A Revised Telephone Transmission Rating Plan

In the years prior to about 1930, performance ratings were assigned to telephone transmission circuits in terms of loss of speech volume or loudness. The adverse effects on transmission quality of such parameters as sidetone, distortion and noise were recognized, but until the 1930's no way was known of incorporating them, together with loudness loss, into a single figure of merit for rating transmission performance. Anti sidetone station sets were first used in quantity in the Bell System in 1932, followed in 1937 by the 302-type combined sets. While these instrumentalities did not yield appreciably greater loudness than their predecessors, they exhibited marked improvements in such characteristics as sidetone, frequency response and nonlinear distortion, which have a marked effect on the ability of a listener to understand as well as to hear transmitted speech. Thus it became necessary to devise a rating system which would give adequate recognition to the contributions of these improvements to over-all transmission performance. There resulted the "effective loss" method of transmission rating. 1 ' 2 The effective loss rating system assigned a figure of merit to a customer-to-customer transmission circuit, based fundamentally on the rate at which listeners requested repetition of what the talkers had said. This basis evaluated transmission circuits from the standpoint of transmitted intelligibility, as well as loudness. In practice, the circuit to be rated was compared with a "working reference system," 2 which consisted of representative subscriber loops and station sets, and a variable, distortionless 1 W.