TELEVISION TRANSMISSION OVER WIRE LINES 



67 



the final stage. From 2 mc up to 3 mc the feedback gradually falls off about 10 db. 

 This arrangement gives the high degree of transmission stability and linearity 

 required for long telephone systems with hundreds of amplifiers in tandem, 

 and satisfactorily meets present requirments. Limited experience with television 

 transmission so far indicates satisfactory performance. The linearity is illustrated 

 in Fig. 6 which shows measurements of 2nd and vkd order modulation products 

 of a 1000-kc signal in a typical amplifier at various signal levels. As in previous 

 coaxial systems, power for operating the amplifiers is transmitted at 60 cycles 

 over the coaxial cable itself from main stations located at about 50-mile intervals. 



64 



1 



2 52 



36 

 32 

 28 

 24 

 20 

 16 



10 II 12 13 14 15 16 17 18 19 20 21 22 23 



FUNDAMENTAL POWER OUTPUT IN DECIBELS fABOVE 1 MILLIWATT) 



Fig. 6 — Amplifier linearity 



24 



Regulation 



In order to compensate for changes in attenuation due to temperature change 

 of the copper conductors, the gain of the amplifier is regulated automatically by a 

 device located at each amplifier point which is operated from a pilot channel. In 

 this system a pilot frequency of 2064 kc is transmitted along the line with the 

 signal. At the output of each amplifier, a high-impedance highly selective 

 crystal filter is bridged on the circuit to select the pilot frequency. This is then 

 amplified, rectified, and used to control the output of an oscillator. The oscillator 

 output in turn is used to control the resistance of one element in the feedback 

 circuit of the amplifier. This variable element is a very tiny thermistor^*^ made 



