SCIENTIFIC RESEARCH APPLIED TO THE TELEPHONE 271 



The first commercial instrument for station use, which demonstrated 

 the possibiHty of carrying out this promise, was the transmitter 

 employed in the handset first supplied by the Bell System in 1927. 

 This transmitter had to meet the requirement of giving the same 

 transmission service as transmitters of the deskstand type, and at 

 the same time meet the very exacting requirements imposed by the 

 handset to make it free from howling and capable of performing over 

 a wide range of positions. The diaphragm resonance was damped to 

 a large extent by the use of paper rings and, by lightening the structure, 

 the point of maximum response was moved up in frequency so that 

 it no longer coincided with the peak of the receiver. The effect of 

 this was not only to broaden the response characteristic and improve 

 intelligibility, but also to reduce the gain in the local howling circuit 

 which is, of course, a maximum when both transmitter and receiver 

 have their greatest efficiency at the same frequency. The same 

 separation of peaks resulted in the received speech being less loud, 

 but in spite of this the overall performance was equivalent to that of 

 the best deskstand type of instrument then available. 



With this accomplishment, further work was directed toward 

 maintaining the lower distortion and increasing the efficiency. The 

 transmitter introduced in 1934 represented a marked improvement 

 along this line. This instrument still further broadens the transmitted 

 frequency range and is used with about the same efficiency in desk- 

 stands, handsets, wall sets, and coin-collect sets. 



A new type of handset will be introduced in the Bell System in 

 1937 which, in addition to having a more pleasing and simplified 

 design, will incorporate the new transmitter mounted in such a way 

 as to make fullest use of its ability to transmit efficiently over a wide- 

 frequency band. 



During this evolution of the transmitter, the knowledge which had 

 been gained as to the importance of transmitting different widths of 

 frequency band over commercial telephone circuits led to the establish- 

 ment of the range from 250 to 2750 cycles for designs of new circuits. 

 It was not the intention in the establishment of this range that circuits 

 should not do better than this where it is possible without materially 

 increasing cost, but that all circuits should be at least as good as this. 

 The establishment of this frequency range took into account a number 

 of factors of which a very important one is that the overall utilization 

 of this range from the sound entering the transmitter to the sound 

 output of the receiver provides a grade of transmission which is highly 

 satisfactory for the reproduction of conversational material. 



