SOUND TRANSMISSION SYSTEM 479 



at a distance must be supplied with a higher transmission level than 

 would a close-up instrument. It thus becomes necessary to provide 

 for greater gain in transmission and greater electrical power capacity 

 than would be required were the instruments held close to the head. 

 The use of the more elaborate transmission facilities is in itself dis- 

 advantageous but it also tends to increase the feed-back from the loud 

 speaker to the microphone; also the effect of any noise at the micro- 

 phone position or at the listening position tends to interfere more seri- 

 ously with the successful conduct of conversation. In the design of the 

 two-way television system recently installed between the Bell Telephone 

 Laboratories at 463 West Street and the American Telephone & 

 Telegraph Co. at 195 Broadway in New York City, it was felt that 

 it would be possible to overcome these technical objections to the 

 distant type instruments and that the advantages mentioned would 

 justify any measures necessary to do so. 



The question of instruments was solved by the use of the Western 

 Electric 394 condenser type transmitter - and a dynamic direct radiator 

 loud speaker. The transmitter is one of the type generally used for 

 phonograph and sound picture recording and for other purposes where 

 good quality, high stability and quietness of operation are essential. 

 The direct radiator type of loud speaker was used instead of the usual 

 horn type because of the limited mounting space available. It con- 

 sists of a dynamic structure with a rigid duralumin diaphragm about 

 3" in diameter flexibly supported at the edge and radiating directly 

 into free air. \\'hile such a structure is not highly efficient and permits 

 of only a small sound power output these considerations are of second- 

 ary importance in this case. The instruments were located in the 

 front wall of the booth about 2' from the position of the user and ad- 

 jacent to the viewing screen in order to enhance the perspective as 

 described above, the microphone being above and the loud speaker below 

 as shown in Fig. 1. These instruments were (in this particular case) 

 connected into a four-wire circuit although in certain cases it might be 

 desirable to use a 2-wire circuit. Such a change would of course be en- 

 tirely feasible. The remainder of the apparatus used consisted of ampli- 

 fiers located at the transmitting end of each channel and an attenuator at 

 the receiving end, the two ends being connected by means of a loop of 

 approximately 3 miles of non-loaded non-equalized cable. The 

 amplifiers and the attenuators were each readily adjustable so that the 

 sounds of different speakers could be reproduced at the optimum loud- 

 ness. Observation of the performance of the system was made 

 possible in each of the control rooms by means of a monitoring head 



^ E. C. Wentc in Physical Review of May 1922. 



