766 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



rate varies when the picture is scanned at the conventional uniform scan- 

 ning rate. The output of the encoder feeds a transmission hne that has 

 a definite channel capacity, and if maximiun efficiency is to be obtained 

 from this transmission mediiun, then the rate of information into it 

 must be held relatively uniform at a value near the channel capacity. 

 It is the job of the encoder to take the varying rate of information from 

 the decorrelator and feed it to the channel at a constant rate. At the 

 receiving end, the decoder must take the constant rate of information 

 and dehver it to the correlator at the variable rate as originally fed into 

 the transmitter's encoder. Thus, to perform this task, a variable or 

 elastic delay to run ahead or behind, depending on the information con- 

 tent of the picture being scanned, is an important part of the encoder. 

 Over a long period of time, the variable delay would average out to 

 some fixed value. Tliis variable delay must never run out, even when the 

 detail is concentrated. There are instances when this condition could 

 not be met, such as an extended reproduction of a snow storm; however, 

 with good design the system should fail "safe" — a slight degradation of 

 picture quality. This condition can be made infrequent enough to cause 

 little concern. 



The encoder design must also account for noise as well as bandwidth 

 of the channel and must consider the ultimate effect of an error that 

 may be introduced by noise along the transmission line. As more re- 

 dundancy is removed to get at the "essence" of the picture signal, the 

 more important it is to guard this "essence," as mistakes presented to 

 the receiver will propagate themselves longer in the absence of correla- 

 tion. Errors can be minimized by rugged systems of modulation such 

 as PCM, where the signal-to-noise ratio of the transmission line deter- 

 mines the base of the PCM system selected. In any event, the encoder 

 must send the information so that the effect of errors will not appreciably 

 disturb the picture. 



DECORRELATION AND LINEAR PREDICTION 



Fig. 2 illustrates, in a general way, a means of decorrelating the sig- 

 nal, Si(t). For purposes of explanation, the encoder and decoder have 

 been omitted, and the transmission between the receiving and sending 

 terminals, idealized. The predictors, P, are identical, and base their 

 prediction, Sp(t), on the signal's past history. In this way, the output 

 of the computer represents the discrepancy between the actual value of 

 the signal sample and the predictor's prediction. By this means we are 

 sending only our mistakes- — the amount by wliich the next pictm'e ele- 

 ment surprises us. For example, if the computer is so designed that it 



