760 Chapter 21
and aimed upward. Never place loudspeakers in front of
microphones as microphones cannot distinguish between
talkers in the room (desired sound sources) and talkers
heard via loudspeakers (undesired sound sources).
When there is talking in the room, automatic micro-
phone mixers can reduce the level of the loudspeaker
signal from the remote site. This is accomplished via
attenuating relays, ducking circuits, etc. By contrast,
Sound Control Technologies offers a system that places
the loudspeaker symmetrically between a pair of micro-
phones that are out of polarity with each other. The
loudspeaker contribution to the send line is claimed to
be reduced by 40 dB with this arrangement.
If sound reinforcement of conversations within the
room (sometimes known as voice lift) must also be
provided in addition to audio conferencing, even more
attention must be given to reducing the audio coupling
between the loudspeakers and the microphones. Such
systems can be very difficult to design correctly and
must be approached with great caution. The use of an
experienced acoustical/audio consultant is highly
recommended in these cases.
21.3.7.5 Send Level Control
Send level—i.e., the audio signal voltage supplied to the
telephone line—should be within acceptable ranges.
Compressors, AGCs and levelers are all devices to con-
sider for this technical requirement.
21.3.7.6 Echo Canceler
Echo cancelers reduce residual echo return in audio
conferencing installations. If the local site returns sig-
nificant signal from its incoming port to its outgoing
port, and there is significant propagation delay due to
the transmission line, the remote site will hear an
annoying echo when someone in the remote site speaks.
The imperfect balancing of hybrids is one path for
echo. Signal reflection within the telephone line is
another source of echo. Echo also occurs acoustically
when loudspeaker sound reaches open (active) micro-
phones that are transmitting speech. The use of satellite
transmission links also makes echo problems worse
because of the long propagation delays.
A line echo canceler attempts to reduce echoes that
are electronic in nature, such as those caused by hybrid
leakage. An acoustic echo canceler looks at the signal
coming into a room and inserts a time-delayed mirror
image of the incoming signal into the outgoing signal
leaving the room. The idea is to cancel any of the
incoming signal that leaks into the outgoing signal path
as a result of the acoustical coupling between loud-
speaker and microphone.
Echo canceler technology has rapidly advanced due
to faster CPU speeds and new research into canceler
algorithms. Early echo cancelers were very expensive
and thus having a single canceler at each conferencing
site was considered adequate. As the price of echo
cancelers has declined, manufacturers such a Gentner
and ASPI now offer devices that have an echo canceler
for each microphone input channel.21.3.7.7 Historical Examples of Teleconferencing
EquipmentTwo historical systems will be described in more detail
in order to show the number of parameters that must be
considered in addition to the usual sound reinforcement
needs. The first is an automatic microphone mixer
approach as exemplified by the Shure ST3000, first
manufactured in the 1980s. The second is the Sound
Control Technologies system that does not use auto-
matic microphone mixing.Shure ST3000—An Analog Speakerphone. A simpli-
fied block diagram of the ST3000 is shown in Fig.
21-33. A conference call connection is made by taking
the telephone handset from its cradle and dialing the
desired number. When it is determined there is a good
connection with the dialed party, the controller confer-
ence switch can be depressed to turn on the conference
system. Green talk LEDs turn on and the handset may
be returned to its cradle. The controller loudspeaker vol-
ume may next be adjusted if necessary. Levels for any
auxiliary equipment may also be adjusted. Use the mute
switches to prevent the called party from hearing local
conversation. Red LEDs indicate muted status. The con-
ference is terminated by depressing the controller tele-
phone switch for at least 1 second.
In Fig. 21-33, the upper left mixer amplifier feeds
the various auxiliary outputs. Below this amplifier, the
conference microphone inputs are shown. Only the
mutable (i.e., for automatic mixing) microphone inputs
feed the send signal path to the hybrid. The receive path
leads to the power amplifier and loudspeaker. Relative
send and receive signals in the room are controlled by
the send/receive switching and suppression logic. The
suppression logic causes either the send amplifier or the
receive amplifier to attenuate its signal depending on
the presence of a receive signal. Because standard voice
quality telephone lines have restricted bandwidth
requirements, bandpass filtering is included in the send