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.
Figure 8. Detail of front panel showing auto null section
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
