Manual

Model 47A User Guide Issue 2, January 2020
Studio Technologies, Inc. Page 25
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 47A 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 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 applied
to those same pins. This ensures that
the Model 47A’s hybrid circuitry remains
stable, preventing objectionable audio sig-
nals 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 con-
dition. 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
applications the auto terminate function
can be disabled. This primarily will come
into play when “splitting” the Model 47A’s
resources into four separate single-chan-
nel interfaces. Moving advanced configu-
ration DIP switch 1, located on the Model
47A’s back panel, to its on (up) position
disables the auto terminate function. To
someone observing only the Model 47A’s
front panel this change would not be read-
ily apparent. 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 47A will never
apply 200 ohm terminations to pins 2 or
3. For the hybrid circuits to remain stable
termination impedances must be provided
by the connected circuits. If these are
not present one should expect the hybrid
circuits to generate a very impolite noise.
This condition, caused by the 2-wire out-
put circuitry being in an “unloaded” state,
will not damage the Model 47A’s circuitry.
But errors in cable “patching,” a discon-
nected cable, or other real-world issues
can lead listeners of the Model 47A’s 4-
wire outputs from being presented with a
rude surprise!
In conclusion, it’s important that technical
personnel working with the Model 47A
be informed when the auto termination
function has been disabled. They will then
be aware of the potential noise issues and
be ready to make corrections should a
problem arise.
Factory Test
Back-panel DIP switch 4 allows a factory
test mode to be enabled. During normal