TRANSMISSION DATA FOR RATING TELEPHONE CIRCUITS 345 



For the normal range of conditions existing in the telephone plant 

 simple algebraic addition of the component ratings gives the com- 

 bined rating with sufficient accuracy provided the individual ratings 

 are obtained under typical conditions. Ratings can be obtained only 

 for complete circuits, since some of these parameters, such as distor- 

 tion, have no meaning except when applied to a complete circuit, and 

 others, such as sidetone, depend on two or more circuit elements. 

 However, by choosing for computation complete circuits in which the 

 elements to be rated are substituted for the corresponding element of 

 the reference circuit, it is possible to determine the effective losses of 

 individual circuit elements in accordance with the definitions given 

 previously. 



The parameters used for describing circuits have been used pre- 

 viously in a qualitative sense and several have quantitative definitions 

 based on listening tests. A problem is presented, however, by the 

 necessity for computing them from physical measurements. Volume 

 loss, for example, has been defined in terms of voice-ear comparisons 

 with an adjustable reference condition. Such a definition is satis- 

 factory for describing this parameter but the testing method is cum- 

 bersome for obtaining the large amount of data required for engineer- 

 ing purposes. If, however, a series of such volume loss measurements 

 is made on a large number of circuit conditions for which the physical 

 characteristics are varied systematically, an empirical formula can be 

 derived for weighting and combining the efficiencies measured over 

 the voice-frequency range. Using this formula, the volume loss of 

 other conditions can be readily computed. Similar methods may be 

 used for deriving empirical formulas for computing noise and sidetone 

 volume losses from the basic physical characteristics. 



The use of distortion in a quantitative sense requires the adoption 

 of a scale for this parameter. The only requirement for such a scale 

 is that any two circuits having the same amount of distortion, all other 

 parameters being equal, will give the same repetition rate. The term 

 distortion factor is applied to the particular scale used in this work 

 and its definition is derived from laboratory articulation tests. These 

 tests are made on a large number of circuits which have equal volume 

 losses but which differ from each other in frequency characteristics. 

 The empirical formula for computing the distortion factor from the 

 basic circuit measurements is set up so that all of the circuits which 

 give the same articulation rate will have the same distortion factor. 

 From service observations made on a number of different types of 

 circuits it has been shown that, with all other parameters constant, 

 circuits having the same distortion factor will give essentially the 



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