WIRE TRANSMISSION SYSTEM FOR TELEVISION 617 



without any deviation from the adjustments which had been worked 

 out in the original design. 



Requirements 



General. The ideal requirement for a transmission line for television, 

 or for that matter any other purpose, is, of course, that it introduce 

 no distortion whatsoever, in which case there could be no question 

 but that the television images obtained in the receiving apparatus 

 after transmission over the long distance line would be identical with 

 the image obtained with the transmission only over a distance of a 

 few feet. Practical transmission lines, however, tend to introduce a 

 certain amount of distortion and the less the allowable distortion 

 which is specified the greater will be the cost of providing a proper 

 line. Before going ahead with the matter of engineering the line 

 required to transmit the television currents from Washington to New 

 York it was, therefore, first necessary that the requirements be set. 

 The requirements were made more severe than strictly necessary in 

 cases where they were easy to meet. 



Frequency Range. In any system for the electrical transmission of 

 intelligence, the required frequency range is, in general, proportional 

 to the speed of transmission. In the case of picture transmission or 

 television, the speed of transmission may be expressed in terms of the 

 number of picture elements which must be transmitted per second, 

 where a picture element is the smallest unit area which it is intended 

 to be able to distinguish in the received picture from its neighboring 

 unit areas. 



When the picture currents are transmitted in the most efficient 

 manner, the frequency range necessary is approximately equal to 

 half the number of picture elements which must be transmitted per 

 second. A simple way of seeing this is to realize that as the picture 

 elements are transmitted in sequence, the greatest possible rate of 

 variation of detail is obtained when alternate picture elements are 

 black and white. A complete cycle corresponds in this case, therefore, 

 to the time interval required to transmit two picture elements. 



According to this relationship this pa;rticular television system in 

 which about 40,000 picture elements per second are transmitted 

 should require a frequency range of approximately 20,000 cycles. 

 As a matter of fact it was found by a laboratory test that due to certain 

 characteristics of the apparatus a frequency range as great as this 

 was ample, just detectable distortion being introduced in the repro- 

 duction of the human face when the range was narrowed to about 

 14,000 cycles. In providing the line circuit, however, extending the 



