Polycom C12, C16, C8, SR12

Design Guide for the Polycom SoundStructure C16, C12, C8, and SR12

B - 20

While side-tone is desirable while talking on a handset, it is not desirable in a

conferencing application. As the line echo (or side-tone) is mixed together with

the audio from the remote telephone talkers' speech, the line echo will be

played into the room over the same loudspeakers in the local room used to

hear the remote talkers. The line echo will sound like an echo of the local

talkers' speech back to them with short delay. If the signal is loud enough in

the room and there is significant loudspeaker to microphone coupling, such as

low ERL, the line echo may cause acoustic feedback to occur in the room as the

local talkers audio is played back into the room, picked up by the microphones

reflected off the telephone line interface and played back into the room. Since

the telephone signal is part of the acoustic echo canceller reference signal, the

AEC will try to echo cancel the side-tone and prevent it from being sent to the

remote side (and hence causing more side-tone) but it may not completely

cancel the signal as the system will be in double-talk, meaning that the echo

canceller will detect both the local talkers speech and the side-tone

(interpreted as the remote talkers' speech). This will prevent the acoustic echo

canceller from converging properly, degrading the conferencing experience.

The line echo canceller shown in the previous figure above is an adaptive filter

that uses the transmit signal to adapt and remove the line echo that appears on

the receive side of the line echo canceller. Similar to the acoustic echo canceller,

there may also be some non-linear suppression to remove residual echoes

when the line echo canceller is not fully converged. The line echo canceller

typically is adapting when there is transmit audio present and no receive

audio, in other words, just the local talkers are speaking. This is in contrast to

the acoustic echo canceller which typically is only adapting when there is

remote audio present and the local talkers are not talking.

Due to the variation in performance of telephone lines across the world,

telephone hybrids will have a country code setting that will be required to

properly interface the hybrid to the local phone line characteristics. If the

phone hybrid settings don't match the communications network, the

performance of the phone hybrid will be degraded, and possibly in violation

of local telecommunications regulatory requirements.

D/A

A/D

2-wire

Telephone Line

Transmit

Receive

LEC