Studio Technologies 76 V.1.04 - Manual (Page 33)

Model 76/77 User Guide Issue 4, September 2008
Studio Technologies, Inc. Page 33
for Surround
normally. Whenever the AC mains input
drops below its nominal operating volt-
age range, the DC “rails” will drop and the
relays will almost immediately go into their
muted state. During testing it was found
that upon AC mains power up the monitor
outputs remained very quiet; during power
down a small to moderate “tick” was the
worst that was heard.
Model 76 to Model 77
Connections
Figure 6 gives a detailed description of
the signals that connect the Model 76
Central Controller to the one or more
Model 77 Control Consoles. The Model
76 provides a 12 volt DC, 200 milliampere
maximum power source for use by one
or two Model 77 units. The DC output is
current-limited to minimize the chance
that a short-circuit condition will damage
the Model 76’s circuitry. An asynchronous,
bi-directional data interface links the
units. This uses an RS-485 hardware
connection scheme that operates at a
rate of 115.2 kbit/s and uses an 8-N-1
data format. The Model 76 communicates
with each Model 77 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; the left
(L) and right (R) input signals experience
a 180-degree inversion in the Model 76’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.