918 



THE 



BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



band the resulting delay distortion introduced into the television band 

 becomes very difficult and expensive to equalize. Below this ''guard 

 band" is the "master group" of 600 telephone channels which is trans- 

 mitted simultaneously with the television signal. 



The detailed allocation of specific frequencies, e.g., TV carrier and the 

 pilot frequencies, results from consideration of effects produced by these 

 frequencies in telephone channels as a consequence of modulation dis- 

 tortion in repeaters. These considerations are described in detail in 

 the system design paper.^ 



Modulation Process 



In the LI coaxial cable system^ frequency translation by the television 

 terminals is accomplished in two stages. The two step process employs a 

 first modulator supplied with a very high frequency carrier to translate 

 all video frequencies to a band far outside the final transmitted band of 



II VIDEO 

 ll INPUT 



LOW-PASS 

 FILTER 



MODULATOR 



VESTIGIAL 

 FILTER 



1 



I 



L3LINE 

 SIGNAL 



I 

 I 



OEMOOULATOR 



LOW-PASS 

 FILTER 



1 



r 4.139 MC 



X 



4.139 MC 



1. 



1. 



r8.278 MC 



Jl 



ll OUTPUT FREQUENCY IN MEGACYCLES PER SECONDS 



Fif?. 2 — Television terminal modulation processes. 



12 



