Practical Application of the Recently Adopted Transmission Unit

HE purpose of this paper is to outline the practical considerations involved in the use of the transmission unit (abbreviated TU), which was recently adopted by the Bell System to replace the mile of standard cable in transmission engineering work. A description of the TU, together with a discussion of the considerations which led to its adoption has been given by M r . Martin in another article in this issue. E F F E C T O F A D O P T I N G THE T U A S R E G A R D S T R A N S M I S S I O N STANDARDS The transmission standards in general use vary from 18 miles of standard cable to about 30 miles of standard cable, depending upon the locality 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 corresponding 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 standards on the new basis are very little different numerically from the same standards on the old basis.