Subjective Evaluation of Delay and Echo Suppressors in Telephone Communications

The effect of transmission delay upon the quality of a telephone circuit was investigated using naturally occurring telephone conversations. Round-trip delays of 600 and 1200 msec went almost unnoticed at first when no echo sources or echo suppressors were present in the circuit. After exposure to pure delays of up to 2400 msec, considerable dissatisfaction, indicated by rejection of the circuit, developed with the 600- and 1200-msec delays. When echo sources and echo suppressors were added to the circuit, some dissatisfaction developed immediately for round-trip delays of 600 and 1200 msec. No adaptation to the effects of delay occurred; in fact dissatisfaction increased with experience under certain conditions.