Subjective evaluation of unsuppressed echo in simulated long-delay telephone communications

Abstract

The paper describes an experiment to measure the subjective effect of unsuppressed echo to determine the improvement in echo return loss required of echo cancellers in long-delay telephone communications. Using a simulated long-delay telephone connection of 16.5 dB nominal overall reference equivalent, tests were carried out over a range of conditions in which the 1-way delay varied between 50ms and 600ms, and the echo return loss between 15 dB and 50 dB. Echo-path frequency characteristics which an echo canceller might cause in practice were represented by a flat characteristic and a charactereistic decreasing in loss with increasing frequency. Because of the large number of test conditions (40) in the experiment, the experimental design is unique and is described.