Handbook for Sound Engineers

Preamplifiers and Mixers 749

all microphones in the system must be below some

maximum value that avoids feedback oscillation. The
safe system gain is set relative to the sum of all micro-
phone signals in the system. If one microphone has
more signal than the average of all signals, then that
microphone channel is given more gain and all the other
channels less gain roughly in proportion to the relative
increase of signal level.

Dugan’s U.S. Patent 3,992,584 describes such a
system where a 3 dB level increase at one microphone
causes that channel gain to go up by 3 dB, while the
gain of the other channels decreases by 3 dB. Speech
from two persons talking into separate microphones
with levels differing by 3 dB (both appreciably above
the background level) would appear at the output of the
system with a 6 dB difference. In other words, the
signal from a microphone with the highest output is
given the most gain, and a signal from a microphone
with the smallest output is given the least gain. With
this operational concept, NOMA is not needed in the
output stage. Theoretically, the system is configured so

that the total gain is constant at a level that safely avoids

feedback oscillation.

Automatic microphone mixers marketed by Dugan,

Lectrosonics, Protech Audio, and Altec Lansing have

used level proportional control based on average input

signal amplitudes, Fig. 21-19.

Figure 21-18. Block diagram of Biamp autoTwo Automatic Mixer. Courtesy Biamp Systems.

Figure 21-19. Protech Audio automatic mixer. Courtesy

Dan Dugan.