Model 76b to model 77b connections, Mono-to-center polarity – Studio Technologies 76B 2008 User Manual
Page 39
Model 76B/77B User Guide
Issue 1, July 2008
Studio Technologies, Inc.
Page 39
for Surround
disconnected, or has changed signifi cantly
from nominal. A combination of hardware
and software is used to monitor one of the
power supply “rails.” Until the AC mains in-
put exceeds its nominal operating voltage
range, electromechanical relays maintain
a short-circuit condition on the monitor
outputs. After the input voltage is recog-
nized as valid, a short delay takes place
before the relays are allowed to function
normally. Whenever the AC mains input
drops below its nominal operating voltage
range, the relays immediately go to their
mute state. During testing it was found
that upon power up the monitor outputs
remained very quiet; during power down a
small to moderate “tick” was the worst that
was heard.
Model 76B to Model 77B
Connections
Figure 6 gives a detailed description of
the signals that connect the Model 76B
Central Controller to the one or more
Model 77B Control Consoles. The Model
76B provides a +12 volt DC power source
for use by the Model 77B’s circuitry. The
DC output is current-limited to minimize
the chance that a short-circuit condition
will damage the Model 76B’s circuitry. An
asynchronous, bi-directional, data inter-
face links the connected units. The RS-485
hardware connection scheme operates at
a rate of 115.2 kbit/s and uses an 8-N-1
data format. The Model 76B communicates
with each Model 77B unit 20 times per
second, receiving button and rotary level
control information and sending LED and
4-digit display status data.
Mono-to-Center Polarity
It’s worthwhile to note that a polarity inver-
sion takes place whenever the stereo to
mono downmix function is active. By de-
sign, the left (L) and right (R) input signals
experience a 180-degree inversion in
the Model 76B’s combining circuitry. This
should not cause a problem as monitoring
audio signals over a single loudspeaker is
considered polarity insensitive. Only during
electrical testing would this condition be
detected.