Telephone Hybrid - Polycom SoundStructure C16 Design Manual

Telephone Hybrid

To use the audio conferencing system, there must be a way to get the local signal to the remote participants
and vice-versa. While only supporting 3.5 kHz of audio bandwidth, the Public Switched Telephone Network
(PSTN) provides the most common and reliable real-time communication network for audio conferencing.
In the conference room, the PSTN network is accessed by a pair of conductors that carry both the transmit
and receive signals over the PSTN. When interfacing external equipment to the public switch network, it is
necessary to separate the transmit and receive signals - this is the task of the telephone hybrid, also known
as a 2-wire (PSTN) to 4-wire converter (separate transmit and receive signals).
The telephone hybrid circuit that interfaces the 2-wire PSTN network to the 4-wire separate transmit and
receive uses a line echo canceller (LEC) which is similar to the acoustic echo canceller to remove line echo
that is caused due to imperfect signal balancing of the transmit and receive circuits onto the 2-wire network.
This imperfect balance means that when a transmit signal is sent to the telephone line, there is some
leakage, or coupling, of the signal back to the receive path. This leakage is heard as a return echo of the
local talker's speech. This is the same echo (commonly referred to as side-tone) that is heard on a telephone
handset when speaking into a telephone - this side-tone echo serves the purpose of providing feedback to
the local talkers as to how loud they are talking and that the phone line is working properly.
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
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