Practical Application of the Recently Adopted 

 Transmission Unit 



By C. W. SMITH 



THK purpose of this i)aper is to outline the practical consider- 

 ations iinolveil in tlie use of the transmission unit (al)l)re\iatecl 

 /T), which was recently adopted liy the Hell System to rei)lace the 

 mile of standard caMe in transmission engineerinj; work. A descrip- 

 tion of the TU, together with a discussion of the considerations whidi 

 letl to its adoption has been given 1)> Mr. Martin in another article 

 in this issue. 



Ki-KixT oi" AooPTiNci THI-; TU .\s Ri:(..\ri)s Tr.wsmission 



St.\.NI).\R1)S 



The transmission standards in general use var\- from 18 miles of 

 standard cable to about 30 miles of standard cable, dejiending upon 

 the Icxality and the class of service such as local and toll. It has 

 become customary among telephone people interested in standards 

 of service to associate certain figures for transmission standards with 

 the corresiX)nding standards of service which they represent, it is a 

 distinct advantage, therefore, to retain the same figures for the same 

 standards of service when changing to the new unit. The zero of 

 reference was so selected, therefore, that 24 TU is equivalent to 24 

 miles of standard cable in volume reproduction. This means that if 

 one talks with the same loudness over a circuit of 24 TU as over a 

 circuit of 24 miles of standard cable, the volume received from each 

 will be the same. As the attenuation corresponding to the TU is 

 only about 6 per cent, less than the attenuation corresponding to the 

 mile of standard cable and 24 miles represents the mean between 

 the highest and lowest standards in common use, transmission stand- 

 ards on the new basis are ver>' little diflferent numerically from the 

 same standards on the old basis. The former 18-mile standard is 

 equivalent in transmission to 17. (j TU and the 30-mile standard is 

 equivalent to 30.4 TU. The same numerical values can, therefore, 

 generally be used for transmission standards in the new system, as 

 in the old, since the greatest differences encountered will be 0.4 TU. 



It is also true that a given transinission loss specified in miles will 

 correspond ver>' closely in numerical value to the same loss expressed 

 in TU. People not directly engaged in transmission work, therefore, 

 may generally disregard the slight difference which exists in con- 

 sidering transmission losses expressed in TU as compared with stand- 

 ard cable. 



409 



