Handbook for Sound Engineers

750 Chapter 21

21.3.4.4 Direction-Dependent Automatic Microphone
Mixers

A direction-dependent automatic microphone mixer
responds to signals having acceptable levels within a
predefined physical space in front of a microphone. By
making the decision as to whether a channel should be
on depends on the relative signal levels at two
back-to-back cardioid microphone capsules in a single
microphone housing. The Shure AMS (automatic
microphone system) responds in part to the location of
the sound source. This mixer works only with its own
unique two-capsule microphones.
When an AMS input channel is activated, the front
facing microphone signal is transmitted to the mixer
output. This mixer functions like a variable-threshold
system with its threshold being a fixed level above the
background ambient noise but with the threshold also
being a function of the sound source location and its
angular relationship to the microphone.

Any input channel may turn on when the signal level
from the front microphone capsule is 9.5 dB above the
level from the rear capsule. Effectively, n SNE of 5 dB
to 7 dB is required for a channel to activate. Of course, a
weaker sound source will not activate the channel. The
level difference of 9.5 dB is derived from the criterion
that a cardioid microphone response at 60° off-axis is
typically one-third of its on-axis response. The activation
angle of the mixer input channel is thus 120°. A sound
source outside of the 120° angle will not activate an
input channel no matter what the sound pressure level.

To keep the AMS microphones compatible with
conventional shielded twisted pair cable while keeping
the two microphone signals separated, an unbalanced
signal path is used. This approach can be more suscep-
tible to induced hum and noise pickup than a conven-
tional balanced signal path. The use of current source
preamplifiers in the microphone and unusually low
impedance inputs in the mixer minimizes this potential
problem.
It is recommended that an AMS microphone be
installed within three feet of each talker, and the talker
must be located within the 120° activation angle. Each
AMS microphone should also be at least three feet from
any wall behind it and at least one foot from objects
behind it such as books, large ashtrays, or briefcases.
This precaution is necessary to avoid unwanted reflec-
tion of the talker’s acoustic signal into the rear facing
microphone capsule. Stray acoustic reflections can lead
to unreliable input activation.

As the direction-dependent automatic microphone
mixer process is covered under U.S. Patent 4,489,442,

this type of automatic microphone mixer has been

marketed only by Shure. In 2000, U.S. Patent 6,137,887

was issued to Shure for a new AMS design. Developed

by Anderson, this patent adds a circuit that guarantees a

single talker will activate only a single input channel,

even if that talker is within the activation angles of

multiple AMS microphones, Fig. 21-20.

21.3.4.5 Noise-Adaptive Threshold Automatic

Microphone Mixers

This concept employs a dynamic threshold unique for

each input channel. Using an inverse peak detector, each

input channel sets its own minimal threshold that con-

tinually changes over several seconds based on varia-

tions in the microphone input signal.

Sound that is constant in frequency and amplitude,

like a ventilation fan, will not activate an input but will

add to the noise-adaptive threshold. Sound that is

rapidly changing in frequency and amplitude, like

speech, will activate an input. The mixer activates an

input when two criteria are met:

1. The instantaneous input signal level from the talker
   is greater that the channel’s noise-adaptive
   threshold.


2. The input channel has the maximum signal level
   for that talker.

Without this second criterion, a very loud talker

might activate more than one input channel.

Note that this system deems any sound that is rela-

tively constant in frequency and amplitude as

nonspeech. Sustained musical notes may activate an

input on attack, but after several seconds the sustained

note will raise the threshold and the input will be attenu-

ated. As previously stated, automatic microphone

mixers are designed primarily for speech applications,

not music.

Developed by Julstrom and covered by the U.S.

Patent 4,658,425, the noise-adaptative threshold config-

uration has been used in automatic microphone mixers

Figure 21-20. Shure AMS8100 mixer. Courtesy Shure

Incorporated.