Customer Evaluation of Telephone Circuits with Delay

Echo occurs on any two-wire or combination two-wire, four-wire telephone circuit. The degrading effect of this echo depends on the time it takes for the echo to return to the speaker -- the delay time. For very short echo delays -- on the order of a few ms -- the echo is masked by the sidetone in the telephone receiver. For somewhat longer delays -- on the order of a few tens of ms -- the echo can be made tolerable by providing loss in the telephone circuit to reduce the level of the echo. For delays longer than this, too much loss is required and echo suppressors are used. Echo suppressors are voice operated devices placed in the four-wire portion of the circuit which insert loss in the return path to suppress the echo. When an echo suppressor is in its suppression mode, it places a large loss in the echo path which, besides suppressing echo, prevents the speech of the second party of the conversation from reaching the first party if both are speaking at the same time, i.e., double talking. If the echo suppression is removed during double talking, the echo is also heard. 1157