A full-duplex echo suppressor using center-clipping

For telephone circuits which include synchronous satellites, conventional echo suppressors of the voice-switching type are less than satisfactory because of speech mutilation and the presence of echo during double talking.¹ We have found that a multiband center-clipping process may be used as an echo suppressor. This echo suppressor is unique in that no double-talking decision has to be made. The near-end signal, plus echo of the far-end signal, is divided into several contiguous bands with each filter output going to a center clipper. A control circuit sets each clipping level equal to or greater than the echo level in that band. A preliminary analogue implementation of this echo suppressor, in which control circuit gains were manually adjusted to match the experimental return loss, was informally demonstrated using a simulated satellite circuit. Although no attempt at quantitative evaluation has yet been carried out and further evaluation is necessary, no echo was reported during this demonstration, even during double talking, for return losses approaching 0 dB. Operation appeared to be full-duplex at all times with little distortion of the speech. For return losses greater than about 15 dB, the center-clipping system was almost indistinguishable from a 4-wire connection with no echo path. In practice, adaptive setting of control circuit gains as a function of return loss would be desirable if this technique is used as a replacement for conventional echo suppressors.