Issue 1, September 2014
Model 46A User Guide
Page 24
Studio Technologies, Inc.
during the process. In addition to warning
users, it might be a good time to ask them
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
obtaining a “deep” null requires that no
extraneous signals be present on the inter-
com circuit.
Advanced
Operation
The Model 46A allows several of the oper-
ating 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
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 ap-
plied to those same pins. This ensures
that the Model 46A’s hybrid circuitry re-
mains stable, preventing objectionable
audio signals from being sent to the 4-wire
output connectors. One caveat does ap-
ply; 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 is not
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 46A’s re-
sources into four separate single-channel
interfaces. Moving advanced configuration
DIP switch 1, located on the Model 46A’s
back panel, to its on (up) position disables
the auto terminate function. To someone
observing only the Model 46A’s front pan-
el 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 46A 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 46A’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 46A’s 4-wire outputs from being
presented with a rude surprise!
In conclusion, it’s important that technical
personnel working with the Model 46A 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.
