StudioTech M45-00151 manual Auto Null, Detail of front panel showing auto null section

device’s nominal level matches the Model 45’s level setting its actual nominal level may be significantly different. With a digital matrix intercom system this problem could be due to an incorrect configuration hav- ing been made to a specific channel or port. For example, the RTS ADAM system has a published nominal level of +8 dBu, but using its configuration software it’s possible to set panels or ports to a level different than nominal. The best solution in this case is to adjust the associated port on the intercom system so that its levels comply with +4 dBu. This should, at least in theory, lead to the best system perfor- mance, including correct interfacing with the Model 45. But often it’s impossible to “clean up” the levels of an existing system.

The “to” 4-wire meters display the level of signals that come from the 2-wire party- line circuit and go out the 4-wire inter- faces. An issue may arise if the signals coming from the connected party-line user devices aren’t at a sufficient level so that a normal meter display can be reached. This could be the result of an incorrectly configured 2-wire nominal audio level.

It’s also possible that an error is pres- ent on the 2-wire party-line circuit. Signal levels associated with 2-wire party-line circuits depend on a single termination being present. This termination, typically 200 ohms, is almost always made at the power supply source. But it’s possible that another device, such as a second active power supply on the same circuit, will cause a “double-termination” condi- tion. 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 45 contains circuitry to auto- matically null the two 2-wire-to-4-wire interfaces. Normally this process is per- formed at the time of initial Model 45 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 45’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 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 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.

Figure 7. Detail of front panel showing auto null section

Issue Preliminary 2, March 2009	Model 45 User Guide
Page 22	Studio Technologies, Inc.