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Issue 2, September 2007 Model 46 User Guide
Page 24 Studio Technologies, Inc.
Auto Null
Each of the Model 46’s dual-channel inter-
faces has circuitry to automatically null the
two 2-wire-to-4-wire interfaces. Normally
this process is performed at the time of
initial Model 46 configuration but there’s
no reason why “auto nulling” can’t be initi-
ated anytime one desires. The only time
that auto null must be performed is if con-
ditions have changed vis-à-vis the inter-
com user devices and wiring connected to
a Model 46 2-wire PL interface connector.
Even a slight change to an intercom cir-
cuit, such as adding or removing a section
of cable, is sufficient to require that the
auto null process be performed.
Two buttons, one associated with each
interface, are provided to activate the
auto null process. Refer to Figure 8 for a
detailed view. To initiate auto null simply
requires pressing and releasing (“tap-
ping”) a button. The process begins by
nulling channel one of an interface and,
when completed, moves on to channel
two. Two LEDs provide a visual indication
of the auto null process, flashing when the
auto null process for its respective chan-
nel is active.
An actual auto null sequence starts by
muting the 4-wire input and output signal
paths associated with the specific channel
to be nulled. Then a short period of 24 kHz
signal is sent out the 2-wire PL intercom
interface channel. This will turn off micro-
phones on those connected user devices
that are compatible with the RTS TW-
series “mic kill” protocol. The actual auto
nulling process will next be performed. A
series of tones will be sent out the 2-wire
interface. Other Model 46 circuitry, un-
der software control, will rapidly perform
adjustments to achieve the best null pos-
sible. After the adjustments are made the
results are stored in non-volatile memory.
Once the process has completed the
4-wire input and output paths are again
activated.
Advanced configuration DIP switch 2,
located on the back panel, allows an
independent auto null button mode to
be selected. If 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
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 to mute any
Figure 8. Detail of front panel showing auto null
section