938 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



tions or, alternatively, from commercial 60-cycle sources. When com- 

 mercial power is used directly a provision is made for switch-over to 

 emergency power in the event of power failure. The emergency power 

 supply comprises a 130- volt battery, an inverter, and automatic switch- 

 over .equipment. If during service the commercial power source fails a 

 switch-over to emergency power is made automatically in a time under 

 one-tenth second. 



Dc and heater power provisions involve two departures from previous 

 practice. First, the rectifiers which supply anode and other dc power 

 requirements are non-regulated metallic, (selenium), rectifiers. These 

 are supplied from magnetic ac line voltage regulators to obtain suppres- 

 sion of line voltage variations. The combination of the magnetic ac 

 regulator and the metallic rectifier provides adequate suppression of 

 hum and line voltage variations with, it is expected, greater reliability 

 than alternative vacuum tube regulated rectifiers. Secondly, the magnetic 

 ac line voltage regulator together with an improved heater transformer 

 design make possible the operation of the vacuum tubes at reduced 

 heater voltage to obtain increased thermionic life expectancy. This ad- 

 vantage can be taken only if close control of heater transformer output 

 voltage is possible. In this case, variations in output voltage expected 

 to occur due to temperature variations of the transformer windings 

 substantially have been eliminated by incorporating thermistor tem- 

 perature compensation in the transformer primary circuit. 



T3 Transmitting Terminal 



A block diagram of the components of the transmitting terminal is 

 shown in Fig. 18. The composite video signal is received from the Bell 

 System television operation center over 124-ohm balanced cable and 



CARRIER LEVEL 

 CONTROL 



LOW -PASS 

 FILTER 



VIDEO 

 AMPLIFIER 



CLIPPER 





MODU- 

 LATOR 



CARR 

 OSCILLATOR 



"ER ThI— 



ATOR L_V| 



HARMONIC 

 PSC ILL ATOR 



VEST. 

 FILTER 



HF 

 AMPLIFIER 



MOP-UP 

 EQUALIZER^ 



PILOT 

 FILTER 



PREDISTORTER 



AMPLIFIER 



Fig. 18 ~ L3 transmitting terminal block diagram. 



