RATING TRANSMISSION PERFORMANCE 127 



Starting with such a circuit, changes can be made in its transmission 

 characteristics such as varying the attenuation of the trunk and its cut- 

 off, varying the length and type of the subscribers' loops, using different 

 types of transmitters and receivers in order to get different efficiencies 

 and kinds of distortion and changing the type of station circuit to get 

 different amounts of sidetone. By using circuits of these various char- 

 acteristics in commercial service and determining the repetition rates 

 obtained, a relation can be established between grade of service and 

 transmission characteristics both for different overall circuit combina- 

 tions and also for the various changes which can be made in such a 

 circuit. An outstanding advantage of selecting the type of circuit 

 which has been indicated, as a w^orking reference circuit, is that it 

 readily permits direct comparisons of the service performance of the 

 working reference circuit, or of circuits having closely similar charac- 

 teristics, with the service performances of various types of commercial 

 circuits. 



It is desirable to go one step further and to express the effects of 

 changes in various transmission characteristics all in terms of changes 

 in some one characteristic of the circuit. For this latter has been 

 chosen the attenuation of the trunk. In accordance with this, then, 

 the effect of such things as differences or changes in cutoff of the trunk, 

 line noise, room noise, transmitter and receiver volume efficiencies and 

 distortions, sidetone, and, in fact, of any transmission characteristics of 

 any part of the circuit can each be expressed in terms of an equivalent 

 change in the attenuation of the trunk on the basis of equality of effect 

 on service performance. Thus the ratings of all such effects can be 

 placed on a basis which makes them readily comparable. For the 

 practical range of variations in these factors it has been found that in 

 general the effects so expressed can be added together with a good de- 

 gree of approximation. Where this is not the case, interrelated sets of 

 effective transmission ratings can be supplied to cover the various 

 typical combinations which are likely to be found in practice. This 

 places the application of the ratings given by this method on a com- 

 parable basis with the application of the old volume ratings, that is, 

 the assignment of a number to each part of the circuit, which numbers 

 can be combined by algebraic summation in arriving at an overall 

 rating for any particular circuit. 



In line with this, the effective transmission of a trunk, for example, 

 is rated in terms of an attenuation loss of so many db plus a rating in 

 db which expresses the effect of the range of frequencies transmitted 

 with respect to some range selected as standard, plus another rating 

 expressed also in db to take account of the noise on this trunk. Simi- 



