638 BELL SYSTEM TECHNICAL JOURNAL 



structed in one of the wings of the building to house the television 

 terminal apparatus. Walls, ceiling and floor were completely covered 

 with No. 24 gage sheet copper lapped about one inch and carefully 

 soldered. The windows were covered with fine copper gauze. The 

 door was covered with sheet copper which was carried around the 

 edges so that in closing it made a firm wiping contact with the sur- 

 rounding frame. Circuits for lighting and miscellaneous power service 

 were led in through two specially constructed transformers fitted with 

 grounded copper shields between the primary and secondary windings. 

 The picture circuits leading to the radio transmitter, the microphone 

 circuits, and the necessary studio signal and control circuits were 

 run in lead cable and in most cases were brought into the room through 

 suitable radio-frequency filters enclosed in metal boxes attached to 

 the copper sheathing. In order to avoid the possibility of the heavy 

 current leads to the arc bringing in radio-frequency energy, and to 

 eliminate the noise and heat from the arc, provision was made for 

 mounting the latter in its metal cabinet outside of the room. The 

 circular opening through which the light beam was projected into 

 the room was protected by the lamp cabinet which was also grounded 

 to the sheathing. Satisfactory acoustic conditions within the studio 

 were obtained by applying celotex wall board over the copper and by 

 the use of suitable floor coverings. 



Transmitting and Receiving Apparatus 



For the television channel, arrangements were made to install a 

 standard Western Electric 5-B Radio Broadcasting Transmitter and 

 to modify it for the purpose. This transmitter is a 5-kilowatt unit 

 (carrier output without modulation) designed for high quality tele- 

 phone transmission in the 500-1 500-kilocycle band. It will transmit 

 signal components ranging from 50 to 5000 cycles without noteworthy 

 discrimination. At 30 cycles and at 10,000 cycles there is some loss 

 in efficiency and beyond these points the characteristic curve falls 

 rapidly. The necessary changes, therefore, involved both the radio 

 and audio circuits, the latter phase of the problem being perhaps the 

 more difficult. 



The schematic circuit of the modified transmitter is shown in Fig. 4. 

 The revised radio frequency circuits were very similar to the standard 

 arrangement, the changes mainly affecting the magnitudes of various 

 coils and condensers. The output circuits were, of course, redesigned 

 to meet the conditions imposed by the transmission line. The circuit 

 was of the master oscillator — modulating amplifier — power amplifier 

 type. The master oscillator employed a 50-watt tube operating in a 



