User Guide
Issue Preliminary 2, March 2009
This User Guide is applicable for serial numbers
M45-00151 and later
Copyright © 2009 by Studio Technologies, Inc., all rights reserved
50216-0309, Issue Preliminary 2
Table of Contents
Introduction................................................................... 5
Matrix Intercom Systems .............................................. 28
Appendix B—Interfacing with Riedel® Artist™
Appendix C—Interfacing with Clear-Com®
Model 45 User Guide
Studio Technologies, Inc.
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Model 45 User Guide
Studio Technologies, Inc.
The connected party-line intercom circuit
can also provide the Model 45’s operat-
ing power. For maximum flexibility the unit
is capable of supplying 30 volt DC power
and AC terminations, thus creating a
2-wire party-line intercom circuit. This
allows direct support for devices such
as intercom belt packs.
Introduction
The Model 45 is designed to interface
2-wire full-duplex party-line (PL) intercom
circuits with 4-wire audio circuits associ-
ated with matrix intercom systems. Appli-
cations for the Model 45 include television
sports and live-event broadcasting, theme
park and theater installations, corporate
AV, and industrial testing environments.
The unit provides one full-featured two-
channel 2-wire-to-4-wire interface. The
interface features hybrid circuitry that
includes automatic nulling capability. The
analog hybrids, under software control,
provide excellent audio quality and high
return-loss.
Configuration settings allow the Model
45’s performance to be optimized. Audio
level meters provide user confirmation
of system performance during setup and
operation. Standard audio connectors
are used for interfacing audio input, audio
output, party-line intercom, and DC power
signals. The Model 45 is housed in a com-
pact “half-rack” form factor that meets
the needs of broadcast “throw-down”
applications. Optional front panels allow
one or two Model 45 units to be mounted
in one space (“1U”) of a standard 19-inch
rack enclosure.
The Model 45 can be powered by an ex-
ternal source of 24 volts DC. The circuitry
is designed so that full isolation from the
connected party-line circuit is maintained.
Figure 1. Model 45 Front Panel
Figure 2. Model 45 Back Panel
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 5
mode the 30 volt, 300 milliampere maxi-
mum output can power devices such as
user stations and belt packs. In many
applications this will eliminate the need
for an external intercom power supply.
Besides reducing total system cost, this
feature can also lower system weight,
reduce required mounting space, and
decrease the mains energy requirement.
2-Wire Party-Line Interface
The Model 45’s two-channel 2-wire party-
line interface is optimized for direct connec-
tion with a dual-channel party-line intercom
circuit. In addition, single-channel party-
line intercom circuits can also be connect-
ed. Many broadcast applications use the
dual-channel TW-series from RTS®, which
includes their popular BP325 belt pack. The
Model 45’s 2-wire interface is configured for
a –10 dBu nominal level, exactly matching
the level of the TW-series. Other industry-
standard single- and dual-channel party-
line intercom systems, including those from
Clear-Com®, are also directly compatible.
While their nominal level is approximately
4 dB lower (–14 dBu) their greater dynamic
range tends to compensate for any opera-
tional differences.
With the 2-wire interface’s ability to supply
up to 300 milliamperes of current a typical
broadcast application which uses two or
three BP325 belt packs can easily be sup-
ported. The circuitry’s output regulation is
such that little change in the output volt-
age will occur over its entire rated output
current. The power source is an “industry-
standard” 30 volts DC, helping to ensure
that applications requiring long intercom
cable runs will function correctly. Also, the
design of the power supply circuitry helps
to minimize the noise and “hiss” often
associated with intercom power supplies.
Under software control the DC power
supply’s output is monitored for over-cur-
rent and short-circuit conditions. This
allows protection shut-down of the output
DC, as well as providing an alert by way of
a visual indicator.
The Model 45’s 2-wire party-line interface
is typically connected to a powered (“wet”)
unbalanced intercom circuit. Powered
party-line circuits have a DC voltage pres-
ent, typically provided by power supplies
from RTS or Clear-Com. This DC power,
normally 30-32 volts, provides energy for
connected devices such as user stations
or belt packs. A configuration setting is
used to select how the Model 45 will impact
the connected 2-wire circuit. In the default
mode the Model 45’s circuitry maintains
a high-impedance load and draws no
power from the party-line circuit. An alter-
nate mode allows the power required by
the Model 45’s circuitry to be supplied by
the connected party-line circuit.
An auto terminate function ensures that
should a party-line circuit not be connect-
ed, the Model 45’s interface circuitry will
remain stable. This unique feature makes
certain that objectionable audio signals,
such as oscillations and “squeals,” will
rarely be sent to the connected 4-wire
device.
A significant capability of the Model 45’s
2-wire interface is its ability to create a fully
functioning 2-wire party-line intercom cir-
cuit, supplying the required DC power and
200 ohm AC terminations. Referred to as
the Model 45’s 2-wire (PL) power source
Analog Hybrids with Auto
Nulling
A key reason that the Model 45 achieves
excellent audio performance is the design
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Model 45 User Guide
Studio Technologies, Inc.
of its 2-wire-to-4-wire hybrid circuits. Each
of the two independent circuits provides
low noise and distortion, good frequency
response, and high return-loss (“nulling”),
even when presented with a wide range
of 2-wire party-line conditions. Unlike tele-
phone-line (“POTS”) oriented DSP-based
hybrid circuits, the Model 45’s analog
circuitry provides extended frequency
response. With a pass band of 100 Hz
on the low end and 8 kHz on the high
end natural-sounding voice signals can
be sent to, and received from, the 2-wire
party-line circuit.
highlight that while the nulling process is
automatic, it only takes place upon user
request. The parameters obtained during
the nulling process are stored in nonvola-
tile memory; power interruptions won’t
require the auto nulling function to be
performed again.
The Model 45 generates a sine-wave
audio tone for use during the auto nulling
process. The signal’s frequency is soft-
ware-controlled to maximize the ability
of the hybrid circuits to reach a “deep”
null. In addition, at the beginning of each
auto nulling sequence a short period of
24 kHz tone is sent to the associated
channel of the 2-wire party-line interface.
This serves as a microphone disable (“mic
kill”) signal for user devices such as the
RTS BP325. By automatically disabling
“open” microphones the auto nulling
process can achieve a better result.
A hybrid’s ability to isolate the transmit
signal from the receive signal in the 2-wire-
to-4-wire interface is critical. The quality of
this isolation, technically known as return-
loss or trans-hybrid loss, is measured in
dB. A high value is important, especially
in applications where multiple 2-wire-
to-4-wire interfaces are used together.
Remote sports broadcast applications
are especially sensitive to this require-
ment. The Model 45’s sophisticated auto
nulling function uses analog circuitry
under microprocessor control to achieve
significant trans-hybrid loss. This return-
loss “null” is achieved by making a set of
adjustments to account for the resistive,
inductive, and capacitive conditions that
are present on the connected 2-wire party-
line circuit. The party-line’s conditions are
the sum of the impact made by the type
and quantity of cable, the connected user
devices, and the intercom power source.
4-Wire Interfaces
Associated with the 4-wire portion of
the Model 45’s dual-channel interface
are analog line-level inputs and outputs.
These are intended to interconnect with a
variety of 4-wire devices, including matrix
intercom systems, audio-over-fiber trans-
mission systems, and specialized audio
equipment. The input and output circuitry
is transformer-coupled to minimize the
chance of hum, noise, or ground “loop”
issues. The nominal input and output
levels are +4 dBu, helping to ensure
compatibility with professional audio
equipment. Some digital matrix intercom
systems use other nominal levels but with
their configuration flexibility they can be
easily adjusted to match the Model 45. For
example, the Riedel Artist® system has a
nominal level of +6 dBu so an adjustment
Whenever a user presses the Model 45’s
auto null pushbutton switch digital circuitry
adjusts the analog hybrids to achieve their
maximum return-loss. The nulling process
takes approximately 12 seconds for each
interface channel. And it’s important to
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 7
of only 2 dB is required. The RTS ADAM™
series of matrix intercom systems has a
nominal level of +8 dBu. This level also
applies to their RVON-I/O VoIP products.
As such, reducing their nominal input and
output levels by 4 dB will ensure optimal
compatibility.
to be essentially identical to that of the
unpowered (“dry”) channel. This unique
situation allows the automatic nulling
circuitry to provide excellent, consistent
results for both the powered and unpow-
ered channel.
Attention to detail is a hallmark of the
Model 45’s design. For example, during
the brief auto nulling process the interface
channel’s 4-wire input and output signals
are normally muted, preventing unwanted
audio from reaching the connected equip-
ment. Associated with the 2-wire party-line
interface is circuitry that, under software
control, applies 200 ohm terminating
impedances to the 2-wire party-line circuit.
This, along with other circuitry that moni-
tors DC voltages present on pins 2 and
3 of the 2-wire circuit, ensures that audio
instability associated with unterminated
circuits will rarely occur.
The Model 45 contains four 5-segment
LED level meters with two displaying the
level of the signals being received on the
4-wire inputs and two displaying the level
being sent out the 4-wire outputs. During
installation and setup the meters are in-
valuable in helping to confirm that proper
operation is taking place. The meters are
also useful during normal operation, allow-
ing confirmation of audio signal flow into
and out of the Model 45.
Pro Audio Quality
The Model 45’s audio circuitry was
designed in the spirit of professional
audio equipment, rather than that found
in typical party-line intercom gear. High-
performance components are used
Special Applications
While the Model 45 is designed to directly
integrate into typical applications, it’s
ready to support the “one-in-a-million”
situations too. To accomplish this five DIP
switches, accessible on the front panel,
allow control over some of the unit’s
features. For example, one DIP switch
changes the way the auto null pushbutton
switch operates. Normally pressing the
button begins the process of auto nulling
both channels. The alternate switch mode
allows each auto null sequence to be
activated independently. One “tap” of the
auto null button will begin the auto null
process for channel 1. Two “taps” of the
button will begin the auto null process
for channel 2. Another DIP switch allows
the auto terminate feature to be disabled.
While this feature can help maintain
throughout, providing low distortion, low
noise, and high headroom. Using passive
and active filters, the frequency response
is limited to nominally 100 Hz to 8 kHz.
This range was selected to provide excel-
lent performance for human speech, while
maximizing the ability of the hybrids to
create substantial “nulls.” When the Model
45’s internal DC power source is selected
to provide 2-wire party-line intercom pow-
er enhanced audio performance can also
be expected. The quality of the DC supply
circuit is very good, with very little noise,
hum, or “hiss” being added to the 2-wire
connection. In addition, the impedance
characteristics of the interface’s DC
powered (“wet”) channel was tailored
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Model 45 User Guide
Studio Technologies, Inc.
audio quality, advanced users might need
to disable it. This would allow full control
over the Model 45’s two hybrid circuits,
enabling them to be used in a completely
independent manner.
Design Philosophy
While the “bits and pieces” that make up
the Model 45 have been described in con-
ventional terms, the real strength of the unit
rests in the way it integrates and performs
in the “real world.” Studio Technologies
learned from conversations with indus-
try experts that installing and configuring
2-wire-to-4-wire interface equipment has
traditionally been a time-consuming, ag-
gravating process, requiring the efforts of an
expert to achieve reasonable results. And
even under those constraints the resulting
audio performance was often mediocre.
This “history lesson” made it clear that any
new design had to start with a unique set
of requirements. This led to an overriding
design goal: create a “new breed of cat,”
fundamentally changing how broadcast
2-wire-to-4-wire interface equipment fits into
actual applications.
Simple Installation
The Model 45 uses standard 3-pin XLR-
type connectors, allowing convenient 2-wire
party-line and 4-wire interconnection in
broadcast and general audio environments.
For flexibility, access to the 2-wire party-line
intercom interface can be made using
either a male or female XLR connector.
In many applications the Model 45 will be
powered by an external source of 24 volts
DC. A compact, lightweight 24 volt DC out-
put power source is supplied with each unit.
The power supply’s universal mains input
capability (100-230 volts, 50/60 Hz) allows
operation virtually anywhere in the world.
The Model 45 can also be powered by the
connected 2-wire party-line intercom circuit.
An important first step was to eliminate
the requirement that a senior technician,
along with a screwdriver, be present dur-
ing every installation. (It was universally
acknowledged that their time can be better
spent elsewhere!) The need to adjust trim
potentiometers, fabricate special cabling
and connector straps, use nulling ear-
pieces, etc. had to be eliminated. For ex-
ample, in virtually all instances, input and
output levels fall within just a few dB of their
nominal values and, as such, could be sup-
ported with one industry-standard nominal
audio level. In addition, it was acknowl-
edged that in this application analog audio
circuitry was capable of providing excellent
audio performance, but that the required
manual nulling process was operation-
ally taxing. By adding digital control to the
analog circuitry, automatic nulling could be
performed—the best of both worlds!
The four LED meters (previously mentioned)
make it simple to confirm operation of the
connected 4-wire inputs, 4-wire outputs,
and 2-wire party-line circuit. Additional LED
status indicators are also provided, offering
a clear view of the 2-wire DC power source,
auto null functions, and input operating
power.
The Model 45 is housed in a rugged,
lightweight aluminum enclosure that is
designed to be “road tough.” The half-rack
unit is ready for portable or stand-alone
“thrown-down” applications. Optional “1U”
front panels allow one or two Model 45
units to be mounted in one space of a
standard 19-inch rack enclosure.
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 9
The next step was to identify resources
that would improve the installation process
and make operation more reliable. This
led to the use of standard 3-pin XLR-type
audio connectors, enabling rapid installa-
tion and troubleshooting in any locale. The
inclusion of LED level meters allowed con-
tinuous monitoring of the input and output
signals. Additional status LEDs were also
deemed to be valuable. Configuration DIP
switches would allow crucial operating
modes to be selected.
getting it ready for use in portable appli-
cations requires only making a few inter-
connections. These include 4-wire audio
input, 4-wire audio output, and 2-wire
party-line intercom connections that use
3-pin XLR-type connectors. In many ap-
plications an external source of 24 volts
DC will be connected using the supplied
power adapter.
For permanent installations one or two
Model 45 units can be mounted in one
of the optional rack panel adapters. The
resulting one-rack-space unit will be
mounted in an equipment rack.
A subtle but critical requirement was the
need for the Model 45 to receive its oper-
ating power from either an external 24 volt
DC source or the connected 2-wire party-
line intercom circuit. It also turned out that
in many applications only a small number
of user devices, such as belt packs, were
typically connected to a 2-wire party-line
circuit. So by adding a DC power source
to the Model 45’s 2-wire interface the need
for an external intercom power supply
could often be eliminated.
Shipping Carton Contents
The shipping carton contains a Model
45 Interface, a universal input/24 volt DC
output power supply, one set of self-stick
protective bumpers, and a user guide. A
North-American standard mains power
cord will be provided for use with the
power supply. If necessary, the dealer or
distributor will provide an AC mains cord
that is appropriate for your location. If a
rack-mount front panel is going to be used
as part of the installation it will typically be
shipped in a separate carton.
The final step was to create a physical
package that would provide significant
resources in a format that allowed simple
and reliable integration with other equip-
ment. This was accomplished by speci-
fying a convenient half-rack form factor
which would be excellent for “thrown-
down” use. And by creating two “1U”
rack-mount options, it would be possible
to install one or two Model 45 units in one
space of a 19-inch rack.
Locating the Model 45
The location of the Model 45 will dictate
the length of the cable runs needed to link
the unit with the other required intercom
devices. This factor really only relates to
the 2-wire party-line intercom circuit.
These circuits carry unbalanced audio
which can be susceptible to interference
and crosstalk issues. And since party-line
intercom circuits typically carry DC power
a voltage drop due to resistive loss can
become an issue. In general, minimizing
the length of the 2-wire party-line intercom
Installation
In this section you will be preparing the
Model 45 for use. Since the standard unit
is housed in a self-contained enclosure,
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Model 45 User Guide
Studio Technologies, Inc.
cables will help to ensure more reliable and
consistent intercom system performance.
The differential (“balanced”) 4-wire circuits
are typically not impacted by the length of
their interconnecting cables. A final location
criterion is to provide access to the Model
45’s front panel. An optimal location will
allow convenient use of the auto null push-
button and easy observation of the level
meters and status LEDs.
front panel, begin the process by remov-
ing the five screws that hold the standard
“throw-down” front panel to the Model
45’s chassis. Using a Phillips screw driver,
remove the 4-40 flat-head Phillips-head
machine screw which is located in approx-
imately the center of the front panel. Then
use a 5/64-inch hex wrench to remove
the four 4-40 button-head hex machine
screws. Save the screws and carefully
protect and store the standard front panel
for possible later use. Using the screws
that were just removed, attach the rack-
adapter front panel to the Model 45’s
chassis. To prevent damage extreme care
is required when aligning the front panel
with the LEDs, DIP switch assembly, and
auto null pushbutton switch that protrude
through the Model 45’s chassis. Tighten
the screws only after careful inspection,
ensuring that all protrude through the front
panel without interference. The center
(Phillips-head) screw should be tightened
first, then tighten the remaining four.
Protecting the Enclosure
The Model 45 is shipped as a self-contained
unit suitable for portable use. Included with
the unit are self-stick “bump on” protec-
tors. These may be applied to the bottom
corners of the unit if it is going to be placed
on surfaces where scratching of either the
Model 45 or the surface could take place.
Rack Mounting the Model 45
For permanent or mobile applications it’s
possible to mount one or two Model 45
units into one space of a 19-inch rack
enclosure. Two rack-mount front panels,
purchased separately, are available from
Studio Technologies. Refer to Figure 3 for
details.
The dual-unit rack-mount panel follows
the same procedure but applies to two
Model 45 units. Store both of the removed
“thrown-down” front panels for possible
later use. Note that on the front-panel
graphics one unit is designated as A while
the other B. This is provided so that each
can be easily identified during installation,
troubleshooting, and operation.
The following provides details on how
to install and use the panels. To attach a
Model 45 unit to a single-unit rack-mount
Figure 3. Model 45 shown in optional single-unit and dual-unit 19-inch rack-mount panels
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 11
Once the desired one or two Model 45
units have been installed in a rack-mount
front panel, the assembly can be mounted
into the designated equipment rack. One
space (“1U” or 1.75 vertical inches) in
a standard 19-inch equipment rack is
required. Secure the unit into the equip-
ment rack using two mounting screws
per side.
and has an impedance of 13 k ohms. The
line inputs are optimized for signals that
have a nominal level of +4 dBu.
Audio sources are connected to the line
inputs by way of 3-pin female XLR-type
connectors which are located on the
Model 45’s back panel. Refer to Figure 4
for a detailed view.
Prepare the mating connectors (males)
so that XLR pin 2 is signal high (+ or hot),
pin 3 is low (– or cold), and pin 1 is shield.
If connecting a source in this manner
results in hum or noise it’s possible that
removing the shield connection from
pin 1 can eliminate the issue; “floating”
pin 1 will remove a ground current path
from the Model 45’s chassis ground point
through the shield of a cable. (Pin 1 on
each of the two input connectors is inter-
nally connected to the Model 45’s earth/
chassis ground point.) Also, if a hum or
noise issue does arise be certain to con-
firm that, unless absolutely necessary, the
mating connector’s “shell” isn’t connected
to the cables’ shield or pin 1. Termination
of this “fourth” pin of a 3-pin XLR connec-
tor is often the cause of seemingly inexpli-
cable noise issues.
4-Wire Audio Inputs and
Outputs
Two audio line input and two audio line
output signals are associated with the
4-wire interface section of the Model 45.
Connections are made using standard
3-pin male and female XLR connectors.
Refer to Appendices A, B, and C for de-
tails on interconnecting with RTS, Riedel,
and Clear-Com matrix intercom systems.
4-Wire Line Inputs
As previously mentioned, the Model 45’s
4-wire interface allows two analog line-
level audio sources to be connected. The
source for these signals will typically be
ports on a matrix intercom system. It’s
also possible that the signals will come
from other devices, such as a fiber optic
or copper-based audio transmission sys-
tem. The 4-wire input circuitry is balanced,
capacitor coupled, transformer isolated,
With an unbalanced source connect XLR
pin 2 to signal high (+ or hot) and both
pins 1 and 3 to shield. If connecting an
Figure 4. Detail of back panel showing line inputs and outputs
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Model 45 User Guide
Studio Technologies, Inc.
unbalanced source in this manner results
in hum or noise, connect XLR pin 2 to high
(+ or hot) and pin 3 to shield; leave pin 1
unterminated.
45’s 2-wire party-line interface can also
serve as an intercom power source and
200 ohm impedance generator, allowing
intercom user devices to be directly con-
nected. The Model 45’s internal 30 volt
DC intercom power source is limited to
300 milliamperes of current. This moder-
ate amount of power requires that the type
and number of connected user devices be
selected appropriately.
4-Wire Line Outputs
The Model 45’s dual-channel interface pro-
vides two analog line-level audio outputs.
These outputs are intended to be connect-
ed to inputs on the devices associated with
the 4-wire audio signals. The outputs are
capacitor coupled, transformer balanced
with a nominal level of +4 dBu. The 4-wire
line outputs are capable of driving inputs
that have impedances as low as 600 ohms,
however connecting to loads of 2 k ohms
or greater is preferred. The line outputs are
connected by way of 3-pin male XLR-type
connectors which are located on the Model
45’s back panel. Refer to Figure 4 for a
detailed view.
For convenience, the 2-wire party-line
intercom circuit or user devices can be
connected to the Model 45 by way of
either a male or female 3-pin XLR-type
connector, both of which are located on
the back panel. Refer to Figure 4 for a
detailed view. The two connectors are
wired in parallel (“multed”) and provide
access to the identical signals.
Dual-Channel Intercom Systems
The mating connectors (females) should
be prepared so that signal high (+ or hot)
is expected on XLR pin 2. Signal low (– or
cold) should be expected on XLR pin 3.
The cables’ shields can be connected to
XLR pin 1. However, in order to minimize
the chance that ground-interaction prob-
lems will arise, pin 1 on each of the line
output connectors is isolated from all Mod-
el 45 circuitry and ground points. “Floating”
pin 1 virtually eliminates the chance that a
“ground loop” problem will occur.
If compatibility with RTS TW-series is
desired the mating connector (one male
or one female) should be wired so that
common is on pin 1, DC with channel 1
audio is on pin 2, and channel 2 audio
is on pin 3. This wiring scheme applies
whether the connected devices are to be
powered by an external source or by the
Model 45’s internal 2-wire intercom power
source.
Single-Channel Intercom Systems
There are two ways of connecting to the
Model 45’s 2-wire (PL) intercom connec-
tors when compatibility with Clear-Com
single-channel intercom devices is de-
sired. The most direct method is to pre-
pare a male or female 3-pin XLR mating
connector so that common is on pin 1,
power is on pin 2, and audio is connected
on pin 3. With this connection scenario
only audio channel 2, associated with
2-Wire Party-Line Intercom
Connections
The Model 45’s 2-wire party-line intercom
interface is designed to connect with a
standard single- and dual-channel party-
line intercom circuit. This type of circuit
typically has positive 28 to 32 volts DC on
pin 2 and common on pin 1. The Model
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 13
pin 3 of the Model 45’s 2-wire party-line
intercom interface, will be utilized. Pin 2,
the Model 45’s audio channel 1, will only
be used for connecting DC power to the
connected devices. While the audio resourc-
es provided by channel 1 will not be used,
the Model 45 can still be used to supply DC
power on pin 2. As previously mentioned,
the maximum current draw on the 30 volt
DC output is 300 milliamperes. Alternatively,
an external intercom power supply can
support the connected devices.
Figure 5. Adapter cable wiring diagram
must be set to its off (down) position. In
addition switch 4, auto terminate disable,
must be placed in its on (up) position.
Refer to the Configuration section of this
user guide for details.
In some single-channel 2-wire party-line
intercom system applications it may be
desirable to take full advantage of the two
channels associated with the Model 45’s in-
terface. The Model 45 can certainly be used
in this fashion, but an adapter will need to
be prepared. The recommended connection
method is to create an adapter that “splits”
Model 45’s 2-wire (PL) intercom connector
into two 3-pin XLR connectors, one for each
audio channel. The adapter should use one
female and two male connectors. Pin 1 of a
female 3-pin XLR intended to mate with the
Model 45 will connect to pin 1 of both 3-pin
male XLR connectors. Pin 2 of the female
XLR will go to pin 3 of the male XLR desig-
nated as channel 1. Pin 3 of the female XLR
will go to pin 3 of the male XLR designated
as channel 2. Refer to Figure 5 for details.
Model 45 Power Source
The Model 45 requires power for operation
of its internal circuitry. This can be provided
by an external source of 24 volts DC or
by way of the connected 2-wire party-line
circuit. As you might expect, if the Model
45 is to serve as the 2-wire party-line power
source an external source of 24 volts DC
must be connected.
When a 24 volt DC power source is
connected it always takes priority. And,
depending on how the Model 45 is con-
figured, the Model 45 can automatically
switch from using the external 24 volt
source to using power from the 2-wire
party-line circuit. While this action can
occur, it is not designed as a power-fail
back-up function. A “seamless” switch
over from using the external 24 volt DC
input to the 2-wire party-line circuit is not
guaranteed. The Model 45 might have a
momentary power outage (approximately
one second) before again going through
the start-up process. Only after this process
has completed will operation again
commence.
Using this adapter the Model 45 can be
directly interconnected with two Clear-Com
intercom circuits. However, power for the
connected devices must be provided by ex-
ternal power sources. The Model 45’s ability
to supply intercom power will not be utilized.
Note: It’s critical that the correct configura-
tion settings be made when using the Model
45’s interfaces to support two independent
intercom circuits. Specifically, the 2-wire
power source configuration DIP switch 2
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Model 45 User Guide
Studio Technologies, Inc.
External 24 Volts as the Power Source
configuration DIP switch 1 must be in its
on (up) position. Please refer to the Con-
figuration section of this user guide for
additional details.
As previously discussed, the Model 45 can
be powered by a source of 24 volts DC. A
“locking” 2.1 x 5.5 mm coaxial power jack
is located on the back panel of the unit.
The center pin of the jack is the positive (+)
connection. While the requirement for the
external source is nominally 24 volts, correct
operation will take place over a 20 to 28 volt
range. The Model 45 requires 750 milliam-
peres for correct operation. (Approximately
500 milliamperes of this current is used for
the 2-wire party-line intercom power source
feature.)
Before blithely going ahead and powering
the Model 45 with a party-line circuit, it’s
important to review the possible pitfalls of
this arrangement. First of all, compared
to most intercom belt packs the Model 45
is a power “hog,” requiring the aforemen-
tioned 175 milliamperes of DC current for
operation. The other requirement is that
the supplied voltage must be 24 volts or
greater as measured directly at the Model
45’s 2-wire (PL) connectors. Whether the
connected party-line intercom circuit can
provide this much energy will depend on
the characteristics of a specific installation.
The relevant factors include the capacity
of the intercom power supply, the number
and type of connected user devices, and
the length and type of the interconnecting
cables.
Included with each Model 45 is a universal
mains input/24 volt DC output power supply.
The power supply’s DC output cable has
been terminated with a Switchcraft® S760K
coaxial power plug. This “locking” type of
plug correctly mates with the Model’s 45’s
24V DC IN jack. The locking feature is im-
portant, allowing the external power source
to be securely attached to the Model 45.
The capacity of the intercom power sup-
ply must meet or exceed the needs of the
connected user devices, including the
Model 45. It’s a good idea to perform a
calculation of the total maximum current
draw prior to attempting to install and use
the unit. As a real-time confirmation often
these power supplies have status indica-
tors that can provide a direct indication of
their ability to supply the connected loads.
A detachable mains cord is used with the
power supply’s 3-pin IEC 320 C14-type
inlet connector. Included with each Model
45 is a mains cord with a North-American
(NEMA 15L) standard plug on one end
and an IEC 320 C13-type connector on the
other. Units intended for use in other desti-
nations require that the appropriate cord set
be obtained from the dealer or distributor.
Using the Connected 2-Wire Party-Line
Circuit as the Model 45’s Power Source
In many cases the length and type
(gauge) of the interconnecting cable will
be the limiting factor as to whether the
intercom circuit can provide adequate
power for a Model 45. This is because the
intercom circuit’s DC voltage will drop in
direct relation to the length of the intercon-
necting cable. And with the Model 45’s
relatively large current draw this voltage
The connected 2-wire party-line intercom
circuit can be used to provide the Model
45’s operating power. The acceptable volt-
age range is 24 to 32 volts DC, with a re-
quired current draw of 175 milliamperes.
Note that for the Model 45 to be powered
by the connected 2-wire intercom circuit
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 15
drop can become appreciable in situations
where belt pack operation wouldn’t nor-
mally be impacted. Performing accurate
calculations in this scenario is a bit more
difficult but might be required to ensure a
reliable installation.
Using the Connected 2-Wire
Party-Line Circuit as the
Model 45’s Power Source
Configuration DIP switch 1 is used to
select whether or not the connected 2-wire
intercom circuit can be used to provide
power for the Model 45. When the switch
is in its off (down) position the intercom
circuit cannot be used to power the Model
45. This ensures that no current will be
drawn from the intercom circuit, some-
thing that can be important if the circuit is
limited in available current, or if the length
of the interconnecting cable is excessive.
When switch 1 is in its on (up) position the
2-wire party-line circuit can serve as the
power source for the Model 45.
Initial Operation
As soon as a source of power is applied
the Model 45 will begin its power-up
sequence. As a “boot-up” indication the
input power, status, and meter LEDs will
light, one at a time, in a “walk-through”
sequence. After the sequence has com-
pleted, one of the input power LEDs will
light indicating which power source is
actively powering the unit. The Model 45
is now fully functional.
It’s important to remember that if an ex-
ternal source of 24 volts DC is connected
it will always be used as the Model 45’s
power source. Even if switch 1 is in its
on (up) position, power will be drawn
from the connected intercom circuit only
if the external source of 24 volts DC is
not present. With that in mind, it’s highly
recommended that unless the application
has been carefully designed, any time an
Configuration
To support the needs of various applica-
tions the Model 45 allows a number of
operating parameters to be configured. A
5-position DIP switch assembly, located
on the front panel, is used to establish the
desired configuration. Refer to Figure 6 for
detailed views.
Figure 6. Details of front panel showing the five configuration DIP
switches and the configuration switches chart
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Model 45 User Guide
Studio Technologies, Inc.
external source of 24 volts is going to be
connected switch 1 should be placed in
its off (down) position. This ensures that a
failure of the connected 24 volt DC source
won’t cause the Model 45 to automatically
revert to pulling power from the intercom
circuit.
in its off (down) position the Model 45 will
not serve as an intercom power source.
This setting is appropriate when an exter-
nal intercom power supply is providing
power and terminations to the connected
2-wire intercom user devices. When switch
2 is in its on (up) position the Model 45
can provide the intercom power source,
with the requirement that an external
source of 24 volts DC is connected to the
Model 45.
Some additional details might be helpful.
Be certain to allow only the connected
intercom circuit to supply power (switch 1
on) if it’s fully able to do so. Trying to draw
current from a circuit that’s not capable
of supplying the needs of the Model 45
will typically result in greatly degraded
intercom audio quality. This is because
the Model 45’s power supply circuit will
always attempt to draw the current it
requires. If it’s able to do so a constant
high-impedance load will be applied to the
circuit and everything will perform correct-
ly. But if the party-line intercom circuit isn’t
capable of providing the required voltage
and current the Model 45’s circuitry will
become unstable and negatively impact
the intercom circuit’s audio quality. Howls,
squeaks, and squeals are the expected
failure mode!No damage will be done
but intercom performance will be greatly
compromised.
Auto Null Button Operation
A pushbutton switch, located on the
Model 45’s front panel, is used to activate
the auto nulling functions. A configuration
parameter allows selection of the button’s
operation. The choices are dual auto null
mode and independent auto null mode.
When dual auto null mode is selected
a single “tap” (press and release) on the
pushbutton switch initiates a routine
that begins with channel 1’s auto null
sequence, followed immediately by
channel 2’s auto null sequence.
The independent auto null mode allows
the auto null function to be initiated for
each channel as desired. A single “tap”
will start the auto null routine for channel
1; two “taps” will start the routine for
channel 2.
In conclusion, the Model 45 can function
very well using the intercom circuit as its
power source. But this requires that the
circuit can meet the voltage and current
requirements.
When configuration DIP switch 3 is in
its off (down) position the dual auto null
mode is selected. This is provided spe-
cifically for cases where the Model 45’s
2-wire party-line interface will be used with
dual-channel intercom user devices. As an
example: the RTS TW-series provides two
independent audio channels, as well as
power, over a single 3-conductor cable.
In a case such as this it’s useful for both
of the hybrid circuits associated with an
Model 45 Provides 2-Wire
Party-Line Power Source
Configuration DIP switch 2 is used to se-
lect whether or not the Model 45 will serve
as an intercom power source, providing
DC power and 200 ohm terminations to
the intercom interface. When switch 2 is
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 17
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.
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.
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.
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.
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
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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Model 45 User Guide
Studio Technologies, Inc.
the Model 45’s 2-wire-to-4-wire converter
circuitry. These audio artifacts will not
cause any damage but will be sent out of
the 4-wire line outputs. Users of the 4-wire
equipment may be less than pleased with
what they hear!
completion, the applicable input power
LED will light. The null settings for the two
hybrid circuits are recalled and used, hav-
ing been previously stored in nonvolatile
memory.
Significant user intervention is typically not
required during normal Model 45 opera-
tion. The unit is designed for continuous
operation with no routine adjustment or
maintenance necessary. Assuming that
the desired configuration has been select-
ed using the DIP switches, only occasion-
ally activating the auto null function should
be necessary. The latter may be warranted
should connected user devices or wiring
associated with the 2-wire party-line inter-
com circuit be changed.
4-Wire Mute during Auto Null
As part of the auto nulling process audio
tones are generated and sent out the
2-wire intercom interfaces. These tones,
depending on the nulled state of the
hybrid circuits, will also appear on the
outputs of the 4-wire interface circuits. To
prevent these unwanted tones from being
sent to the inputs of the connected 4-wire
devices by default the 4-wire outputs mute
when the auto nulling process is active.
External 24 Volt DC Power
Source
There may be cases, primarily at the fac-
tory, where it’s desirable to have the null-
ing tones appear on the 4-wire outputs.
Configuration DIP switch 5 allows the mut-
ing of the 4-wire outputs to be disabled.
In many applications an external source
of 24 volts DC will be connected to the
Model 45. Typically this will be in the form
of the external power supply provided
with the unit by Studio Technologies. The
24VDC input power status LED will light
whenever the external source is providing
power for the unit’s circuitry. In this sce-
nario the Model 45 can be configured as
to whether or not it acts as a power source
for the 2-wire party-line interface circuit.
When switch 5 is in its off (down) posi-
tion the Model 45 operates in its standard
fashion. When switch 5 is in its on (up)
position the 4-wire outputs will not mute
during the auto nulling process. As ex-
pected, when the Model 45 is deployed
in the field switch 5 should remain in its
off (down) position. However, no damage
to the Model 45 or connected equipment
will occur when muting during auto null is
disabled.
When the Model 45 is not Serving as
the 2-Wire Intercom Power Source
When an external source of 24 volts DC is
connected and the Model 45 is configured
to not provide 2-wire party-line intercom
power (configuration DIP switch 2 set to
off), the unit will not provide power to, nor
take power from, pin 2 of the 2-wire (PL)
connectors. The 4-wire inputs and out-
puts, as well as the internal power supply,
Operation
Upon power-up the Model 45 will always
go through a short initialization sequence
before normal operation will begin. The
LEDs will light sequentially and, upon
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 19
are fully isolated from the 2-wire intercom
circuit. In this situation the Model 45’s only
function is to route, by way of the hybrids,
the 2-wire interface’s audio signals to and
from the 4-wire outputs and inputs. In this
mode of operation the Model 45 acts as a
typical user station on the connected inter-
com circuit.
When the Model 45 is Serving as the
2-Wire Intercom Power Source
When configuration DIP switch 2 is set
so that the Model 45 provides 2-wire (PL)
power its interface supplies 30 volts DC
on pin 2 of the 2-wire (PL) intercom con-
nectors. A maximum current draw of 300
milliamperes is available. This current is
sufficient to power various intercom user
devices such as small user stations and
belt packs. A common broadcast appli-
cation would be to use RTS BP325 belt
packs. Select the connected devices so
that their total current doesn’t exceed 300
milliamperes. That’s not always the easi-
est figure to calculate but a web search
will generally find specifications for all
commonly used devices. For example, a
search finds that the original version of
the BP325 consumes a maximum of 85
milliamperes of current. According to this
figure up to three of these units can be
connected to each Model 45 Interface.
Newer versions of the BP325 use surface
mount component technology and have
a lower maximum current draw of 65 mil-
liamperes. Four of these “modern” BP325
units can easily be supported.
As a connection confirmation, hardware
and software in the Model 45 monitors pins
2 and 3 of the 2-wire (PL) intercom connec-
tors for the presence of DC voltage. A level
greater than approximately 18 volts DC on
pin 2 will cause the status LED labeled
PIN 2 to light. A level greater than 18 volts
DC on pin 3 will cause the status LED
labeled PIN 3 to light.
The author is aware that user intercom
devices almost always draw power from
pin 2 rather than pin 3. However, in many
broadcast applications power is provided
on all intercom paths so that flexible chan-
nel assignments can be made. In the ex-
ternal 2-wire power mode the Model 45
is simply an observer; whether intercom
power is present on pin 2, pin 3, or both
pins 2 and 3 is not significant.
In order that the Model 45’s 2-wire-to-4-wire
interface circuits remain stable and don’t
generate audio artifacts an auto terminate
feature is implemented. This maintains a
200 ohm termination on both pins 2 and
3 as long as neither of the 2-wire power
status LEDs is lit. To clarify, if either or both
the LEDs are lit then both terminations are
removed. It was felt that this method would
provide a fairly accurate indication that an
active intercom circuit is connected. In this
case the 200 ohm termination is expected
to be provided elsewhere, typically as part
of the external power supply, and the Mod-
el 45’s termination is “lifted.”
The Model 45’s intercom power supply
circuit operates under software control.
This allows detection of fault conditions
and protection of both the Model 45’s
circuitry and connected intercom user
devices. Upon initial Model 45 2-wire inter-
com power up no monitoring of the inter-
com power output takes place for a period
of five seconds. This allows the Model
45’s circuitry and the connected intercom
user devices to stabilize. The LED associ-
ated with pin 2 will light to indicate that the
output is active. After this initial 5-second
period monitoring becomes active. A fault
condition is detected if the power on
Issue Preliminary 2, March 2009
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Model 45 User Guide
Studio Technologies, Inc.
pin 2 falls below 24 volts for a continuous
1- second interval. The hardware and
software responds to this condition by
turning off the power source to pin 2 and
flashing the pin 2 LED as a warning. After
a 5-second “cool-down” interval the output
returns to the same condition as upon
initial power up. Power is again applied
to pin 2, the pin 2 status LED will light, and
monitoring won’t begin for another five sec-
onds. A full short-circuit condition applied
to the Model 45’s 2-wire connectors will
result in a continuous cycle of six seconds
on (five seconds for start up and one sec-
ond for detection) and five seconds off.
as a support aid during installation, con-
figuration, operation, and troubleshooting.
The meters represent the strength of the
signals coming in from, and going out to,
the 4-wire connections. The meters are or-
ganized in two groups each representing
one input and one output. They are cali-
brated to reflect the level in dB relative to
+4 dBu. This is similar to the way in which
now-legacy VU meters functioned. As an
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.
As a diagnostic aid the 2-wire (PL) power
status LED associated with pin 3 always
remains active, even when the Model 45
is supplying power to the 2-wire party-line
intercom circuit. Whenever DC in excess
of approximately 18 volts is present on
pin 3 the LED will light. This condition will
typically not exist but could prove useful
in special circumstances.
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.
2-Wire Circuit Serves as the
Unit’s Power Source
If no external source of 24 volts DC is
connected and configuration DIP switch 1
is in its on (up) position, the connected
2-wire party-line circuit can provide power
for the Model 45. If this condition is active
the 2-wire (PL) input power LED will light.
The 2-wire (PL) power status LED will also
be lit, indicating that power in excess of
18 volts is detected on pin 2. The pin 3
LED will also light if the same condition
is present on pin 3.
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 incor-
rect settings on the equipment connected
to the 4-wire inputs and outputs. It’s pos-
sible that although a connected 4-wire
Level Meters
The Model 45 contains four 5-segment LED
level meters. These meters are provided
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 21
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.
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.
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.
Figure 7. Detail of front panel showing auto null
section
Issue Preliminary 2, March 2009
Page 22
Model 45 User Guide
Studio Technologies, Inc.
An auto null sequence begins with the
muting of the 4-wire input and output
signal paths associated with the channel
being nulled. This is followed by a short
period of 24 kHz signal that is sent out
the 2-wire party-line intercom interface
channel. This will turn off microphones
on those connected user devices that
are compatible with the RTS TW-series
“mic kill” protocol. The actual auto null-
ing process is performed next. A series of
tones will be sent out the 2-wire interface.
Other Model 45 circuitry, under software
control, will rapidly perform adjustments
to achieve the best null possible. After the
adjustments have been made the results
are stored in nonvolatile memory. Once
the process is complete the 4-wire input
and output paths are again activated.
to many user devices it may not apply to
all. Muting microphones is significant as
obtaining a “deep” null requires that no
extraneous signals be present on the inter-
com circuit.
Auto Terminate Disable
As previously discussed in this user guide,
the auto terminate function can come into
play when the 2-wire party-line interface
is configured to not supply power. Using
a combination of hardware and software,
pins 2 and 3 of the 2-wire (PL) intercom
connector are monitored for the presence
of DC voltage. If a level of greater than ap-
proximately 18 volts DC is not detected on
either pin, 200 ohm terminating networks
are applied to those same pins. This en-
sures that the Model 45’s hybrid circuitry
remains stable, preventing objectionable
audio signals from being sent to the 4-wire
output connectors. One caveat does ap-
ply: there may be a few seconds of severe
noise whenever an interface moves from a
DC present condition to a DC not present
condition. But other than that period, the
audio paths will remain “clean.”
Configuration DIP switch 3 allows an
independent auto null button mode to
be selected. If switch 3 is in its on (up)
position, the front-panel pushbutton will
function in quite a different manner. In the
independent mode, a single “tap” to a
switch will cause channel 1 to auto null.
Two “taps” will cause channel 2 to auto
null. By observing the operation of the two
auto null status LEDs it will become readily
apparent which of the button modes has
been selected.
As a visual aid, LEDs on the front panel
will display the DC power status of pins
2 and 3. But when auto terminate disable
mode is active (configuration DIP switch 4
set to on) the LEDs will no longer indicate
the intercom circuit’s termination status.
If possible, prior to performing an auto null
it’s polite to warn all personnel who are ac-
tively using the connected intercom devic-
es. The tones sent to the 2-wire intercom
circuit during the nulling process are not
excessively loud or obnoxious, but most
users might want to remove their headsets
during the process. In addition to warning
users, it might be a good time to ask them
to mute any active microphones. While
the automatic “mic kill” signal will apply
For special applications the auto terminate
function can be disabled. This will primar-
ily come into play when “splitting” the
Model 45’s resources into two separate
single-channel interfaces. Moving con-
figuration DIP switch 4 to its on (up) posi-
tion disables the auto terminate function.
To someone observing only the Model
45’s front panel this change would not be
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 23
readily apparent. The front-panel LEDs will
continue to display the DC power status of
pins 2 and 3. But whether or not either or
both LEDs are lit, the Model 45 will never
apply 200 ohm terminations to pins 2 or
3. For the hybrid circuits to remain stable
termination impedances must be provided
by the connected circuits. If these are not
present one might find the hybrid circuits
generating very unpleasant audible noise.
This condition, caused by the 2-wire out-
put circuitry being in an “unloaded” state,
will not damage the Model 45’s circuitry.
However, without the auto terminate func-
tion a disconnected cable or other real-
world wiring problem could present users
with a rude surprise!
is interesting to “hear” the nulling process
take place. But unless there’s a really good
reason, configuration DIP switch 5 should
always remain in its off (down) position.
Technical Notes
Software Version Display
A special Model 45 power-up sequence
allows the unit’s software version number
to be displayed. This is useful when work-
ing with factory personnel on application
support and troubleshooting situations. The
five “from” 4-wire LEDs are used to display
the major release number with a range of
1 through 5. The five LEDs associated with
“to” 4-wire channel 1 are used to display
the release sub-number which ranges from
1 through 5. Refer to Figure 8 for a detailed
view of the LEDs and the corresponding
software version numbering scheme. The
Model 45’s initial software release is ver-
sion 1.1 which is represented by the bot-
tom LED of each column being lit.
In conclusion, it’s important that technical
personnel working with the Model 45 be
informed when the auto termination func-
tion has been disabled. They will then be
aware of the potential noise issues and be
ready to make corrections should a prob-
lem arise.
4-Wire Mute During Auto Null
To display the Model 45’s software version
is very simple. From the powered-down
state, press and hold the auto null button.
While continuing to press the auto null but-
ton, apply power either by connecting an
external 24 volt DC power source or a pow-
ered intercom circuit. The normal power-up
sequence will not occur but instead one
LED in the status column will be lit and one
LED in the column associated with “from”
4-wire channel 1 will also be lit. As previ-
ously described, these two LEDs represent
the unit’s current software version. After the
software version number has been “read”
the auto null button can be released. At this
time the unit will begin its normal power-up
sequence.
Normally, the 4-wire outputs are muted
whenever the auto null process is taking
place. This ensures that the test tones
generated as part of the nulling process
will not be heard by users of the signals
associated with the 4-wire outputs. For
special applications configuration DIP
switch 5 allows the automatic muting
mode to be disabled. This mode is pro-
vided primarily for use by factory person-
nel or during demonstration use. During
normal operation switch 5 should remain
in its off (down) position. When switch 5 is
in its on (up) position 4-wire muting dur-
ing auto null will not take place. While not
appropriate during actual Model 45 use, it
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Model 45 User Guide
Studio Technologies, Inc.
Major Release Number
Release Sub-Number
O 4
O 3
O 2
1
.4 O
.3
.2 O
.1 O
Figure 8. Detail of front panel showing the status LEDs that display
the software version. In this example, the software version is 1.3.
Note that while it’s easy to determine
Two Units can be a TW-12B
Replacement
which software version is loaded into
the Model 45 a trip back to the factory is
required to update it. The 8-bit micro-
controller that provides the unit’s logic
“horsepower” also includes internal
Two Model 45 units with their 4-wire in-
terface connectors linked in a crossover
fashion can act as a universal 2-wire-to-
2-wire “bridge.” This should, in theory,
create an updated and improved version
of the venerable Clear-Com TW-12B unit.
In this arrangement two independent
2-wire party-line intercom systems can
function as one while still maintaining full
electrical isolation.
FLASH memory. This nonvolatile memory
is used to store the operating software
(“firmware”). Re-programming this mem-
ory requires using a specialized program-
ming unit. While not outrageous in price, it
still costs in the range of US$500. The pro-
grammer uses a ribbon cable and socket
to interface with a 6-pin “header” on the
Model 45’s printed circuit board. And, as
you would guess, once connected repro-
gramming takes only a matter of seconds.
But unfortunately the programmer is not
something that would be found in a typical
“field shop” or repair facility.
The connection scheme is very simple,
using just four male-female 3-pin XLR
patch cords. The 4-wire outputs of the
first Model 45 would be connected to the
4-wire inputs on the second Model 45; the
4-wire outputs on the second Model 45
would connect to the 4-wire inputs on the
first Model 45. A 2-wire party-line intercom
circuit would connect to each Model 45
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 25
unit. Power can be provided by the con-
nected intercom circuits or by way of an
external source of 24 volts DC. The final
step to achieve correct performance is for
the auto null function on each Model 45 to
be activated. Refer to Figure 9 for details.
Figure 9. Two Model 45 units connected as a universal 2-wire-to-2-wire bridge
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Model 45 User Guide
Studio Technologies, Inc.
Meters: 4
Specifications
Function: displays level of 4-wire inputs and
outputs
General Audio:
Type: 5-segment LED, modified VU ballistics
Frequency Response: ±2.5 dB 100 Hz to 8 kHz
Connectors:
Distortion (THD+N): <0.2%, measured at 1 kHz,
4-wire input to 2-wire interface pin 2
4-Wire Line Inputs: 3-pin female XLR-type
4-Wire Line Outputs: 3-pin male XLR-type
Signal-to-Noise Ratio: >55 dB, measured at
1 kHz, 4-wire input to 2-wire interface pin 2
2-Wire (PL) Interface: 3-pin male and female
XLR-type
2-Wire Party-Line (PL) Intercom Interface:
External 24 Volt DC Input: coaxial power jack,
2.1 x 5.5 mm, locking bushing, compatible with
Switchcraft® S760K plug
Type: two-channel party-line, unbalanced
(pin 1 common; pin 2 DC with channel 1 audio;
pin 3 channel 2 audio)
Compatibility: single- and dual-channel intercom
systems such as from RTS® and Clear-Com®
Power Requirements:
External: 24 volts DC nominal, acceptable range
20 to 28, maximum required current 750 milliam-
peres @ 20 volts. Universal mains input/24 volt DC
power supply shipped with each unit.
Impedance – Normal: >10 k ohms
Impedance – 2-Wire (PL) Power Source Mode:
200 ohms
2-Wire Party-Line (when serving as source):
Nominal Level: –10 dBu
24 to 32 volts DC, 175 milliamperes
“Mic Kill” Signal: square wave, 24 kHz, ±1%
Dimensions – Portable “Throw-Down” Version
(Overall):
2-Wire Power Source: 30 volts DC nominal, 300
milliamperes maximum
8.7 inches wide (22.1 cm)
1.72 inches high (4.4 cm)
8.3 inches deep (21.1 cm)
Hybrids: 2
Topology: 3-section analog circuitry compensates
for resistive, inductive, and capacitive 2-wire party-
line loads
Mounting Options: single- or dual-unit rack-mount
front panel; uses one space in a standard 19-inch
rack
Nulling Method: automatic upon user initiation,
processor implements digital control of analog
circuitry; null settings stored in nonvolatile memory
Weight: 1.8 pounds (0.82 kg), rack-mount front
panel adds 0.2 pounds (0.09 kg)
Nulling Line Impedance Range: 120 to 350 ohms
Nulling Cable Length Range: 0 to 3500 feet,
typical
Specifications and information contained in this
User Guide subject to change without notice.
Trans-Hybrid Loss: >40 dB, typical at 800 Hz
4-Wire Inputs: 2
Type: transformer-coupled, capacitor isolated
Impedance: 13 k ohms
Nominal Level: +4 dBu
Maximum Level: +22 dBu
4-Wire Outputs: 2
Type: transformer-coupled, capacitor isolated
Impedance: 50 ohms nominal
Nominal Level: +4 dBu
Maximum Level: +20 dBu into 2 k ohms
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 27
Appendix A
Interfacing RTS® Matrix Intercom Systems with the Model 45 Interface
ADAM™ Matrix Intercom System Analog Ports to
Model 45 Interface
RVON-I/O I/O Connections to Model 45 Interface
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Model 45 User Guide
Studio Technologies, Inc.
Appendix B
Interfacing Riedel® Artist™ Matrix Intercom System Analog Ports with the
Model 45 Interface
Model 45 User Guide
Studio Technologies, Inc.
Issue Preliminary 2, March 2009
Page 29
Appendix C
Interfacing Clear-Com® Matrix Intercom System Analog Ports with the
Model 45 Interface
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