Handbook for Sound Engineers

Preamplifiers and Mixers 759

The conference bridge operates in a similar manner.
Good balance in both the bridge and the hybrid is neces-
sary. This involves well-controlled and constant tele-
phone impedance. Unless these devices can adapt to
variable telephone line conditions, signal leakage may
be retransmitted through them. If the boardroom has not
been correctly treated acoustically, the combination of
room echo and signal leakage creates an undesired feed-
back path. Many hybrids suppress leakage by less than
15 dB whereas 35 dB to 40 dB is regarded as the
minimum acceptable for loudspeaker receive confer-
ence installations. The paths of signal leakage are
shown in Fig. 21-31. Active hybrids are supplied by
manufacturers such as Gentner Electronics, ASPI, and
Telos. These products provide means for optimizing the
impedance match to the telephone line, thereby giving
additional suppression of signal leakage. Active hybrids
can make the difference between a marginal and an
acceptable teleconference.

Typical telephone line impedances range from 600:

to 900:. Telephone equipment expects send levels of

0 dBm. The receive level standard is 6 dBm, but these

levels are reported to vary widely, 10 dBm is

frequently experienced. The standard telephone line has

48 Vdc (some private exchanges use 24 Vdc) for system

control that must be blocked with a transformer or

capacitors. The dc current through the off-hook relay

keeps the line open while the connection is active.

21.3.7.2 Microphone Considerations

For a small group in a conference room, it may be pos-

sible to use only one omnidirectional microphone on a

table top, typically of the surface-mount type. How-

ever, even for a group of four to six people, the equiva-

lent of several directional microphones with an

automatic microphone mixer is preferred to reduce the

number of open microphones to the minimum necessary

for the discussion. Three cardioid microphones in a cir-

cle, spaced at 120° intervals, is a typical approach.

There are a number of surface-mount microphones that

can be used, provided that the distance to the talkers is

acceptably short. The typical participant in a teleconfer-

ence expects, at minimum, the sound quality heard from

a handset where the microphone is within inches of the

talker’s mouth. Thus, keeping the microphones close to

the talkers is very important.

21.3.7.3 Microphone Mixing

Larger groups inevitably require a large number of

microphones to keep the participant-to-microphone dis-

tance within the limits set by the room’s critical dis-

tance. Some form of automatic microphone selection

and mixing is essential in this case. Systems can be

designed using an automatic microphone mixer (as

described earlier in this chapter) connected to a tele-

phone line interface device. Or a system can be imple-

mented using an integrated device where the automatic

mixing and the telephone interface are contained in the

same chassis. Depending on the complexity required,

there are many suitable approaches to system design.

Consult the equipment manufacturers for specific

design suggestions, Fig. 21- 32.

21.3.7.4 Loudspeaker Considerations

Direct feedback from loudspeakers to microphones in

any of the conference sites must be avoided; therefore,

loudspeaker placement is critical. Loudspeakers should

be placed in the null of the microphone pickup patterns.

For cardioid microphones pointing in a horizontal direc-

tion, loudspeakers can be placed behind the microphones

Figure 21-31. The paths for signal leakage and undesired

feedback in a typical teleconference system.

Telephone

hybrid

Incoming audio is reflected

and retransmitted on

telephone line

Telephone line

Signal leakage (caused

by impedance mismatch

between lines)

Telephone

line

Signal leakage in conference

bridge causes retransmission

of received audio on telephone

lines

Figure 21-32. The configuration of a multimicrophone

audio conference installation, without sound

reinforcement.

Mic

Amp

Power

Amp

Telephone

Telephone line interface

from bridge