Manual

Model 45A User Guide Issue 3, August 2015
Studio Technologies, Inc. Page 23
on the same circuit, will cause a “double-
termination” condition. This leads to a
nominal 100 ohm line impedance which
will result in a level drop of at least 6 dB.
Removing the unwanted termination is
really the only valid means of correcting
the problem.
Auto Null
The Model 45A contains circuitry to auto-
matically null the two 2-wire-to-4-wire inter-
faces. Normally this process is performed
at the time of initial Model 45A configuration
but there’s no reason why “auto nulling”
can’t be initiated anytime one desires. The
only time that auto null must be performed
is if conditions have changed vis-à-vis the
intercom user devices and wiring connected
to a Model 45A’s 2-wire (PL) connectors.
Even a slight change to an intercom circuit,
such as adding or removing a section of
cable, is sufficient to require that the auto
null process be performed.
A pushbutton switch is provided to activate
the auto null process. Refer to Figure 11
for a detailed view. To initiate auto null re-
quires simply pressing and releasing (“tap-
ping”) the button. The process begins by
nulling channel 1 of the interface and, when
completed, moves on to channel 2. Two
LEDs provide a visual indication of the auto
null process, flashing when the auto null
process 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 possi-
ble. After the adjustments have been made
the results are stored in nonvolatile memo-
ry. Once the process is complete the 4-wire
input and output paths are again activated.
Configuration DIP switch 3 allows an inde-
pendent auto null button mode to be se-
lected. 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 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
Figure 11. Detail of front panel showing auto
null section