TELEVISION TRANSMISSION OVER WIRE LINES 



69 



Interference from all sources, both external and internal, is very low in this 

 system. The largest contributions of such interference are from tube noise and 

 from thermal agitation in the conductors and circuit elements. The effect of 

 interference from external sources so far encountered is lower than the above, al- 

 though the presence of radio broadcasting stations can be detected. Intermodula- 



Fig. 8 — Complete repeater 



tion of signals traversing the system simultaneously has been very carefully meas- 

 ured because of its importance for multichannel telephony and television. In 

 telephony, because of the large number of modulation products, principally 2nd 

 and 3rd order, these appear as random interference. 



The method of measurement of interference from all sources was to transmit 



