Issue Preliminary 2, March 2009
Model 45 User Guide
Page 18
Studio Technologies, Inc.
interface to be auto nulled at approxi-
mately the same time. The dual auto null
mode allows a single “tap” of the button to
initiate nulling of both hybrid circuits.
When configuration DIP switch 3 is in its
on (up) position the independent auto null
mode is selected. This is appropriate for
applications in which the two audio paths
associated with the Model 45’s intercom
interface are used with independent party-
line intercom circuits. This situation might
arise when two Clear-Com single-chan-
nel intercom circuits are connected to the
Model 45. The hybrid circuit associated
with each channel can be auto nulled as
desired. Another example would be in
an RTS TW-series application that uses
source assignment panels. The “SAP”
panels would be used to route multiple
intercom channels to various sets of user
devices as desired. In this case, the two
channels associated with the Model 45’s
interface will often end up not routed to
the same user device. Independent auto
nulling is certainly desired.
Auto Terminate Disable
The auto terminate function is designed
to ensure that each 2-wire-to-4-wire in-
terface circuit remains stable under most
operating conditions. It can only be active
when the Model 45 is not serving as the
intercom power source. The auto termi-
nate function applies 200 ohm imped-
ances to both pins 2 and 3 of the 2-wire
party-line interface when no source of
DC power is detected on either of those
pins. This function is directly compatible
with RTS TW-series intercom applications
in which the three conductors of a cable
support both DC power and two channels
of audio. It’s also compatible in situations
where all three conductors of a Clear-Com
single-channel intercom circuit are con-
nected to the Model 45. In this latter case
only one of the Model 45’s audio channels
will be used.
There may be situations where it’s neces-
sary for the two channels associated with
the Model 45’s interface to be used with
separate 2-wire party-line circuits. An ex-
ample of this might be where two “loops”
of single-channel belt packs, such as units
from Clear-Com, need to be connected. In
this case the intercom circuit’s DC power
will not be connected to the Model 45, but
only to the user devices. In this scenario,
a DC voltage won’t be present on pin 2
or pin 3. Another example of this might
be where adapter boxes with DC blocking
are used to interconnect intercom circuits
with the Model 45. In both these cases a
DC voltage won’t be present on either
pin 2 or 3 and the Model 45 will automati-
cally apply 200 ohm terminations. These
terminations would be in error, resulting
in incorrect audio levels due to a double-
termination condition.
To prevent this problem the auto terminate
function can be disabled. When configura-
tion DIP switch 4 is in its off (down) posi-
tion the auto terminate function is active.
When switch 4 is in its on (up) position the
auto terminate function associated with
each interface is disabled.
The auto terminate disable function should
be used only when absolutely necessary;
it’s possible that a significant downside
could be experienced. With auto terminate
disabled it’s important that properly termi-
nated 2-wire party-line intercom circuits be
connected to pins 2 or 3 of the Model 45’s
2-wire (PL) interface. If they are not con-
nected, it’s likely that audio oscillations,
noise, and distortion will be generated in
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