StudioTech Intercom System M45 00151 User Manual

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  
 
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Issue Preliminary 2, March 2009  
Page 4  
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  
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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  
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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  
ADAMMatrix 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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