Specifications

CHAPTER SEVEN
AUTOMATIC MIXERS
The function of an automatic mixer is twofold: 1) to
automatically activate microphones as needed and 2) to
automatically adjust the system gain in a corresponding
manner. With some automatic mixers, ordinary micro-
phones are used and the actual control is provided by the
mixer. In others, special microphones are integrated with
the mixer to provide enhanced control.
The reasons for using an automatic mixer relate to
the behavior of multiple microphone systems. Each
time the number of open or active microphones
increases, the system gain also increases. This results
is a greater potential for feedback as more microphones
are added, just as if the master volume control
were being turned up. Also, when multiple open
microphones pick up the same talker, a degradation of
audio quality occurs, called comb filtering. Since sound
travels at a finite speed, the talker’s voice arrives at the
microphones at different times. When electronically
combined in a mixer, these “out-of-step” microphone
signals produce a combined frequency response very
different from the frequency response of a single
microphone. The aural result of comb filtering is an
audio signal that sounds hollow, diffuse, and thin.
In addition, unwanted background noise increases with
the number of open microphones. Here, the effect is a
loss of intelligibility as the background noise level rises
closer to the level of the desired sound.
The solution is to activate microphones only when they
are addressed and to keep them attenuated (turned down)
when not being addressed. In addition, when more than
one microphone is addressed at a time, the system gain
must be reduced appropriately to prevent feedback and
insure minimum noise pickup.
There are several techniques used to accomplish
channel activation or gating in an automatic mixer. In most
mixers, a microphone is gated on when the sound it picks
up is louder than some threshold or reference level. When
the sound level falls below the threshold, the microphone
is gated off. This threshold may be fixed, adjustable,
or even automatically adjustable.
Certain other automatic mixers, with integrated
microphones, can actually sense the location of the
sound source relative to the ambient noise and activate
microphones only when the sound comes from the
desired direction. These directional gating systems do not
require any threshold adjustments.
There is another circuit within every automatic mixer
that continuously senses the number of open microphones
(NOM) and adjusts the gain of the mixer accordingly. With
a properly functioning automatic system, if each individual
microphone is adjusted to a level below the feedback point,
then any combination of microphones will also be below
the feedback point.
Many automatic mixers have additional control
circuitry, often in the form of logic connections. These are
electrical terminals that can be used for a variety of
functions, including: microphone status indicators, mute
switches, loudspeaker attenuation, and the selection
of priority channels. Some automatic mixers have a
selectable off-attenuation control: instead of gating the
microphone completely off, it can be turned down
by some finite amount which makes the gating effect
less noticeable in certain applications. Another control
included on some units is an adjustable hold time: when
the desired sound stops, the channel is held on for a short
time to avoid gating off the microphone between words or
short pauses. Finally, most automatic mixers are able to
be expanded by adding individual channels and/or by
linking multiple mixers together to control large numbers
of microphones simultaneously.
27
MEETING FACILITIES
Audio Systems Guide for
SCM810 automatic microphone mixer