THE L3 SYSTEM — DESIGN 795 



Fig. 9 may also be used to obtain relative modulation performance. 

 For this purpose, the following factors must be considered; (1) the 

 magnitude of the signal generating the interference ("black" or 'Svhite" 

 carrier magnitude); (2) whether the interference is proportional di- 

 rectly or to the square of the carrier magnitude; (3) relative interference 

 sensitivity in black or white portions of the picture; and (4) deviations 

 from the Weber-Fechner law as the brightness is varied over its full 

 range. The relationships among these factors were used to establish 

 that for all cases of interest, bar patterns due to cross modulation are 

 always more interfering in either black or white portions of a picture 

 than in an intermediate gray area. 



Table III shows the relative system performance for the five carrier 

 frequency wave forms of Fig. 9. For comparison purposes, the signal- 

 to-noise and signai-to-bar pattern ratios are all related to Fig. 9(f). 



Table II — Television Viewing Tube Characteristics Assumed 



FOR L3 SIGNAL-TO-NOISE ANALYSES 



1. Brightness-grid voltage characteristic of viewing tubes follows 5/2 power law: 



B a el". 



2. Maximum high light brightness of viewing tubes will be 150 foot lamberts. 



3. Contrast ratio of viewing tubes will be 150:1. 



4. Viewing tubes will have interference sensitivities which vary with brightness 



in accordance with the characteristic of Fig. 10. 



5. The visibility of bar patterns will decrease with frequency in accordance with 



the characteristic of Fig. 11. 



6. Deviations from the Weber-Fechner law may be assumed to follow the curve 



of Fig. 12. This law states that "the minimum change in stimulus necessary 

 to produce a perceptible change in response is proportional to the stimu- 

 lus alread.y existing." 



It is obvious from Table III that the signal is transmitted most effi- 

 ciently at an excess carrier ratio of one half. The wave form of Fig. 9 

 (f), which illustrates excess carrier of one half, is the one used in L3. 

 Television terminal circuit problems arising from this choice of carrier 

 frequency wave form are discussed in another paper. 



2.16 Signal Levels and Repeater Spacing 



In a broadband system like L3, the problem of determining the re- 

 peater spacing is made complex by the large number of parameters that 

 must be considered. The approach to this problem that has been used to 

 advantage in the L3 design is to assume several reasonable values of 

 repeater spacing and determine for each the system performance achiev- 

 able with various combinations of important parameter values. This 

 method also permits evaluation of the effects on repeater spacing due 

 to variations in parameters so that it is possible to form judgements as 

 to the most economic design. 



