Theatre Design & Technology - Summer 1983 - 18

mixer just turns the mike on or off.
That might seem easy, but it is complicated by signal source: is it an actor talking into the mic or just audience noise?
A number of methods allow the mixer to
tell the difference. Some are as simple as
a level sensing circuit that assumes an
actor will be louder than the audience.
This type of circuit has a "threshold"
level and only turns the microphone
"on" when the level goes above the
threshold (usually set by the operator).
Some automatic mixers use more complex methods. In all automatic mixers,
however, some type of on/off function
is combined with a circuit to reduce the
master volume control by 3dB every
time the NOM is doubled. The differences between mixers are in the ways
they perform these two functions, the
additional functions they perform, and
the way they allow the operator to adjust the functions.
Footlight Microphones, A Simple
Theatre Application
The trouble with footlight microphones is that they are often too far
from the performers to do a lot of good.
Many undesirable things happen as a
result. For example, the audience noise
level at the microphone may be picked
up along with the actors' voices. Or the
level of an orchestra picked up by the
microphone may exceed the actors'
voice levels in the footlight microphones.
Another problem resulting from too
much distance between the performers
and the microphones is the all-toofamiliar problem of feedback. When
you turn up the mike far enough to effectively reinforce an actor's voice, you
are so close to feedback that you hear a
ringing every time the actor speaks.
Turn up a second mike and feedback is
unavoidable (the number of open microphones problem).
A third problem with footlight microphones is what we often call "phasing." When an actor is close enough to
two microphones to be picked up by
both, the sound may be "hollow" and
lack "sparkle." As the actor moves between the microphones, the sound quality changes in a way that sounds like a
kind of special effect known as "phasing." Unless the actor is exactly the
same distance from each microphone,
the sound will arrive at one microphone
very slightly ahead of the time it arrives

at the other microphone. The result is a
ragged frequency response that changes
as the actor moves.
The question is, can an automatic
mixer help solve any of these problems?
The answer is, yes it can, within limits.
The first problem was one of signal
to noise ratio. The signal (the actors'
voices) was low compared to the noise
(the audience or orchestra) at the microphone. Thus, unwanted sounds were
picked up. If an automatic mixer were
able to discriminate between actors'
voices and audience noise, it could turn
on only those microphones which were
picking up voices and turn off those
microphones that were picking up mostly noise. Manufacturers have developed
some very sophisticated circuits for doing just that. Some, of course, are as
simple as the "threshold" circuit mentioned earlier. Some automatic mixers
have circuits that are able, to some degree, to tell whether the sound reaching
the microphone is a human voice or
some other noise. One new system, by
Shure, even includes a patent-pending
rear-discrimination sensing circuit that
reponedly will not turn on the microphone in response to any sound reaching
the rear of the microphone (the audience
and orchestra).
Ultimately, by turning on only those
microphones actually receiving actors'
voices, the automatic mixer could help
prevent unwanted noise pickup. The
limits are obvious: none of the circuits
are a/ways going to be able to tell whether the sound reaching the microphone is
a human voice or unwanted noise. And,
when the microphone does turn on, if
the level of the actor's voice is no louder
than the unwanted noise, both will be
amplified equally. 0 automatic mixer
can prevent that.
The second problem was feedback.
The automatic mixer may help solve that
problem because of its" OM" reduction feature. The automatic mixer prevents feedback and that is a very desirable result, indeed. The trouble is, that
the overall volume decreases by 3dB.
ow, a 3dB reduction in volume is probably acceptable, but what happens if
four microphones turn on instead of
two? The circuit reduces the master volume by another 3dB for a total of 6dB,
and a 6dB reduction in overall level is
noticeable by nearly everyone. Th us, if
you have a feedback problem, an auto-

matic mixer can help avoid that feedback, but not without side effects.
The third problem was "phasing"
caused by an actor's voice reaching two
microphones at slightly different times
resulting in ragged frequency response.
If only one of those microphones were
on, the problem would disappear. That
very desirable result could occur if the
actor's voice reaches one microphone
with a sound level that is slightly below
the "threshold" for the automatic mixer and reaches the second microphone at
a level which is slightly above the threshold. In this case, only the second microphone would turn "on" and the phasing
problem would be eliminated. This
slight difference in level, however, is
critical and would probably never occur
in practice. The actor would have to be
exactly the right distance from the two
microphones. A more likely result is
that the two microphones would either
both turn on (causing the phasing problem) or both turn off (an obviously undesirable result).
The Human Operator, A Comparison
An experienced human operator
could probably solve all three problems.
The human could see which microphones are closest to the actors. Even
when scenery hides the microphones, the
human operator can probably tell which
microphone to use as well as or better
than the automatic mixer. If the house
mixing console (non-automatic) allows
the operator to "preview" the microphones (listen to them through headphones without turning them on in the
house) he can easily decide whether the
sound reaching the microphone is an actor's voice or just audience noise. As far
as avoiding feedback, the human operator can tryout the controls before the
petfonnance to see where the feedback
points are. During the performance, the
experienced operator will simply avoid
turning up the controls beyond this
point. An experienced operator will
have developed an "ear" for the phasing problem and will select the appropriate microphone. Finally, the human operator can (politics allowing) instruct the
actors to move closer to the microphones or to talk louder, both of which
would improve things more than any
electronic solution!
Thus, in this comparison, the human
operator wins-hands down.

USITT Summer 1983 Theatre Design & Technology

Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1983