TRANSMISSION DATA FOR RATING TELEPHONE CIRCUITS 333 



choose the faciHties for each individual element separately so that its 

 loss would not exceed the permissible loss. In a particular area, for 

 example, the transmitting loss of each loop might be limited to 8 db 

 or less, the receiving loss to 3 db or less, the total office losses on a 

 connection to 1 db or less and the losses of an interoffice trunk to 6 

 db or less. In this case, no interoffice connection would have a loss of 

 over 18 db regardless of which stations in the area were involved. 

 This method makes it possible to design the circuit elements separately 

 and also simplifies the presentation of data for the very large number 

 of combinations of facilities available for the telephone plant. 



A method has been developed for assigning effective loss ratings to 

 parts of a circuit and presenting them in a form very similar to that 

 used for the volume loss data. The advantages of this form are 

 retained, therefore, even though the data include the effects of dis- 

 tortion, sidetone and noise in addition to volume losses. 



The second problem, the preparation of numerical transmission 

 rating data for the various types of facilities available for use in the 

 telephone plant, requires a somewhat indirect attack because of the 

 large amount of data required. Theoretically it would be possible to 

 obtain relative effective ratings directly by means of repetition counts 

 for all the circuit and instrument combinations which might be of 

 interest in plant design. These ratings of complete circuits could then 

 be broken down into ratings for the individual circuit elements. This 

 method of attack, however, is entirely impractical because there is an 

 almost infinite number of combinations of circuit elements and it would 

 take some weeks of observation time on each combination. It is 

 necessary, therefore, to make observations on a relatively small num- 

 ber of circuits chosen to cover the whole range of conditions and to 

 obtain ratings for other circuits by interpolation between the ratings 

 which have been obtained directly. 



The method of interpolating which has been found practicable is 

 based on the fact that the performance of a complete circuit can be 

 described, with sufficient accuracy for most engineering work, as a 

 function of the characteristics, volume loss, sidetone, distortion and 

 noise. The magnitude of all of these characteristics, for any circuit 

 using conventional types of instruments, can be derived from physical 

 measurements. Since this can be done for each of the circuits used 

 in the repetition tests, relations can be obtained for converting changes 

 in noise, sidetone, or distortion into equivalent distortionless changes 

 in volume loss. For any other complete circuit, it is necessary only 

 to determine the magnitude of these characteristics by measurements 

 and computations, and to convert them to effective ratings by means 



