Handbook for Sound Engineers

758 Chapter 21

microphone and a loudspeaker with associated amplifi-
cation. A hybrid interface between the send and receive
lines and the telephone line serves to reduce loop gain
within the room by reducing sidetone leakage.
Possible feedback loops are shown. Not only is there
potential oscillation in the sending room, but also the
coupling through the line to the receiving room and
back is an equally probable feedback loop. Basic speak-
erphones use voice-activated gates to capture the line
and permit transmission in only one direction at a time
and thus interrupt the feedback path from the remote
site. This can cause frequent dropouts in a conversation
and forces the communication into a half-duplex mode
of operation. Half duplex transmission refers to trans-
mission in only one direction at a time.
The ideal is full duplex, which allows transmission in
both directions all the time. A phone call from tele-
phone handset to telephone handset provides full duplex
communication. Full duplex is preferred for audio
conferencing because there are no missing words or
sentences, and conversations can be conducted in a
normal manner. Control of reverberation and room
noise is essential in any full duplex system.

An alternative connection system uses four wires as
shown in Fig. 21-28. One pair of wires is used for each
direction of transmission, thus eliminating the often
troublesome hybrid sidetone leakage. As can be seen,
there is still the possibility of feedback through the
room at either end. However, there is usually cleaner
signal transmission with the added expense of a second
telephone line. Four-wire systems make full duplex
communication possible.
Frequently audio conferences involve several sites
giving rise to point-to-multipoint or multipoint-to-
multipoint telephone interconnections. A conference
bridge is used to connect a number of telephone lines so
that all participants will be tied together. Bridging over
20 phone lines is now quite common. The actual
bridging may be provided by an external bridging

service company or bridging devices may be part of the

on-site teleconferencing equipment.

A typical conference bridge limits the number of

open ports to two because signal leakage in the bridge

can cause retransmission of received audio on telephone

lines. As a result, only one two-way conversation can

occur and others can only listen. Also, the uncertain and

variable quality of telephone connections can result in

having a noisy line tying up the system and preventing

access since the bridging control depends on signal-acti-

vated switching.

21.3.7 Teleconferencing Equipment

21.3.7.1 Telephone Interface

The telephone interface for a typical two-wire site is the

hybrid. It converts the two-wire transmission of the con-

necting lines to internal four-wire paths to isolate the

send and receive signals. A hybrid passes the micro-

phone send signal (two of the four wires within the

room) to the two-wire telephone line but attenuates it to

the receive line. Conversely, a signal being received

from the telephone line passes to the receive line (the

other two of the four wires in the room) and is attenu-

ated to the microphone send line. For many years, the

hybrid in a standard telephone set was a transformer;

now electronic equivalents are common.

Figure 21-29. Teleconference system with two-wire telephone connection, showing feedback paths.

Microphone Microphone

Loudspeaker Loudspeaker

Acoustic

coupling

Acoustic

coupling

Sidetone

leakage

Sidetone

leakage

Send

Receive

Hybrid

Send

signal

Send

Receive

Hybrid

Send

signal

Figure 21-30. Teleconference system with a four-wire tele-

phone connection.

Microphone

Microphone

Loudspeaker

Loudspeaker

Send

Send

Receive

Receive