Studio Technologies 47A - Manual (Page 21)

Model 47A User Guide Issue 1, September 2014
Studio Technologies, Inc. Page 21
in excessively high audio signals being
sent to 2-wire party-line circuits. Setting
the nominal level for too high a value will
reduce the signal-to-noise performance.
And, as expected, it may also result in
audio signals being sent to the 2-wire
circuits at too low a level. Begin a review
by confirming that the two 4-wire level DIP
switches associated with each interface
are set as required.
If the Model 47A’s 4-wire nominal levels
are set correctly but the meters still reflect
sub-optimal levels, the issue may be relat-
ed to incorrect settings on the equipment
connected to the 4-wire inputs and out-
puts. It’s possible that although a connect-
ed 4-wire device’s nominal level matches
the Model 47A’s level setting, its actual
nominal level may be significantly differ-
ent. With a digital matrix intercom system
this problem could be due to an incorrect
configuration having been made to a spe-
cific channel or port. For example, the RTS
ADAM system has a published nominal
level of +8 dBu, but using its configura-
tion 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
intercom system so that its levels comply
with its own published nominal value. This
should, at least in theory, lead to the best
system performance, including correct
interfacing with the Model 47A. But often
it’s impossible to “clean up” the levels of
an existing system. In this case it may be
necessary to revise the Model 47A’s 4-wire
nominal level settings to match the reality
of the connected signals.
The TO 4-wire meters display the level
of signals that come from the 2-wire party-
line circuits 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.
Confirm that the DIP switch associated
with each interface is set to match the
nominal level of the connected devices.
RTS user devices have a nominal level
of –10 dBu; Clear-Com devices typically
have a –14 dBu nominal level.
It’s also possible that an error is present
on the 2-wire party-line circuit. Signal levels
associated with 2-wire party-line circuits
depend on a single termination being pres-
ent. 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” 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.
Connecting Party-Line Devices
The Model 47A’s two 2-channel interface
circuits allow two sets of user devices to
be connected. Depending on the setting
of the configuration DIP switches, either
an external intercom power source or the
Model 47A’s internal 2-wire power sources
have been designated for supporting the
connected user devices.
External Power Source
When set for an external 2-wire power
source, the Model 47A will not provide
power on pins 2 and 3 of the 2-wire con-
nectors, nor will the Model 47A use any