The Transmission Unit and Telephone 

 Transmission Reference Systems' 



By W. H. MARTIN 



Synopsis: C'onsideralion is given to the method of determining and 

 expressing the transmission efficiencies of telephone circuits and apparatus, 

 and of the desirable qualifications for a unit in which to express these effi- 

 ciencies. The" transmission unit " described in this paper has been selected 

 as being much more suitable for this purpose under present conditions than 

 the "mile of standard cable" which has been generally used in the past. 



'^r^ 1 1 1-2 "niilf of stanciaril cable" has been uswl in telephone en- 

 A gineering in this country for over twenty years, and during that 

 time has been adopted in other countries, as the unit for expressing 

 the transmission efficiency of telephone circuits and apparatus. In 

 the present state of the telephone art, this unit has been found, how- 

 ever, to be not entirely suitable and it has recently been replaced in 

 the Bell System by another unit which for the present, at least, has 

 been called simply the "transmission unit." Before considering the 

 reasons for such a fundamental change and the relati\'e merits of the 

 two units, it may be well to review briefly the general method of deter- 

 mining the efficiency of such circuits and the apparatus associated 

 with them. 



The function of a lelei)lume lirciiii is to reproduce at one terminal 

 the sj)eech sounds which are impressed upon it at the other terminal. 

 The input and output of the circuit are in the form of sound and its 

 efficiency as a transmission system may be expressed as the ratio 

 of the sound power output to the sound power input. For commercial 

 circuits, this ratio may be of the order of 0.01 to 0.001. 



In the operation of the system, the sound power input is con- 

 verted by the transmitter into electrical power, which is transmitted 

 over the line to the recei\'er and there reconverted into sound power. 

 The efTect of inserting a section of iiiu- or piece of apparatus or of 

 making any change in the circuit can lie determined in terms of the 

 variation which it produces in the ratio of the sound power output 

 to the sound power input, or, if this latter is kept constant, in terms of 

 the ratio of sound power (iiii|)iii after the change to that obtained 

 before the change was made, li should be noted particularly that 

 the change in the output power of the system is the real measure 

 of the effect of any part of the circuit on the efficiency of the system 

 and iliai the ratio of the power leaving any part to that entering it 

 is not necessarily the measure of this effect. For example, a pure 



' Reprinted from the Jouni. A. I. E. E., for June, l'>24. 

 400 



