Studio Technologies 220 2008 - Manual (Page 37)

Model 220 User Guide Issue 4, October 2008
Studio Technologies, Inc. Page 37
with the talkback 1 button. Pin 3 is con-
nected to the circuitry associated with the
talkback 2 button.
P18: Pin 1 is connected to the common
point of the Model 220’s circuitry. Pin 2 is
connected to the circuitry associated with
the main out button. Pin 3 is connected to
the circuitry associated with the auxiliary
pushbutton input. (This function is not cur-
rently supported in software.)
The input circuitry is “active low,” with a
10 k ohm resistor connected to +5 volts
acting as a pull up. A combination of resis-
tors and capacitors provide ESD protec-
tion. A qualified technician can use these
inputs for special applications. Contact the
factory for additional details.
Compressor Circuit
In this section some general information
about the Model 220’s compressor circuit
will be provided. As previously discussed,
the output of the microphone preamplifier
circuit is connected to a studio-quality
compressor circuit. The output of the
compressor is used by the talkback out-
puts and, if configured, the main output.
(In most cases the main output will utilize
the signal coming directly out of the micro-
phone preamplifier.) The gain element in
the compressor circuit is a laser-trimmed
voltage-controlled-amplifier integrated
circuit. It provides accurate, low-noise,
low-distortion performance. The threshold
of the compressor circuit is 2 dB above
the Model 220’s nominal internal operating
level of –2 dBu. A 5:1 compression ratio is
implemented and, like the threshold level,
is not field adjustable. The threshold and
ratio settings were selected so that excel-
lent talkback audio would be provided.
By controlling the dynamic range, intel-
ligibility can be improved and overloading
of connected devices can be avoided.
An LED indicator lights whenever the
compressor’s threshold has been reached
and the circuit is actively controlling the
dynamic range. This LED is provided as
an aid when setting the gain of the micro-
phone preamplifier.
Auxiliary Relay
The Model 220 provides an auxiliary
relay for use in specialized applications.
Some “head scratching” or “brainstorm-
ing” should lead to a number of interest-
ing ways to take advantage of this unique
resource. To implement any auxiliary relay
application does require the services of a
qualified technician. This is because the
Model 220’s enclosure must be disas-
sembled and the desired wiring scheme
implemented. The relay operates under
software control, following the configura-
tion selected using two of the DIP-type
switches. Four operating modes are avail-
able: relay disabled, relay follows main
output status, relay follows talkback output
1 button status, and relay follows talkback
output 2 button status. These choices
were previously discussed in this user
guide and should be reviewed.
The relay provides two “form-C” contacts,
each consisting of a common, normally
open (not shorted), and normally closed
(shorted) connection. Obviously the two
form-C contacts change state in unison;
two independent relay functions are not
provided. These relay contacts are
accessible on the Model 220’s main
printed circuit board assembly by way of
3-position header connectors. The con-
tacts are titled A and B, but there is no
significance between the two. The header
labeled P10 provides access to auxiliary
relay contact A. Header P9 provides ac-
cess to auxiliary relay contact B. With both