r 



TELEVISION BY PULSE CODE MODULATION 



35 



If it were necessary to transmit all of the continuous values present in the 

 sampled wave, it would be necessary to use a large, or even worse, an infi- 

 nite number of digits. Of course, this is not done. Instead the sampled wave 

 which momentarily may have any value is represented by one of the 32 

 values that are permitted by the five-digit code. This process is known as 

 amplitude quantization. The quantized amplitudes are shown in the first 

 column of Fig. 1. In the examples shown any number between 17.5 and 

 18.5 would be represented as 18 and likewise for the other values shown. 

 There is some uncertainty as to the correct value exactly one-half way be- 

 tween permitted values. Here an arbitrary choice is much to be preferred 

 to faulty operation which may give a large error signal. More will be said 

 about this point later. 



H-(C) CODE GROUPS 

 t--f-(e) AUDIO WAVE 



UULJi 



25 



\aT~ 4 4 



^d) DECODED PAM PULSES 



Fig. 2 — PCM wave forms. 



Ul 



25 



28 



Each of these code groups, here illustrated as a 5-digit group, represents 

 a single sample of the ten million per second that are needed to represent the 

 television signal. These digits may be transmitted over a single circuit. 

 Figure 2 is an example of this method of transmission for an audio wave a. 

 The samples are represented by the PAM wave b. The code groups are 

 shown in the wave c while the decoded pulses are shown on the wave d. The 

 original audio wave, delayed by one sampling interval, is shown as wave e. 

 It will be noted that the quantized PAM pulse waves d do not fit exactly on 

 the curve. This is the result of the quantization process previously mentioned. 



For a five-digit 4 kc telephone channel forty thousand digit pulses per 

 second are used in the transmission medium. For television, the same wave 

 forms apply, and a five-digit 5 mc signal uses fifty million pulses per second 

 in the transmission medium. Figure 2 illustrates a PCM system where the 



