British Telecommunications Corporation

British Telecommunications Public Limited Company

Michael D Carr

David G Morrison

Anthony R Leaning

Richard C Nicol

Mohammad S Azam

John Martin O'Donnell

Maurice G Perini

Anthony B Leaning

Predictive coding and decoding with error drift reduction

Signal coding

An analog-to-digital converter 2 for video signals has a sensitivity (reference) input 4 controlled by a controller 10 in response to the converter's digital output so as to maintain the grey level with a predetermined range. Preferably the controller employs an irregular subsampling of the digital values, and responds differently to extreme and mid-range samples. A scale factor may be forwarded to a digital-to-analog converter for reconversion.

Video level control

A television picture transmission system using conditional replenishment employs a scanning rate at the transmitter which differs (for asynchronous operation, slightly; or for operation between systems using different standards, considerably) from the scanning standard at the the receiver. The frame rate conversion is effected by the updating loops at the transmitter or receiver by, respectively, periodically stopping the coding loop for a field or frame so that no data is produced to produce a lower frame rate or periodically repeating a field or frame in the loop without adding any updating data so as to produce a higher frame rate. Resulting variations in data rate are accommodated by utilizing the buffering inherent in conditional replenishment systems. The jerkiness of motion in the received picture resulting from stopping the loop or repeating a field or frame can be reduced by the use of temporal interpolation in the generation of the updating data to anticipate the stopping or repeating of a field or frame, while conversion of the number of lines per frame can be effected by spatial interpolating using a transversal filter.

Television signal standards conversion

A conditional replenishment video data transmission system involves the transmission of digital values representing the brightness changes at picture elements of a raster scanned image since the previous frame. Since nothing is transmitted for those picture elements without a significant brightness change, the amount of data to be transmitted will depend on the extent of movement in the image, and buffer stores are provided to allow the use of a constant transmission rate. If the extent of movement in the image is very high the buffer store at the receiver will tend to fill up and might overflow. In order to avoid this occurring, reduction of the amount of data to be transmitted by element subsampling has been proposed in which in a cluster of changing elements data relating to only some, for example alternate ones, of the elements are transmitted, and interpolation is used at the receiver to calculate the brightness change data for the omitted elements. The blurring of vertical edges in the image resulting from the interpolation is avoided by suspending sub-sampling where vertical edges are detected in the image. When in the sub-sampling mode, in order to avoid having to mark those normally omitted elements for which data are transmitted the receiver is arranged to detect the presence of a vertical edge and respond by treating the data as if they relate to all elements in the vicinity of the edge and not just the sub-sampled ones. Transmission error can result in the receiver being incorrect in the detection of edges, and to avoid this the number of quantizing levels during sub-sampling is reduced from 8 to 4, using one set of 4 codes for the normally present elements and the other set of 4 codes for the normally omitted elements. Since a vertical edge will usually appear in more than one line, the receiver may be arranged to check the previous line for a vertical edge and anticipate the inclusion of normally omitted elements if such an edge has occurred.

Video data transmission

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Ipswich, United Kingdom

Michael D Carr