Manual

Issue 3, August 2015 Model 45A User Guide
Page 24 Studio Technologies, Inc.
to mute any active microphones. While
the automatic “mic kill” signal will apply to
many user devices it may not apply to all.
Muting microphones is significant as ob-
taining a “deep” null requires that no extra-
neous signals be present on the intercom
circuit.
Auto Terminate Disable
As previously discussed in this user guide,
the auto terminate function can come into
play when the 2-wire party-line interface
is configured to not supply power. Using
a combination of hardware and software,
pins 2 and 3 of the 2-wire (PL) intercom
connector are monitored for the presence
of DC voltage. If a level of greater than
approximately 20 volts DC is not detected
on either pin, 200 ohm terminating net-
works are applied to those same pins. This
ensures that the Model 45A’s hybrid circuit-
ry remains stable, preventing objectionable
audio signals from being sent to the 4-wire
output connectors. One caveat does
apply: there may be a few seconds of
severe noise whenever an interface moves
from a DC present condition to a DC not
present condition. But other than that
period, the audio paths will remain “clean.”
As a visual aid, LEDs on the front panel will
display the DC power status of pins 2 and
3. But when auto terminate disable mode is
active (configuration DIP switch 4 set to on)
the LEDs will no longer indicate the inter-
com circuit’s termination status.
For special applications the auto terminate
function can be disabled. This will primarily
come into play when “splitting” the Model
45A’s resources into two separate single-
channel interfaces. Moving configuration
DIP switch 4 to its on (up) position disables
the auto terminate function. To someone
observing only the Model 45A’s front panel
this change would not be readily apparent.
The front-panel LEDs will continue to dis-
play the DC power status of pins 2 and 3.
But whether or not either or both LEDs are
lit, the Model 45A will never apply 200 ohm
terminations to pins 2 or 3. For the hybrid
circuits to remain stable termination imped-
ances must be provided by the connected
circuits. If these are not present one might
find the hybrid circuits generating very
unpleasant audible noise. This condition,
caused by the 2-wire output circuitry be-
ing in an “unloaded” state, will not damage
the Model 45A’s circuitry. However, without
the auto terminate function a disconnected
cable or other real-world wiring problem
could present users with a rude surprise!
In conclusion, it’s important that technical
personnel working with the Model 45A be
informed when the auto termination func-
tion has been disabled. They will then be
aware of the potential noise issues and be
ready to make corrections should a prob-
lem arise.
4-Wire Mute During Auto Null
Normally, the 4-wire outputs are muted
whenever the auto null process is taking
place. This ensures that the test tones gen-
erated as part of the nulling process will not
be heard by users of the signals associated
with the 4-wire outputs. For special appli-
cations configuration DIP switch 5 allows
the automatic muting mode to be disabled.
This mode is provided primarily for use by
factory personnel or during demonstra-
tion use. During normal operation switch
5 should remain in its off (down) position.
When switch 5 is in its on (up) position
4-wire muting during auto null will not take
place. While not appropriate during actual
Model 45A use, it is interesting to “hear”