1284 Chapter 34
The significance of this equation is that, while it
performs the two functions mentioned previously, it
does not mention the word threshold nor the word
switch. A Dugan system automatic mixer varies the
microphone levels in a continuous manner depending
only on the relationship of each individual channel level
to the sum of all the channel levels.
A user sets up the Dugan mixer by adjusting each
individual volume control to a position suitable for the
person talking. That means the volume control for a
quiet talker’s microphone will have a higher setting than
that for a loud talker. These volume control settings
assure that the circuitry treats all microphones equally
in the equation. After this initial volume control adjust-
ment, the user ceases interacting with the mixer. New
talkers, of course, or significant changes in talker input
level, require human intervention. A dummy micro-
phone modification helps keep the mixer’s automatic
circuitry from being fooled by ambient noise.
Other automatic mixers switch microphones on (to a
volume control level preset by the user) when someone
talks into the microphone and off when no one talks into
the microphone. They also reduce the master volume
control by approximately 3 dB when the number of in
use microphones doubles. Most of these mixers incor-
porate sophisticated digital circuitry to make the deci-
sions about when to turn a microphone on or off and
exactly how much to attenuate the master volume
control. As a result, a well-designed automatic mixer of
either type can be successfully used in a system
designed for automatic mixing.
Today, automatic mixers may be digital and auto-
matic mixing functions are often included in multi-
channel DSP devices.
34.4.3.1 Special Features
A number of additional functions/features are either stan-
dard or optional on most automatic mixers. Specific fea-
tures, of course, depend on the make and model chosen.
For example, on the switching-type mixers, users
may have the option of adjusting the threshold setting.
The threshold is the LP at which the mixer turns on a
microphone channel. In very high noise areas, for
example, a user could increase the threshold to reduce
the problem of microphones turning on from ambient
noise input. Another feature available on some mixers is
adjustment of the amount of off attenuation. That is, the
off state can be redefined from no attenuation at all to
infinite attenuation (true off). By selecting no attenua-
tion, the on-off switching feature is defeated, and the
mixer functions only as a number of open microphones
(NOM) attenuator.
One valuable feature found on most mixers is a logic
output. This logic output is a dc voltage output, usually
compatible with TTL circuitry levels, and it goes high
when the microphone is on and goes low when the
microphone is off. This logic output can be used to acti-
vate relays for zone paging or to activate complex
microphone priority switching in conference systems.
Most automatic mixers also allow the user to defeat
the automatic circuitry on an individual input channel.
This allows a tape machine or other nonmicrophone
input to be added to the mix without affecting (and
without being affected by) the automatic mixing of the
system microphones.34.4.3.2 Applications for Automatic MixingIn systems with undemanding, predictable mixing
requirements, the automatic mixer may be able to com-
pletely replace the human operator. Examples are con-
ference and courtroom systems and speech-oriented
systems in religious facilities. In these systems, the
installer sets up the system volume controls and
instructs the user simply to turn the entire system on and
off since the automatic mixer will take care of every-
thing else.
In actuality, systems like these are rare. More
common are systems where the automatic mixer
becomes an operator aid rather than completely
replacing the human mixer. Any of the previously
mentioned systems where different talkers use the same
microphone require some human intervention. But the
automatic mixer can also aid the human operator in
more sophisticated systems including entertain-
ment-oriented systems and dramatic (live) theater
presentations.
Most automatic mixers are unsuitable for mixing
musical material. However, most can be used effec-
tively for voice mixing of footlight microphones in a
theater, and some automatic mixers may find use in
submixing of instruments or vocals in an entertain-
ment-oriented system. In all cases, the ability of the
mixer to sense in-use microphones and attenuate (or to
turn off) other microphones is a valuable aid in reducing
unwanted noise pickup. In addition, the ability to help
reduce the possibility of feedback (the NOM function) is
welcome in any system.