Handbook for Sound Engineers

748 Chapter 21

simple level compression circuits. If there is no signal,
the AGC gain holds at its last value.

21.3.4 Types of Automatic Microphone Mixers

An automatic microphone mixer can have an analog cir-
cuit design, a digital circuit design, or a combination of
the two. Though a digital design might offer more
design flexibility due to software control, the digital
automatic microphone mixer is not inherently better
than an analog automatic microphone mixer. Be it ana-
log or digital, an automatic microphone mixer will fall
in one of the following functional groups:

1. Fixed threshold.


2. Variable threshold.


3. Gain sharing.


4. Direction sensitive.


5. Multivariable dependent.

21.3.4.1 Fixed Threshold Automatic Microphone
Mixers

A detector circuit in the automatic microphone mixer
activates an input channel when a microphone signal is
present and attenuates the input when the microphone
signal ceases. This basic function is often called a noise
gate. To activate the input, the signal must be larger than
a threshold preset for the channel during installation.
This method has several shortcomings. First, there is the
dilemma of where to set the activation threshold. If it is
set too low, it will respond falsely to room noise, rever-
beration, and room-reflected sound. If the threshold is
set too high in an effort to avoid false activation, desired
speech signals may be chopped or clipped. The threshold
should be set high enough to avoid activation by random
noises, but low enough to turn on with desired speech
signals. These are frequently contradictory requirements,
and compromise is generally not satisfactory.
A more serious problem is that any number of input
channels may activate with a very loud talker. One solu-
tion is a first-on inhibiting circuit that permits only one
input channel to be on at a time. One-on-at-a-time oper-
ation is generally unacceptable for conversational
dialog because the hold time needed to cover speech
pauses will keep the second talker off.
Fixed threshold automatic microphone mixers have
fallen out of favor and are now rarely employed. Early
examples of fixed threshold activation products include
the Shure M625 Voicegate (1973), the Rauland 3535
(1978), the Edcor AM400 (1982), and the Bogen
AMM-4 (1985).

21.3.4.2 Variable Threshold Automatic Microphone

Mixers

One attempt at overcoming the problems of a fixed

threshold is to set the activation threshold based on a

signal from a remote microphone. This microphone

would be located in an area that is not expected to pro-

duce desired program input and is presumed to provide

a reference signal that depends on variations in room

noise or reverberation. Any desired talker input must

then exceed this level by some preset amount. It is

assumed that the desired talker signal will be louder

than the reference. However, this may not be true, espe-

cially when the reference signal from a randomly

selected microphone location does not represent the

ambient sound in the vicinity of the talker’s micro-

phone. This is the basis of a system described by Dugan

in U.S. Patent 3,814,856. An alternative source of refer-

ence threshold may be derived from the sum of the out-

puts of all the microphones in the system.

The discontinued JBL 7510 automatic microphone

mixer employed a variable threshold design to override

a fixed threshold. This design assumed that if a common

acoustical disturbance was sensed at several micro-

phone input channels, an input channel should not be

activated. Instead, the overall system threshold should

be raised. A talker must then be loud enough at the

microphone to override the new raised threshold. Both

the fixed threshold and the contribution of the back-

ground threshold reference would be set at installation.

Release time, input attenuation, and gain were also

necessary adjustments for each input channel. Varia-

tions on this concept of variable threshold design have

been used in automatic microphone mixers from Audio

Technica, Biamp, IED, Ivie, Lectrosonics, and TOA.

The Biamp autoTwo Automatic Mixer, Fig. 21-17,

includes adaptive threshhold sensing to minimize false

gate triggering, a speech frequency filter to minimize

false gating due to noise, logic outputs from channels

for switching external circuits, and 6 dB of hysteresis to

reduce gate fluttering when near threshold. The block

diagram is shown in Fig. 21-18.

21.3.4.3 Gain-Sharing Automatic Microphone Mixers

A gain-sharing automatic microphone mixer works

from the premise that the sum of the signal inputs from

Figure 21-17. Biamp autoTwo Automatic Mixer. Courtesy

Biamp Systems.