300 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



synchronized by means of a vacuum tube oscillator located at one 

 end of the line and delivering a frequency of 1,275 cycles per sec- 

 ond at a low-power level. This frequency is transmitted over a 

 separate pair of wires. At the receiving end this frequency, through 

 vacuum tube means, controls the field strength of the motor and 

 thereby holds its speed exactly proportional to the frequency. In 

 the same way, the speed of the motor at the transmitting end is 

 controlled by a similar vacuum tube circuit so that its speed is also 

 proportional to the frequency of the same oscillator, and thus the 

 motors driving the scanning disks at both ends of the line are held 

 in synchronism. By using a frequency of 1,275 cycles per second, 

 the degree of synchronization is held within sufficiently close limits to 

 keep the picture at the receiving end central within its frame within 

 a small fraction of the picture width. Novel features of this syn- 

 chronizing system are the use of mechanically damping couplings 

 between the disks and the motor shafts to improve the steadiness of 

 the image, and of an electrical phase shifter for framing the images. 



The acoustic portion of the 2-way television system is unusual 

 in that it permits simultaneous 2-way conversation without re- 

 quiring either person to make any apparent use of telephone in- 

 struments. It is obviously desirable to arrange the acoustic system 

 in this way because the ordinary telephone instrument conceals part 

 of the face and would thus prevent the system from approximating 

 to the conditions of ordinary face-to-face conversation. The elimi- 

 nation of telephone instruments is accomplished by the use of a 

 microphone sensitive to remote sounds and a loud speaker concealed 

 near the television image at each station. The microphone at one 

 station is connected through suitable vacuum-tube amplifiers and a 

 telephone circuit to the loud speaker at the other station. This 

 permits conversation in one direction while a similar connection 

 between the other microphone and loud speaker permits conversa- 

 tion in the other direction. The persons using the system then 

 communicate as if face to face and with no telephone system appar- 

 ently involved. 



In order that the transmitted sounds be familiar and natural, dis- 

 tortion in the sound-transmission system has been reduced to a 

 minimum. The microphones are of the condenser type used exten- 

 sively in radio broadcasting and sound-picture recording. Being of 

 small size, they are readily concealed near the television image in 

 the most advantageous position for picking up the voice. The loud 

 speaker, also of small size but capaljle of reproducing a broad fre- 

 quency range, is likewise concealed near the television image, so 

 that the sounds produced appear to emanate from the image itself. 



