Studio Technologies 45A - Manual (Page 22)

Issue 2, November 2011 Model 45A User Guide
Page 22 Studio Technologies, Inc.
example let’s take the situation where an
output (“to 4-wire”) LED labeled 0 dB is lit.
(For this example, the level on the 2-wire
party-line circuit is just large enough to
light the 0 LED.) This indicates that a sig-
nal with an approximate level of +4 dBu
is present on the associated 4-wire output
connector.
Each level meter contains four green LEDs
and one yellow LED. The four green LEDs
indicate signal levels at or below +4 dBu.
The top LED is yellow and indicates a sig-
nal that is 6 dB or greater than the +4 dBu
nominal level. An audio signal that causes
the yellow LED to light doesn’t necessarily
indicate an excessive level condition, but it
does provide a warning that at some stage
reducing the signal level may be prudent.
Typical operation with normal signal levels
should find the meters lighting near their
0 dB point. Signal peaks may cause the
yellow LEDs to flash. But a yellow LED that
lights fully during normal operation will
typically indicate a signal level or configu-
ration problem.
If the meters consistently display lower
or higher levels than their 0 dB points it’s
possible that a configuration issue exists.
This would typically be related to incorrect
settings on the equipment connected to
the 4-wire inputs and outputs. It’s possible
that although a connected 4-wire device’s
nominal level matches the Model 45A’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 45A. 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 45A 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 45A
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