The Picturephone System: Digital Encoding of the Video Signal

Pulse code modulation (PCM) is used for processing of analog signals for digital transmission. Quantizing noise and increased bandwidth are the penalties for the noise immunity inherent in digital transmission systems. Differential systems 1 such as delta modulation 2-4 and the more general differential pulse code modulation (DPCM) have been investigated because of the ability to shape the spectrum of the noise and to take advantage of the sample-to-sample correlation in the signal. In a series of papers in 1952, B. M. Oliver, 5 E. R. Kretzmer 6 and C. W. Harrison 7 considered the use of linear prediction for "decorrelation" of video signals. In 1956 W. F. Schreiber 8 performed entropy measurements on television signals to determine the predominant redundancies present. In 1958 R. E. Graham 9 demonstrated by computer simulation the feasibility of using 3-bit D P C M for the television transmission of still black and white pictures. More recently J. B. O'Neal, Jr.; has analyzed delta modulation 10 and D P C M 1 1 for the transmission of video signals. The noise structure of D P C M systems in slope overload has been investigated by E. N. Protonotarios. 12 Subjective evaluations have been recorded by R. C. Brainard. 13 These studies have led the way for the application of D P C M to Picturephone transmission.