Studio Technologies 761 - Manual (Page 36)

Issue 1, March 2008 Model 761/771 User Guide
Page 36 Studio Technologies, Inc.
for Surround
damage to the monitor loudspeakers
during the time when AC mains voltage is
connected, disconnected, or has changed
signifi cantly from nominal. A combination
of hardware and software is used to moni-
tor one of the power supply “rails.” Until
the AC mains input exceeds its nominal
operating voltage range, electromechanical
relays maintain a short-circuit condition
on the monitor outputs. After the input volt-
age is recognized as valid, a short delay
takes place before the relays are allowed
to function normally. Whenever the AC
mains input drops below its nominal oper-
ating 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 761 to Model 771
Connections
Figure 7 gives a detailed description of
the signals that connect the Model 761
Central Controller to the Model 771
Control Console. The Model 761 provides
a +12 volt DC power source for use by the
Model 771’s circuitry. The DC output is
current-limited to minimize the chance that
a short-circuit condition will damage the
Model 761’s circuitry. An asynchronous,
bi-directional, data interface links the two
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
761 communicates with the Model 771
20 times per second, receiving button and
rotary level control information and send-
ing 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 761’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.