Studio Technologies 47 - Manual (Page 25)

Model 47 User Guide Issue 1, August 2010
Studio Technologies, Inc. Page 25
selected. If DIP switch 2 is in its on (up)
position, the front-panel pushbuttons will
function in quite a different manner. In the
independent mode, a single tap to a but-
ton will cause channel 1 to auto null. Two
taps will cause channel 2 to auto null. By
observing the operation of the two auto
null status LEDs it will become readily
apparent which of the button modes has
been selected.
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 47 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 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 47’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
applications the auto terminate function
can be disabled. This primarily will come
into play when “splitting” the Model 47’s
resources into four separate single-chan-
nel interfaces. Moving advanced configu-
ration DIP switch 1, located on the Model
47’s back panel, to its on (up) position
disables the auto terminate function. To
someone observing only the Model 47’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 47 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