Transmitting Television as Clusters of Frame-to-Frame Differences

The introduction of Picturephone service has stimulated a great deal of interest in techniques for transmitting video signals more effectively than the way normally used for broadcast television. Since digital transmission over large networks has been found to be more economical for video signals than analog transmission, digital coding is presently receiving most of the attention. One of the best known improvements on simple P C M is differential coding, which takes advantage of the similarity between consecutive samples along a scanning line. Differential coding is simple and yields a bit-rate savings of around 50 percent depending on the picture quality desired. This paper describes a method that makes use of the similarity of pictures in successive frames as well as the similarity of adjacent samples. Until recently, using the correlation between frames to improve efficiency required complex and costly equipment; but now, with low-cost memories and integrated circuits, it is economically attractive and seems to be particularly suitable for the visual telephone application. All techniques that reduce the required channel capacity by anticipating a redundancy in the signal have a limitation, in that signals which do not contain the expected redundancy cannot be transmitted unless some alternative process is provided for them. This limitation is possibly the reason why redundancy-reducing techniques have not been used extensively for broadcast television, where there is need to display unusual scenes to attract viewers and entertain them.