Preamplifiers and Mixers 745An automatic microphone mixer cannot improve the
performance of microphones. Its primary benefit comes
from limiting the number of microphone signals fed to
the mixer output. A side benefit is often the apparent
increase of critical distance in a multiple microphone
system. (Critical distance is defined as a point in the
room where the direct signal of the talker equals the
reflected signal of the talker, i.e., 50% direct signal and
50% reverberant signal.) Because unused microphones
remote from the talker are attenuated, room reverbera-
tion and ambience that would otherwise be amplified
are reduced.
21.3.3 Controls and Features of Automatic
Microphone Mixers
Automatic microphone mixers have many of the same
controls and features of manual microphone mixers.
Examples are:
- Level control for each input channel.
- Master level control for each output channel.
- Input signal attenuation (“trim”).
- Phantom power.
- Two or three band equalization for each input channel.
- Output level metering.
- Output signal level limiter.
- Nonautomatic auxiliary inputs.
- Headphone output with level control.
These controls and features may be configured in
hardware–e.g., switches, potentiometers, LED strings—
or they may be configured in software. In either case,
the function of the control or feature remains the same.
21.3.3.1 Controls and Features Unique to Automatic
Microphone Mixers
As automatic microphone mixers typically perform
more functions than a manual microphone mixer, there
are controls and features that are unique to automatic
microphone mixers.
Input Channel Threshold. Determines at what signal
level a gated automatic microphone mixer input passes
the incoming microphone signal to the mixer’s output.
Input Channel On Indicator. Illuminates to indicate
that an input channel is passing the microphone signal
onto the mixer output.
Direct Output for Each Input Channel. Provides an
isolated output for each input channel that is unaffected
by the automatic microphone mixer action.
Last Microphone Lock On. Keeps on the most
recently activated input channel on a gated automatic
microphone mixer until another input channel is acti-
vated. This maintains room ambience when the auto-
matic microphone mixer is used to provide a broadcast
feed, a recording feed, or a feed to an assistive hearing
system.Hold Time. Keeps an activated input channel on a
gated automatic microphone mixeron for a period of
time after speech has ceased. This feature bridges the
natural gaps that occur in speech patterns.Input Attenuation. Determines how much gain reduc-
tion is applied to an input channel of a gated automatic
microphone mixer when the channel is not activated.
Typical range of adjustment is 3 dB to 70 dB of attenua-
tion, with 15 dB being a common value.Decay Time. Establishes the time required for an input
of a gated automatic microphone mixer to be lowered
from the activated state to the attenuated state. Decay
time is always in addition to the hold time.Manual/Auto Select. Allows the automatic microphone
mixer to operate in a nonautomatic (manual) mode.21.3.3.2 External Control Capability and Status
Indication of Automatic Microphone MixersMost automatic microphone mixers include the ability
to be controlled by external switches, potentiometers,
touch screens, personal computers, and other types of
control devices. These devices are connected to the
automatic microphone mixer via screw terminals or
multipin connectors on the mixer’s rear panel. The con-
trollable functions and the communication protocol
depends upon the manufacturer and model of the auto-
matic microphone mixer. Examples of automatic micro-
phone mixer functions that can be externally controlled
follow.Gain of an Input Channel or the Master O\output.
In a courtroom, the court clerk could control the volume
level of the witness microphone or the entire sound sys-
tem using a potentiometer located at a distance from the
automatic microphone mixer.Mute an Input Channel. In a city council chamber, a
council member could have a privacy or “cough” switch
located near the microphone.Global Mute of All Input Channels. In a government
hearing room, the presiding member could mute all
inputs to regain control of a meeting.