A Full-Duplex Echo Suppressor Using Center-Clipping

During investigations of a multiband center-clipping process for use in reverberation reduction 2 it occurred to us t h a t this process, which can remove the effects of long-time reverberation or echoes in a room, could also be used to remove echoes in telephone lines resulting from imperfect hybrid junctions. 3 Independently, J. R. Pierce also sug1619 1620 THE BELL SYSTEM TECHNICAL JOURNAL, M A Y - J U N E 1971 gested that this process could be applied to echo suppression and proposed a scheme for controlling the levels of the center clippers in a conventional split echo suppressor configuration. 4 One end of a conventional split echo suppressor is shown in Fig. 1. It is located in the 4-wire section of line near the hybrid junction to the 2-wire loop of the near-end customer. A similar configuration is inserted at the other end of the 4-wire trunk. Because of imperfect balancing of the hybrid, part of the received signal from the far-end talker feeds through the hybrid to the transmit side of the 4-wire line. The return loss of the hybrid is typically 15 dB, that is, the echo level at the echo suppressor is 15 dB below the normal transmit signal level of the near-end talker measured at the same point. The conventional echo suppressor is a voice-operated switch. The logic and control circuit detects the presence of received signal and causes a loss of at least 50 dB to be inserted in the path of the echo signal on the transmit side. Since the loss would also attenuate the signal from the near-end talker, and temporarily make the connection one way, the logic and control circuit also detects the presence of double talking and puts the suppressor into a "break-in" mode which allows an interruption to take place.