794 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



The solution to each of these problems required an understanding of 

 how the various alternatives would be affected by the system noise 

 and linearity performance and an understanding of representative tele- 

 vision viewing tube performance with respect to susceptibility to differ- 

 ent types of interference. In analyzing the effect of system performance 

 on these problems, it was found that non-linearity (cross modulation) 

 would produce interferences in the television band which, while very 

 complex electrically because of the effect of cross modulation involving 

 line scan components of the signal, would produce the same effect on 

 viewing tubes as single frequency interferences, i.e., bar patterns. Fur- 

 ther simplifications were made in the analysis when it was found that 

 such interferences were most visible in relatively large areas of tele- 

 vision pictures having essentially constant brightness. During the time 

 intervals corresponding to such areas, the video frequency voltage of 

 the television signal is essentially constant and therefore, in the cases of 

 interest, it could be assumed that the magnitude of the television carrier 

 would also be constant during such intervals. Thus, to compute the 

 magnitude of any modulation product which falls into the television band 

 and which has as one of its components the television signal itself, it is 

 found convenient to use in the computation the magnitude of the tele- 

 vision carrier corresponding to either black or white portions of a picture 

 signal. (The reason for intermediate shades of gray being less susceptible 

 than either black or white is discussed below). 



To evaluate the effect of television viewing tubes on wave-form prob- 

 lems, a number of tests were made to determine blank field threshold 

 values of single frequency interference as a function of frequency for 

 typical viewing tubes. Furthermore, judgements were made as to what 

 might be expected of future viewing tubes with respect to achievable 

 high light brightness, contrast ratio, and operating characteristics. As 

 a result of these tests and judgements, a series of requirements w^ere 

 derived on the basis of long range objectives to be met for these pro- 

 jected characteristics. The results of these tests and judgements are 

 summarized in Table II. 



Using the parameters and methods of analysis outlined in the pre- 

 ceding paragraphs, the relative system performance achievable with 

 each of the carrier frequency wave forms of Fig. 9 was computed or 

 determined by observation. For example, these wave forms are all drawn 

 to the same peak-to-peak amplitude. If we assume that the coaxial 

 system is limited only by the peak amplitude transmitted we may use 

 Fig. 9 to determine relative signal-to-noise performance directly by 

 measuring the peak-to-peak magnitude of the composite signal voltage 

 (sync tip to white) transmitted. 



