Specifications
With a single microphone fully "on", the increase in system
gain due to incomplete attenuation in the "off microphones
is given by:
Where:
G'
is the system gain increase in dB compared to
full attenuation, Att is the Off-Attenuation in dB,
and m is the total number of microphones.
For an eight-microphone system:
The recommended setting of
-15
provides the benefit of
finite Off-Attenuation while degrading available gain-before-
feedback by less than
1
dB. The Variable position set to
-8
[actually -8.5 dB, fully clockwise] provides very smooth
gating in applications where an increase in room noise and
degradation of gain-before-feedback of up to 3.0 dB is ac-
ceptable. Automatic adjustment of the Off-Attenuation level
maintains these relationships when additional microphones
are added through the linking of additional units. The
-00
set-
ting is useful when a muted microphone should be fully off.
The above discussion assumed an idealized installation
with all the microphones operated under identical conditions.
In practice, of course, the microphones may not be operated
at identical gains, and the acoustical environment will vary at
different microphone locations. Because of its acoustical
location or the need for a higher gain setting, one
microphone will usually reach its feedback threshold before
the others and will establish the limit on system gain. The
AMS will act to prevent an increase in system gain beyond
that of the worst-case microphone operated alone.
The formulas previously given in this section, which deter-
mine the system gain controlling action, assume random
phase relationships among the sound fields at the
microphones. The approximation is valid for a large number
of microphones, but not for just a few. The sound fields at
two microphones will be in phase at some frequencies. The
combination of the two microphones will tend to increase
system gain by
6
dB at these frequencies, instead of the
3
dB for which the AMS compensates. Feedback can occur at
one of these frequencies with the gating on of the second
microphone, if the gain and phase criteria for feedback are
met. Although unlikely, this possibility should encourage the
installer to check different combinations of "on"
microphones before declaring the system to be free of feed-
back.
APPENDIX
Ill
AMS MIXERS AND CONMNTIONAL MICROPHONES
If a conventional low-impedance microphone is connected
to the input of an AMS mixer, that channel will operate like a
normal mixer channel with the following exceptions:
1
.
Normally no gating action will occur; the gate will be per-
manently off.
2.
The channel signal will be (compared to the properly
operating AMS channel] somewhat degraded through
loss of low-frequency signals [bass
rolloff of about
6
dB/octave below 500 Hz], increase in noise, and loss
in signal level.
However, the conventional microphone signal
will
be passed
if:
1
.
That channel's Level control is sufficiently high.
2.
The Master Gain control is sufficiently high.
3. The Off-Attenuation control is
not
set for minus infinity
[-a].
Since the channel remains gated off, the Off-Attenuation
control determines the level at which the conventional
microphone signal will be passed.
A more desirable method of using an AMS mixer channel
with a conventional microphone is to use the Override In logic
terminal to force the channel on. In this mode, the
microphone signal is typically
1
5 dB higher than in the gated-
off mode. Only the Channel Level and Master Gain controls
affect the microphone signal; the Off-Attenuation control
does not affect the signal because the channel is gated on.
Note that the use of a conventional microphone in one chan-
nel will not affect other AMS mixer channels. They will con-
tinue to operate properly with AMS microphones.
.Many phantom powerable condenser microphones may
operate with the low dc voltage supplied by each AMS mixer
channel. For instance, Shure's SM85 will operate; however,
the microphone's clipping level is significantly reduced
because the powering voltage is lower than the minimum
rated voltage.
As this section indicates,
a
conventional microphone
can
be used in an AMS mixer channel. However, it must be
noted that signal degradation and loss of all automatic action
for that channel are the price paid for not using an AMS
microphone.