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

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.