Subjective Evaluation of Delay and Echo Suppressors in Telephone Communicaions

The preceding two papers describe the nature of the problems introduced into telephone circuits by delay and echo and the attempted solution of these problems through the design of echo suppressors. The present paper describes recent determinations of the degradation of transmission quality caused by pure delay and delay plus echo and echo suppressors. The studies reported were all done by the human factors research department of Bell Telephone Laboratories. Other groups have studied the effect of delay upon voice transmission. These include the research department of Bell Telephone Laboratories, Stanford Research Institute, Italian Telecommunications Administration and the British Post Office. Unfortunately, none of this work has yet been published. In general, these studies have found little degradation with pure delay, even for round-trip delays over one second, although some objection did occur in one experiment with natural conversations with 1410-msec round-trip delay. These same subjects did 2919