Transmitted Frequency Range for Telephone Message 



Circuits 



By W. H. MARTIN 



IN the Bell System the general objective which has been set up for 

 the transmitted frequency range for new designs of telephone mes- 

 sage circuits is a range having a width of 2,500 cycles, extending from 

 about 250 cycles to about 2,750 cycles. In determining the frequency 

 range of such a circuit, the cutoff points are taken as those at which 

 the attenuation reaches a value 10 db greater than that at 1,000 cycles. 



This frequency range for design is taken in general as applying to 

 the overall transmission characteristic of the circuit between the ter- 

 minal central offices of a connection. Where such offices are connected 

 by a direct trunk this frequency range applies to the individual trunk. 

 Where two or more trunks are used in tandem the frequency range 

 of the overall connection will tend to be somewhat less than that of 

 the component parts. For this reason then, to meet the specified range 

 for an overall multi-switch connection, it will be necessary to have the 

 frequency ranges of the trunks which are used as parts of built-up con- 

 nections, somewhat greater than the specified range for the overall 

 circuit. 



In view of the relatively lower cost of toll switching trunks and other 

 similar trunks from toll offices to local central offices, it is desirable that 

 these terminal circuits have a broader frequency range than that speci- 

 fied above, so as to avoid their narrowing the range transmitted by the 

 toll trunks with which they are connected. 



It may be stated that the general purpose in working to the trans- 

 mitted frequency range given above is that each individual trunk 

 should have a frequency range at least as great as that specified and 

 that the frequency ranges of those trunks which are frequently used 

 as parts of built-up connections should be somewhat greater than the 

 specified range. 



In setting up the requirements for the various transmission char- 

 acteristics of telephone message circuits, the aim is to arrive at the 

 combination of requirements which will give the most economical tele- 

 phone system for furnishing the desired grade of transmission service. 

 Since the effects of many of the factors entering into the determina- 



483 



