RATING TRANSMISSION PERFORMANCE 121 



correlations between transmission service performance and transmis- 

 sion characteristics, these latter can of course be used to indicate 

 service performance. 



In addition to specifying any kind or grade of circuit performance on 

 the basis of performance results there is the method, which has had 

 important practical application, of indicating performance in terms of 

 types of instruments and circuits and of the conditions of their use. 

 For example, a statement of the types of transmitters, receivers, 

 station sets and cord circuits and of the length and types of loops and 

 trunk, together with specific conditions of use, provides an indirect 

 specification of a performance. This method, which is extensively 

 used in many fields, may be termed the "instrumentality designation 

 method." An outstanding application of this method in telephone 

 transmission work is the Standard Cable Reference System ^ which 

 was so widely employed to provide a scale of performance. This 

 method has many present applications where physically determined 

 characteristics are unavailable or are difficult of definite determination 

 and specification. Also, the designation of instrumentalities is con- 

 venient in many cases because it provides a ready means of specifying 

 a practical combination of various kinds of transmission characteristics. 

 While this method is often expedient practically, taken by itself, it is 

 inherently cumbersome for the development of improved instrumental- 

 ities because of the lack of physical indication of the features to be 

 investigated. 



Transmission Characteristics 



The usefulness to the listener of the speech sounds reproduced over a 

 telephone circuit is a function of their loudness, of their distortion or 

 degree of departure from facsimile reproduction, and the magnitude 

 and character of the extraneous sounds or noise which accompany 

 them. Transmission characteristics are therefore directed primarily to 

 indications of the effects of the circuit and its parts on the reproduction 

 of sounds in these three respects. As already indicated, transmission 

 characteristics are determined not only for the path from transmitter 

 at one end to receiver at the other, but also for the sidetone and echo 

 paths. 



Speech sound transmission characteristics, that is, expressions of the 



relations between impressed and reproduced speech sounds, while they 



have been extensively used, present some difficulty in quantitative 



determination and specification because of their complex nature. 



Also, the human element is involved in the persons used as generators 



1 "Master Reference System for Telephone Transmission," Martin and Gray, 

 Bell System Technical Journal, July 1929. 



