Use of Public Address System with 

 Telephone Lines, 



By W. H. MARTIN and A. B. CLARK 



SYNOPSIS: The combination of the public address system and the tele- 

 phone lines makes it possible for a speaker to address, simultaneously, 

 audiences located at a number of different places. Such a combina- 

 tion has been used in connection with several public events and a descrip- 

 tion is given of the system as used on Armistice Day, 1921, when large 

 audiences at Arlington, New York and San Francisco joined in the cere- 

 monies attending the burial of the Unknown Soldier, at the National 

 Cemetery, Arlington, Virginia. 



More recently the public address system has been used in conjunction 

 with telephone lines to attain two-way loud-speaker service. This ar- 

 rangement permits the holding of joint meetings between audiences in 

 two or more locations, separated by perhaps thousands of miles, in such a 

 manner that speakers before each of the audiences can be heard simul- 

 taneously by the other audiences. A demonstration of two-way opera- 

 tion was given at the mid-winter convention of the American Institute of 

 Electrical Engineers in February, 1923, and took the form of a joint meeting 

 between 1,000 members in New York and 500 in Chicago. 



The electrical characteristics of any telephone line which is to be used 

 in conjunction with loud-speaker equipment must receive special atten- 

 tion. In commercial telephone service the main requirement is under- 

 standability, while with the loud-speaker naturalness of reproduced speech 

 is very important. People are accustomed to hearing through the air with 

 very little distortion and naturally expect the same result with loud speakers. 

 A s itisfactory line for this purpose must show freedom from transients, echo 

 effects, etc., as well as good uniformity of transmission over the proper 

 frequency range. 



The public address system, apparatus and methods has also been applied 

 to radio broadcasting. The combination of the public address system with 

 lines and radio makes it possible for one speaker to address enormous num- 

 bers of people located all over the country. — Editor. 



THE public address system which is described in the preceding 

 paper by I. W. Green and J. P. Maxfield, was developed and first 

 used for the purpose of extending the range of the voice of a speaker 

 addressing an audience. With the aid of this system enormous crowds 

 extending from the speaker's stand to points a thousand feet and more 

 distant have in reality become an audience and have easily understood 

 the speaker whose unaided voice covered only that portion of the 

 crowd within a hundred feet or so from him. 



When this system, consisting of a high quality telephone transmitter, 

 distortionless multi-stage vacuum tube amplifiers, powerful loud 

 speaking receivers and projectors, had so shown its capabilities in 

 reproducing speech sounds, a logical extension of its application was 

 to use it with telephone lines. By connecting the transmitting and 

 receiving elements of the public address system through a suitable 



1 Presented at the Midwinter Convention of the A. I. E. E., New York, N. Y., 

 February 14-17, 1923. Published in the Journal of the A. I. E. E. for April, 1923. 



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