A Differential Pulse-Code-Modulation Codec for Videotelephony Using Four Bits Per Sample

A video codec is described that encodes a 1-MHz analog Picturephone® signal into a 6.312-Mbit/s digital form, suitable for transmission on the Bell System T2 digital facility. Through efficient coding of the horizontal sync format, the bulk of the horizontal blanking period is available for sending active video information. The signal is differentially encoded into 4 bits per sample with a quantizer whose levels are spaced nonlinearly. Excess bits are stored in a small buffer to be transmitted during the time made available in the blanking region. The nonlinear quantizer characteristics are subjectively matched to visual processes to minimize the viewer's sensitivity to coding inaccuracies for a broad range of pictures. All the necessary automatic maintenance and alarm functions required for a digitally switched network are included in the codec design.