Studio Technologies 45A - Manual (Page 23)

Model 45A User Guide Issue 2, November 2011
Studio Technologies, Inc. Page 23
changed vis-à-vis the intercom user de-
vices and wiring connected to a Model
45A’s 2-wire (PL) connectors. Even a slight
change to an intercom circuit, such as add-
ing or removing a section of cable, is suf-
ficient to require that the auto null process
be performed.
A pushbutton switch is provided to activate
the auto null process. Refer to Figure 7 for
a detailed view. To initiate auto null requires
simply pressing and releasing (“tapping”)
the button. The process begins by nulling
channel 1 of the interface and, when com-
pleted, moves on to channel 2. Two LEDs
provide a visual indication of the auto null
process, flashing when the auto null pro-
cess for its respective channel is active.
An auto null sequence begins with the mut-
ing of the 4-wire input and output signal
paths associated with the channel being
nulled. This is followed by a short period
of 24 kHz signal that is sent out the 2-wire
party-line intercom interface channel. This
will turn off microphones on those connect-
ed user devices that are compatible with
the RTS TW-series “mic kill” protocol. The
actual auto nulling process is performed
next. A series of tones will be sent out the
2-wire interface. Other Model 45A circuitry,
under software control, will rapidly perform
adjustments to achieve the best null pos-
sible. After the adjustments have been
made the results are stored in nonvolatile
memory. Once the process is complete
the 4-wire input and output paths are
again activated.
Configuration DIP switch 3 allows an
independent auto null button mode to
be selected. If switch 3 is in its on (up)
position, the front-panel pushbutton will
function in quite a different manner. In the
independent mode, a single “tap” to a
switch 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 inter-
com devices. The tones sent to the 2-wire
intercom 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 micro-
phones. 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.
Auto Terminate Disable
As previously discussed in this user guide,
the auto terminate function can come into
play when the 2-wire party-line interface
is configured to not supply power. Using
a combination of hardware and software,
Figure 7. Detail of front panel showing auto null
section