors so that the substitute vector is closer to the wanted value.

Abstract: A decoder for video signals, such as MPEG, which uses motion-compensated bidirectional predictive coding, performs concealment of lost or corrupted portions of a picture. For this purpose, it estimates missing motion vectors by combining the two vectors which accompany a bidirectionally coded frame to create a substitute vector. An encoder can be modified to enhance this decoder operation, including forcing at least one frame per group of frames to be coded using bidirectional prediction, and constraining the two vectors so that the substitute vector is closer to the wanted value.

Abstract: In order to measure the rate at which a terminal 2 can receive data from a source 5, 6, 7 the source temporarily increases it transmitting rate. The time taken to receive these higher-rate transmissions is measured at the terminal (or acknowledged by the terminal and measured at the transmitter) and the rate calculated. Inter alia, these measurements can be used for control of layered coding where apparatus for transmitting signals to video or other terminals 2 has coding means 5 for receiving the signals to generate a base signal stream B which is a coarse representation of the signal to be transmitted and at least one higher order stream E1, E2 containing finer detail of the signal not present in any lower order stream. These streams are sent by transmitting means 6 to the terminals, but at least some of the terminals do not receive all of the higher order streams.

Abstract: Compressed recorded video is to be transmitted over a variable bit-rate link, using bandwidth reservation. In order to determining the transmission rate that is to be used (and reserved) at any given time, in such a way as to make the two match so as to minimise wasted bandwidth, the data stream is split into chunks chosen such that the average bit rate for the chunk as a whole is no less that the average bit rate for any shorter chunk starting at the same point. The chunk can then be transmitted using this average rate without any buffering problems. Preferably one chooses chunks so that the average bit rate for the chunk is no less than that for any shorter or longer chunk starting at the same point. This has the benefit that a request for bandwidth never has to ask for an allocation that is higher than that specified in any previous such request. In systems that switch between streams of different degrees compression.

Abstract: A measure of quality of compressed video signals is obtained without reference to the original uncompressed version, but generated directly from the coded image parameters, thereby avoiding the need to decode the compressed signal. A first measure is generated (3) from the quantiser step size and a second generated (10, 11) as a function of the number of blocks in the picture that have only one transform coefficient NC. The two measures are combined at 414. Adjustments may be made to the step-size based measure to compensate for spatial or temporal masking effects.

Abstract: In order to measure the rate at which a terminal 2 can receive data from a source 5, 6, 7 the source temporarily increases it transmitting rate. The time taken to receive these higher-rate transmissions is measured at the terminal (or acknowledged by the terminal and measured at the transmitter) and the rate calculated. Inter alia, these measurements can be used for control of layered coding where apparatus for transmitting signals to video or other terminals 2 has coding means 5 for receiving the signals to generate a base signal stream B which is a coarse representation of the signal to be transmitted and at least one higher order stream E1, E2 containing finer detail of the signal not present in any lower order stream. These streams are sent by transmitting means 6 to the terminals, but at least some of the terminals do not receive all of the higher order streams.

Abstract: Transcoding is achieved by decoding a received video signal which has been coded according to a first coding scheme employing motion compensation and carries coded data and motion compensation information, and encoding the decoded according to a second coding scheme also employing motion compensation. Estimated motion vectors are generated for a current frame of the video signal, using vectors which, in the received signal, accompanying at least one other frame of the video signal. These may be used directly or used to define a search area for motion estimation.

Abstract: A decoder for decoding coded video signals includes a base layer and an enhancement layer which includes a cell loss detector for detecting when cells are missing from the enhancement layer. In such an event the enhancement data from the immediately previous video signal displaced by a displacer according to the current motion vectors provides interpolation data for the missing cells. The interpolation data is added to the decoded base layer as indicated by an adder. The enhancement data is obtained by subtracting the immediately previous base video signal stored in a frame store from the immediately previous decoded video signal stored in another frame store as indicated by a subtracter.