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