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Model 46 User Guide Issue 2, September 2007
Studio Technologies, Inc. Page 25
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 obtaining a
“deep” null requires that no extraneous
signals be present on the intercom circuit.
Advanced
Operation
The Model 46 allows several of the operat-
ing parameters to be configured to meet
the needs of specific applications. The
following paragraphs provide details about
the auto terminate disable and factory
test modes.
Auto Terminate Disable
As previously discussed in this user guide,
the auto terminate function can come into
play when a 2-wire interface is configured
for external power. Using a combination
of hardware and software, pins 2 and 3
of each 2-wire PL intercom connector are
monitored for the presence of DC voltage.
If a level of greater than approximately
18 volts DC is not detected on either pin,
200 ohm terminating networks are applied
to those same pins. This ensures that the
Model 46’s hybrid circuitry remains stable,
preventing objectionable audio signals
from being sent to the 4-wire output con-
nectors. 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 the LEDs will no longer
indicate the intercom circuit’s termination
status.
For special externally powered 2-wire ap-
plications the auto terminate function can
be disabled. This primarily will come into
play when “splitting” the Model 46’s re-
sources into four separate single-channel
interfaces. Moving advanced configuration
DIP switch 1, located on the Model 46’s
back panel, to its on (up) position disables
the auto terminate function. To someone
observing only the Model 46’s front panel
this change would not be readily appar-
ent. The front-panel LEDs will continue to
display the DC power status of pins 2 and
3. But whether or not either or both LEDs
are lit, the Model 46 will never apply 200
ohm terminations to pins 2 or 3. For the
hybrid circuits to remain stable termina-
tion impedances must be provided by the
connected circuits. If these are not pres-
ent one should expect the hybrid circuits
to generate a very impolite noise. This
condition, caused by the 2-wire output
circuitry being in an “unloaded” state, will
not damage the Model 46’s circuitry. But
errors in cable “patching,” a disconnected
cable, or other real-world issues, can lead
users listening to signals that originate in
the Model 46’s 4-wire outputs from being
presented with a rude surprise!
In conclusion, it’s important that technical
personnel working with the Model 46 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.