Handbook for Sound Engineers

Preamplifiers and Mixers 761

channel. Bandpass filtering in the receive channel

reduces the possibility of extraneous noise from the

telephone line.

In the early 1990s, digital technology replaced

analog devices such as the Shure ST3000. Polycom is

now one of the prime suppliers of digital, full duplex

speakerphones.

Sound Control Technologies Ceiling Systems. Two
configurations have been supplied by Sound Control
Technologies. Loudspeakers and microphones are

mounted in the ceiling over the conference participants.

In one configuration, two loudspeakers are driven in

antiphase (180% out of polarity) and a small micro-

phone is mounted midway between them. Direct sound

from the loudspeaker to the microphone is balanced for

a null of 20 dB for receive signals. The basic element is

shown in Fig. 21-34.

The second configuration uses a microphone and a

loudspeaker mounted precisely 12 inches apart in

reflecting baffle ceiling-mounted panels. Pairs of these

loudspeaker/microphone units are placed above the

conference table. All loudspeakers are driven in the

same phase, while the microphones of symmetrically

located units are mixed and balanced antiphase. A block

diagram of such a system is shown in Fig. 21-35.

The microphone signals being mixed and balanced

in antiphase feed the bus from which both the sound

reinforcement (voice lift) and telephone send signals are

derived. Notch filters are used for adjustment of spec-

trum balance. Delay may be included in the reinforced

sound feeds if the room is large. The telephone return

signal also feeds the sound reinforcement loudspeakers.

An echo canceler is included to reduce the effects of

telephone line echo or room acoustic echo.

As with the Shure system described previously, a

telephone connection is made with a handset. Upon

completion of the connection, the status of the line is

determined by transmission of a group of tone bursts

that allows the hybrid to electronically balance for the

complex impedance of the telephone line. A push-

button switch converts the connection to conference and

the handset may be placed in its cradle.

Figure 21-33. Block diagram of an analog teleconferencing system using automatic microphone control techniques.

Courtesy Shure Incorporated.

Headphone

Aux output

Mic level out

Output level

Microphone

signals

Not mutable

mutable

Input

level

Aux input

Loudspeaker

Power

amplifier Limiter

Receive

suppression

Volume

Send/receiver

switching &

suppression

control

logic

Receive

compression

Bandpass

filtering

Limiter Hybrid

Bandpass

filtering

Send

suppression

Receive

Send

Telephone

Conference

Telephone

set

Telephone

line

Figure 21-34. A method of using acoustic cancellation to
reduce acoustic leakage in a conference room by driving
loudspeakers out of phase.

Loudspeaker Microphone Loudspeaker

Microphone

Loudspeakers

driven out

of phase

+

+