SOUND TRANSMISSION SYSTEM 



481 



The ordinates of this curve represent variations in sound pressure from 

 the loud speaker for constant pressure on the transmitter diaphragm. 

 These data were obtained with the loud speaker located in a heavily 

 damped room. The measurements were made on the sound axis at a 

 distance of 2', representing the relative position of the observer under 

 conditions of actual use. 



In setting up such a system the chief consideration is in regard to the 

 acoustic feed-back from the loud speaker to the microphone and in 

 this connection the design of the booth is an important factor. The 

 booth must necessarily be so shaped that the user, looking at the view- 

 ing screen, can be satisfactorily scanned and the light reflected from 



463 WEST ST. 



I CONTROL ROOM 



CONDENSER 

 MICRO- I- 

 PMONE 



CONDENSER 

 MICRO- 

 PHONE 



AMPLIFIER 



LOUD 

 SPEAKER 



VOLUME 

 INDICATOR 



MAIN 

 AMPLIFIER 



MONITORING 

 RECEIVER 



MONITORING 

 ATTENUATOR 



195 BROADWAY 

 CONTROL ROOM 



RECEIVING 

 ATTENUATOR 



MONITORING 

 ATTENUATOR 



MONITORING 

 RECEIVER 



LOUD 

 SPEAKER 



MAIN 

 AMPLIFIER 



VOLUME 

 INDICATOR 



CONDENSER 

 MICRO- 

 PHONE 



AMPLIFIER 



CONDENSER 

 MICRO- 

 PHONE 



Fig. 2 — Circuit diagram for sound transmission system for two-way television. 



the scanned areas will strike the banks of photoelectric cells required 

 for the reproduction of the visual likeness. This requires that the 

 person scanned be located in close proximity to the scanning disc and 

 to the photoelectric cells as well as to the microphone and loud speaker. 

 Such an arrangement is objectionable from an acoustic standpoint in 

 that in the present state of development the cells are necessarily large 

 and poor absorbers of sound. They thus tend to cause part of the 

 sound output from the loud speaker to reflect back into the microphone. 

 If the sound so reflected or fed back is equal or greater in magnitude 

 than the original sound picked up and is of the proper phase relation, 

 the system will "sing" and the sound system become of no practical 

 use. A further eff"ect of the design of the booth is that as a closed 

 cavity, it tends to cause sounds of a certain pitch range to be accen- 



