Sound Transmission System for Two- Way Television* 



By D. G. BLATTNER and L. G. BOSTWICK 



In this paper is described the speech transmission part of the two-way 

 tele\'ision system described in companion papers. The system is designed 

 to produce the best possible illusion of face-to-face communication between 

 speakers located at a distance. Some of the novel features of the system 

 described include the use of distant pick-up transmitters and loud speakers 

 concealed in the wall of the booth, also the use of heavy glass windows 

 through which the scanning beam and the reproduced image are projected 

 as a means of preventing the admission of noise into the booth. 



IN the design of a sound transmission system to be correlated 

 with a visual system, the requirements as to perfection of results 

 desired are no more stringent than for other high grade sound rep- 

 producing systems ^ that have been described in the literature from 

 time to time. Rather in this case the peculiarities of the system are 

 largely those incidental to the adaptation of old technique to meet 

 new conditions. 



The principal limitation of the sound system imposed by the visual 

 system is that the user be relieved of all necessity of holding a telephone 

 in close proximity to the head. Such a limitation is highly desirable 

 in order to secure the most natural pose of the features and the most 

 satisfactory scanning. Obviously, the best way of meeting this lim- 

 itation is by the use of telephone instruments of the type adapted for 

 picking up and reproducing sounds at a distance. The use of such 

 instruments has the further advantage that they can be located near 

 the vision screen and so reproduce any peculiarities in tone quality 

 that would result if the speaker were actually located at the position 

 of the image. Of, course, the sharpness of this perspective effect 

 obtained is influenced by the loudness of both the original and the 

 reproduced sounds but the matter of location of instruments is also 

 very important. 



It would thus seem that the use of distant pick-up and distant pro- 

 jecting instruments offers certain rather fundamental advantages but 

 it is also true that it presents certain other disadvantages. One of the 

 disadvantages is that the distant pick-up microphone gives less output 

 than a close-up device because of the reduced sound pressure on the 

 diaphragm; also a sound producing device to give suitable reception 



* Presented at June 1930 meeting of A. I. E. E., Toronto, Canada. 

 1 "Public Address Systems" by J. P- Maxfield and I. W. Green in A. I. E. E., 

 Feb. 14, 1923. Also "High Quality Recording and Reproducing of Music and 

 Speech" by J. P. Maxfield and H. C. Harrison in A. I. E. E., Feb. 1926. 



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