PRODUCTION OF TELEVISION SIGNALS 583 



the data on interference and on permissible signal to noise ratio 

 which were used in the design of the terminal transmitting amplifiers 

 to be described in the latter part of this paper. 



In considering the frequency range of lines, it was apparent in the 

 beginning that the wire channel might include sections of cable. 

 With existing loading systems for such cables a frequency range of 

 not over 40,000 cycles appeared available. The terminal apparatus 

 was therefore designed to deliver a generated signal whose essential 

 components lay well within this limit, and the laboratory tests men- 

 tioned in the preceding section showed that this requirement was met. 



A lower frequency limit was imposed by the necessity of a trans- 

 former for joining the transmission line to the terminal equipment. 

 Fortunately it proved possible to design transformers as described 

 in the final part of this paper in which this limit was at or below the 

 essential low frequency limit found in the preceding discussion of the 

 signal wave. 



(f) Requirements Which Transmission Channels Must Meet in Order 

 to Carry Television Signals. We have shown that a certain band 

 width of frequency components is essential to the adequate repro- 

 duction of the image. This sets the frequency limits of the trans- 

 mission channel which must be provided. It is essential, however, 

 that within these transmission limits the channel should present a 

 reasonably uniform attenuation, and that the phase relations should 

 be fairly accurately maintained. The problem as presented to the 

 transmission engineers of wire, radio and terminal equipment for the 

 recent demonstration was to meet the following requirements: 



First, transmission must be provided for frequencies between about 

 10 cycles and 20,000 cycles. 



Second, the amplitude frequency characteristics within this range 

 should be uniform to about ± 2 T U. 



Third, the phase shift through the range should be maintained so 

 that the slope of its characteristic as a function of frequency is constant 

 to ± 10 or 20 micro-seconds over all but the lowest part of the fre- 

 quency range. There, about 50 times this limit was considered the 

 maximum permissible. 



These requirements were arrived at by considerations based on 

 theory and experiments on television and analogy to similar require- 

 ments in telephotography. The first requirement follows directly 

 from the discussion of the essential frequencies in the signal. The 

 following paragraphs are intended to illustrate the significance of the 

 remaining requirements. 



As we have as yet no quantitative measure of the goodness of 

 38 



