Subjective Evaluation of Delay and Echo Suppressors in Telephone Communication

Abstract

The effect of transmission delay upon the quality of the telephone circuit was investigated by means of psychometric methods using experimental telephone circuits. The results reveal that 600 msec of one-way transmission-time delay is well tolerable when no echo is included. Annoyance of talker and listener echo was studied. The just-tolerable minimum echo attenuation increased rapidly as the delay time increases, and, above 300 msec of roundtrip delay, the attenuation stays at the value around 20 dB. The evaluation was made on three types of conventional echo suppressors and an experimentally made cancellation-type echo suppressor. It was found that only one among the three conventional echo suppressors is good enough to give the tolerable quality up to 600 msec of one-way transmission delay. Since the cancellation-type echo suppressor in which the echo is cancelled out with the signal synthesized by convolution calculation circuit has no speech chopping, the speech quality with it was found to be better than that with all the three conventional echo suppressors.