Studio Technologies 47A - Manual (Page 24)

Issue 1, September 2014 Model 47A User Guide
Page 24 Studio Technologies, Inc.
If possible, prior to performing an auto
null it’s polite to warn all personnel who
are actively using the connected intercom
devices. The tones sent to the 2-wire inter-
com circuit during the nulling process
are not excessively loud or obnoxious,
but most users might want to remove
their headsets 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 intercom 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 ap-
plied to those same pins. This ensures
that the Model 47A’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 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-channel
interfaces. Moving advanced configuration
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 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 47A 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 present
one should expect the hybrid circuits to
generate a very impolite noise. This condi-
tion, caused by the 2-wire output circuitry
being in an “unloaded” state, will not dam-
age the Model 47A’s circuitry. But errors in
cable “patching,” a disconnected 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