A System of Effective Transmission Data for Rating 

 Telephone Circuits 



By F. W. McKOWN and J. W. EMLING 



A previous paper ' introduced the idea of rating the transmission per- 

 formance of telephone circuits on the basis of repetition observations and 

 outlined briefly a method for expressing such ratings. The present paper 

 describes in some detail a system for presenting these data in a form suit- 

 able for engineering use and the steps required for obtaining these data 

 from the repetition observations. 



Introduction 



A TELEPHONE circuit may be described In terms of its physical 

 characteristics, but these characteristics do not in themselves 

 indicate the transmission results which will be obtained by its users 

 in service. Laboratory talking tests, such as articulation tests,^ indi- 

 cate the ability of the circuit to transmit speech sounds under the 

 conditions of the tests. In service, however, a wide and complex 

 range of conditions is encountered and in the case of a new kind of 

 instrument or circuit the service conditions may be modified in an 

 unpredictable way due to the users' reactions. The complexity and 

 a priori uncertainty of these conditions point to the advantage of 

 ratings obtained during actual service. 



The real criterion for rating a circuit is its transmission performance 

 when in actual use as a link in the extremely complicated and variable 

 communication channel between the brain of one telephone user and 

 the brain of another telephone user. The paper by Mr. Martin ^ fully 

 developed this idea and described a quantitative method for providing 

 ratings on this basis of the transmission performance of a circuit, 

 which method includes the effects of such circuit characteristics as 

 volume loss, noise, distortion and sidetone. This method uses as a 

 measure of circuit performance the number of repetitions requested 

 by normal telephone users per unit time while using the circuits in 

 actual service, on the basis that this number is a direct quantitative 

 measure of the success with which telephone users carry on conversa- 

 tions. The previous paper also discussed other methods of rating the 

 transmission performance of telephone circuits such as articulation 



^ "Rating the Transmission Performance of Telephone Circuits," \V. H. Martin, 

 B. S. T. J., Vol. X, p. 116. 



^ "Articulation Testing Methods," H. Fletcher and J. C. Steinberg, B. S. T. J., 

 Vol. VIII, p. 806. " Developments in the Application of Articulation Testing," T. 

 G. Castner and C. W. Carter, Jr., this issue of the B. S. T. J. 



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