540 Chapter 16
16.6.12.3 Zoom Microphone Video Camera Linkage
In order to obtain a good matching of picture and sound,
a mechanism that synchronizes the optical zooming and
acoustical zooming becomes inevitable. Electrical
synchronization would also be possible by using
voltage-controlled amplifiers (VCA) or
voltage-controlled resistors (VCR).
16.6.13 Automatic Microphone Systems
There have been many advances in automatic mixers
where the microphone is normally off until gated on by
a signal, hopefully, a wanted signal. Many operate on an
increased level in one or more microphones with respect
to the random background noise, see Chapter 21.
While the Shure Automatic Microphone System
(AMS) is a discontinued microphone, it is still used in
many venues. The system turns microphones on and off
(with automatic gating), greatly reducing the reverberant
sound quality and feedback problems often associated
with the use of multiple microphones. The AMS micro-
phones are gated on only by sounds arriving from the
front within their acceptance angle of 120°. Other sounds
outside the 120° angle, including background noise, will
not gate the microphones on, regardless of level. In addi-
tion, the AMS adjusts gain automatically to prevent
feedback as the number of on microphones increases.
The Shure Model AMS22 low-profile condenser
microphone, Fig. 16-105, is designed for use only with
the Shure AMS. Unlike conventional microphones, it
contains electronic circuitry and a novel transducer
configuration to make it compatible with the Shure
AMS mixers. The microphone should not be connected
to standard simplex- (phantom-) or non-simplex-
powered microphone inputs because they will not func-
tion properly.
AMS microphones, in conjunction with the special
circuitry in the AMS mixers, uniquely discriminate
between desired sounds that originate within their 120°
front acceptance angle and all other sounds. Sounds
from the front of a microphone are detected and cause it
to be gated on, transmitting its signal to the mixer
output. Sounds outside the acceptance angle will not
gate the microphone on. When an AMS22 is gated on, it
operates like a hemi- or half-cardioid microphone
because half the cardioid pattern disappears when the
microphone is placed on a surface, Fig. 16-106. Each
AMS microphone operates completely independently in
analyzing its own sound field and deciding whether or
not a sound source is within the front acceptance angle.
The microphone should be placed so that intended
sources are within 60° of either side of the front of the
microphone—that is, within 120° acceptance angle.
Sources of undesired sound should be located outside
the 120° acceptance angle. Each microphone should be
at least 3 ft from the wall behind it, and items such as
large ashtrays or briefcases should be at least 1 ft behind
it. If the microphones are closer than that, reflections
will reduce the front-to-back discrimination and, there-
fore, make the microphone act more like a conventional
cardioid type.16.6.14 PolarFlex¥ Microphone SystemThe PolarFlex system by Schoeps models any micro-
phone. The system features two output channels with
two microphones per channel, Fig. 16-107. The stan-
dard system consists of an omnidirectional and a figure
8 microphone for each channel and an analog/digital
processor.
Essential sonic differences between condenser
microphones of the same nominal directional pattern are
not only due to frequency response, but also to the fact
that the polar pattern is not always uniformly main-
tained throughout the entire frequency range particu-
larly at the lowest and highest frequencies. Though
ostensibly a defect, this fact can also be used to advan-
tage (e.g., adaptation to the acoustic of the recording
room). While the frequency response at a given pickup
angle can be controlled by equalizers, there was no way
to alter the polar pattern correspondingly. The only wayFigure 16-105. Shure Automatic Microphone System
(AMS) model AMS22 low-profile microphone. Courtesy
Shure Incorporated.