IMAGE TRANSMISSION SYSTEM 



461 



transformer, the transformer winding is shunted by a battery and a 

 resistance in series. The resistance is made high, so that the trans- 

 mission loss due to bridging it across the circuit is small; the voltage 

 of the battery is made equal to the potential drop across the resistance 

 due to the plate current of the tube, so that the average voltage 

 across both the battery and the resistance, and hence across the trans- 

 former winding, is zero. 



A vacuum thermocouple is connected in series with the line winding 

 of the output transformer, serving as level indicator for the trans- 

 mitting amplifier. The level indicator for the receiving amplifier is 



12 



II 



10 



- 

 U 



u 



10 



100 1,000 10,000 



FREQUENCY IN CYCLES PER SECOND 



100,000 



Fig. 10 — Insertion loss characteristic of cable cricuits which transmit the image 

 signal, measured between 125 ohm resistances. 



a vacuum thermocouple in series with the grid resistance of the two 

 250-watt tubes. 



The electrical control panels associated with one terminal of the 

 television apparatus are shown in Fig. 11. 



Transmission Circuits 



Two special requirements for the two-way television transmission 

 circuits are to be emphasized. The first, which has already been 

 referred to, is the wide frequency transmission band, from 18 cycles 

 to 40,000 cycles, which must have a high degree of uniformity of trans- 

 mission efficiency and freedom from phase distortion. The second is 

 the necessity for two circuits for the television images. This arises 

 from the fact that the two parties to the conversation must both see 

 and be seen at all times. There can be no interruption of one face by 

 the other, comparable with the alternation of the role of speaker and 

 listener in telephony which permits the use of a single circuit for 

 ordinary speech communication. 



The terminal stations of the two-way television system are con- 



