Television by Pulse Code Modulation* 



By W. M. GOODALL 



Transmission by pulse code modulation presents inviting possibilities in the 

 field of television in that information may be relayed by many repeater sta- 

 tions without deterioration. In a PCM system, the information signal is periodic- 

 ally sampled and its instantaneous amplitude described by a group of pulses 

 according to a pre-set code. These pulse groups occur at the sampling rate and 

 constitute the transmitted signal. In this process an operation known as ampli- 

 tude quantization is required. 



This paper will include a discussion of time sampling, amplitude quantization, 

 binary coding and decoding of a television signal. The operation of the equip- 

 ment used to perform these functions is described. 



The results obtained with an experimental system for different numbers of 

 digits (i.e., maximum number of pulses per group) from one to five are illus- 

 trated by photographs. The television signal used in these tests was obtained 

 from a special low noise film scanner. As was expected, the number of digits 

 required depends upon the amount of noise in the test signal. 



THE papers that have so far appeared on pulse code modulation have 

 dealt primarily with the transmission of speech. The present work deals 

 specifically with the problems involved in the transmission of television, but 

 in its general aspects it is pertinent to the transmission of any broadband 

 signal by PCM. The chief difference between a system for telephony and 

 one for television resides in the required speeds of operation. The use of the 

 wide band required for this system would be justified by the well known ad- 

 vantages of a pulse-code system which have been pointed out by Oliver, 

 Pierce and Shannon^ Regenerative repetition of the on and off binary pulses 

 at repeater points permits the relaying of the signal to great distances with- 

 out introducing any significant degradations due to noise or distortion aris- 

 ing in the medium. In addition, the coding process permits the trading of 

 bandwidth for noise advantage on a very favorable basis. 



General Considerations 



As is well known, PCM is a form of time-division modulation. The in- 

 formation to be transmitted is sampled at regular intervals. This process 

 results in a definite and limited number of amplitudes per unit of time which 

 replace the original wave in subsequent operations. When the sampling 

 frequency is at least twice the highest frequency present in the original 

 wave, the resulting distortion falls outside the desired band and can be re- 

 moved by a low-pass filter in the output of the system. For a system of fixed 



* Presented orally before the I.R.E. National Convention, New York City, March 

 1949. 



II See "Philosophy of PCM"— OHver, Pierce and Shannon— Proc. I.R.E., Nov. 1948. 



