Preamplifiers and Mixers 747(21-3)
where,
N is the number of activated microphones.While NOMA helps maintain stable gain in a sound
system as the number of activated microphones varies,
it does not limit the number of microphones that can be
activated.21.3.3.5 Restricting the Number of Open Microphones
(NOM)Recent developments in automatic microphone mixer
design have led to a feature best described as a NOM
restrictor. This feature restricts the number of active
input channels to a predetermined amount. For example,
in a large legislative system with 100 microphones, it
makes little sense to allow all 100 microphones to be
active at any instant, even if all 100 legislators are
talking.
Restricting the NOM to 5 microphones of the 100
allows spirited debate while not subjecting the audience
to the cacophony of 100 open microphones.21.3.3.6 Input Channel AttenuationGating automatic microphone mixers use some form of
input channel attenuation to turn off unused micro-
phones. The activation of an input channel becomes
audibly apparent if the level change from the off state to
the on state is too great. Practical experience has shown
that a 15 dB change from off to on is a good compro-
mise. However, as the number of microphones in the
system increases, more input channel attenuation may
be required for system gain stability. Adjustment of
input channel attenuation is available on most automatic
microphone mixers. This adjustment can be on an input-
by-input basis or for all inputs at once. The relationship
between gain before feedback, input channel attenua-
tion, and the number of microphones is calculated by
the following equation:(21-4)where,
'G is the gain improvement in dB with only one micro-
phone activated,N is the total number of microphones,
A is the attenuation for all input channels in dB.
Fig. 21-16 shows the relationship in graphical form.
Note the asymptotic maximum value of gain improve-
ment with infinite attenuation—i.e., all but one channel
turned off. Also note that input channel attenuation
greater than 30 dB offers little improvement for systems
with up to 256 microphones.21.3.3.7 Automatic Gain ControlAutomatic gain control (AGC) of an input or output is a
feature of a few automatic microphone mixers. A sound
engineer rides gain to bring up weak signals or reduce
overly loud signals and attempts to do this without
destroying the inherent dynamic range of speech. An
AGC in an automatic microphone mixer is typically
designed to reduce gain only should the input signal level
increase. The AGC is adjusted so that the quietest talker
has maximum gain (without feedback). All louder talkers
will force the AGC to bring down the overall level.
The IRP Level-Matic circuit is an example. It auto-
matically adjusts the master gain to maintain a uniform
output level for input signal variations up to 10 dB. A
loud talker causes the gain to steadily decrease. When
the talker stops, the gain holds as established by his or
her average talking level. If a quiet talker then speaks,
the gain steadily increases to a new value set by his or
her average speaking level.
Gain control is based on loudness versus frequency
and loudness versus time response of the ear. Gain
adjustments are made at a constant dB per second rate
to minimize the pumping and breathing effects ofAttenuation in dB= 10 logN'G 10 N
1 N 1– 10Ae 10
+= log-------------------------------------------Figure 21-16. Gain improvement with different channel-off
attenuations in a mixer that has a number of microphones
and only one channel on.2520151050
1 2 4 8 16 32 64 128 2568.2 dB
6.7 dB
5.2 dBGain increase before feedback–dB 0 dBChannel attenuation =5 dBNumber of microphones in systemInfinite30 dB
25 dB20 dB15 dB10 dB