Handbook for Sound Engineers

Preamplifiers and Mixers 751

manufactured by Shure, including the now obsolete
FP410 battery-operated portable mixer, Fig. 21-21.

21.3.4.6 Multivariable-Dependent Automatic
Microphone Mixers

The automatic microphone mixer methods described so
far essentially use input signal amplitude as the activa-
tion variable. The relative timing of signals at each
input is another variable that can be employed. A multi-
variable-dependent system makes its activation decision
from both input signal amplitude and the time sequence
of the input signals.

Peter’s U.S. Patent 4,149,032 is such a design. The
instantaneous positive signal amplitudes of all inputs
are simultaneously compared to a threshold voltage (dc
ramp) that falls 80 dB in 10 ms (or less) from a high
value to a low value. Initially, all input channels are
held in an attenuated state. The first input channel that
has an instantaneous amplitude equal to the instanta-
neous value of the falling threshold is activated, while
the other inputs remain attenuated. This activated
channel remains so for 200 ms.

Once an input channel is activated, the threshold
voltage is reset to its high value and immediately starts
to fall again in search of another input to activate. If all
talkers are silent and an amplitude match is not found,
the threshold search progresses the full 80 dB in 10 ms
and then resets. However, this scenario is not typical.
Most of the time, a signal on one of the inputs will

produce a threshold amplitude match early in the

search. In practice, the average input activation time is

3 or 4 ms. Since the threshold resets every time an input

is activated, the frequency of the threshold searches will

be also every 3 or 4 ms on average.

As mentioned, the input activation is maintained for

200 ms. If on the second search the same input still has

the largest signal amplitude, its activation status is

renewed for another 200 ms. If during a future threshold

search, a different input channel has the higher ampli-

tude, it is activated for 200 ms. The first input activated

times out and attenuates if not reactivated by a future

search within the 200 ms. As long as a talker keeps

speaking, his input is continually renewed for 200 ms

intervals. This rapid response enables conversational

dialog to be conducted and also permits easy activation

of weaker sound sources during gaps in speech.

Since the activation gain of all input channels is the

same, any signal source on an active channel has the

same gain, and the relative levels of different talkers is

preserved in the mixer output.

When multiple talkers vie for access to the system,

the probability of all of them obtaining access decreases

in proportion to the number. This effectively limits the

maximum number of input channels that can be activated

at any given time. For example, ten equally loud talkers

will each be on 88% of the time. But as more than three

or four persons talking at the same time is not intelli-

gible, this limitation is normally of little consequence.

Also unique to the Peters design is the variable

known as the access ratio. Simply put, the access ratio

Figure 21-21. Block diagram of Shure FP410 mixer. Courtesy Shure, Incorporated.

Balanced

mic/line

input

Phantom power Low-cut Channellevel

Channel 1 (typical Channellevel

indicators Intellimix

control

Auto

Manual

Gatedsignals

Non-gatedsignals

Channels 2, 3, 4

In Out

Link jacks

Monitor in

Atten

Mic

Line

1 kHzoscillator

MasterLevel Limiter/

indicator

Atten Tape

out

Mic

Balancedmic/line

outputs

Line

Mic

Line

Peak/VU/batterymeter

Headphoneslevels

Headphones