SCIENTIFIC RESEARCH APPLIED TO THE TELEPHONE 269 



to the condition that when speaking into the transmitter one hears 

 his own voice in the receiver. Local speech so heard is designated as 

 sidetone. The Laboratories have carried on research in order to 

 determine the effect of sidetone on the overall efficiency of the circuit. 

 We find that sidetone above a certain volume decreases the conversa- 

 tional efficiency of the circuit. Parallel with the study of the effects 

 of sidetone, research has been carried on on methods which could be 

 applied to limit sidetone in amount to more nearly its optimum value. 

 This has led to the development of what are known as anti-sidetone 

 circuits, which do not eliminate sidetone but reduce it to an amount 

 which is more nearly that found to be desirable. 



An important step in the association of the transmitter and the 

 receiver is represented by the handset which provides a rigid mechan- 

 ical connection between the two units. This rigid mechanical con- 

 nection introduces mechanical coupling between the receiver and the 

 transmitter, which had to be given very serious consideration in order 

 to avoid speech distortion. 



Trends in Instrument Development 



I have broadly indicated to you fields of research which underlie 

 the development and design of the telephone transmitter and receiver. 

 It will now be of interest for us to note what application is likely to 

 be made of the results of what has amounted to an enormous total of 

 scientific effort. In this connection, it may be well again to emphasize 

 that station apparatus is intimately associated with the user, and has 

 therefore to be designed to fit him, his habits of using the telephone, 

 and the conditions attending such use. The handset has to be 

 designed to fit his head, the holes in the dial to fit the size of his finger, 

 the bell to be loud enough, and so on. Our effective transmission 

 rating system has been set up in an attempt to rate the performance 

 of the telephone when employed by the customer in the way he wants 

 to use it, under the conditions surrounding him. For this reason, 

 this method of rating has been found particularly valuable in the 

 development work on instruments. 



Because of the wide range of customer usage and conditions, a 

 number of factors have to be taken into account in the design of the 

 apparatus. Also, because this apparatus is located on the customer's 

 premises, where it is relatively inaccessible to the telephone personnel, 

 it must be capable of standing up without undue trouble under this 

 wide range of usage and conditions. To strike a proper balance in 

 meeting all these factors requires an intimate knowledge of the field 

 conditions as well as of the development and manufacturing possi- 



