In audio teleconference systems, speech quality may be degraded due to acoustic echo in the conference room. Echo control devices, such as echo cancellers, can be used to improve the speech quality. This paper describes the echo return loss required for acoustic echo controllers used in audio teleconferencing based on subjective assessment. In particular, far-end or talker echo is subjectively evaluated using a simulated audio teleconference system constructed in our laboratory. Test conditions are given as combinations of transmission delay and insertion loss. The subjective tests were also evaluated under three different reverberation times of echo path. The thresholds of detectable and/or objectionable echoes and mean opinion score in view of acoustic echo and reverberation in audio teleconferencing are clarified. From this, we can ascertain that an echo objection limit that 90% of the subjects consider to be permissible leads to a required echo return loss of more than 40 dB when the overall round-trip delay time Tord is 100 ms and the reverberation time Trev is 400 ms.

A transmission performance design method for digital telephone sets is described, considering talker echo resulting from reflection at the four-wire to two-wire hybrid point in mixed digital-analogue networks. The method provides for complete international telephone connection, talker echo suppression and improved speech quality. To develop a design method with these features, first, the effect of talker echo on speech quality is evaluated. Next, digital telephone set loudness ratings are regulated as follows: 8.3 dB sending loudness rating 10 dB, 0.5 dB receiving loudness rating 4 dB. As a result, the Mean Opinion Score (MOS) for digital telephone sets in the ISDN is from 2.3 to 2.7 according to tests, considered acceptable for about 90% of subscribers.