StudioTech Network Router M46 00180 User Manual

User Guide  
Issue 2, September 2007  
This User Guide is applicable for serial numbers:  
M46-00180 and later  
and units upgraded with software version 1.3 and higher  
Copyright © 2007 by Studio Technologies, Inc., all rights reserved  
50110-0907, Issue 2  
 
This page intentionally left blank.  
Issue 2, September 2007  
Page 4  
Model 46 User Guide  
Studio Technologies, Inc.  
 
The Model 46 mounts in one space  
of a standard 19-inch rack enclosure  
and requires 100-240 volts, 50/60 Hz for  
operation.  
Introduction  
The Model 46 is designed to interface  
2-wire full-duplex party-line intercom cir-  
cuits with 4-wire audio circuits associated  
with matrix intercom systems. Other spe-  
cialized audio system interfacing applica-  
tions can also be supported. The Model  
46 provides two independent full-featured  
2-channel interfaces. Each interface con-  
tains two hybrid circuits which include  
automatic nulling capability. The analog  
circuitry, under software control, provides  
excellent audio quality and high return-  
loss. The interfaces are compatible with  
powered and unpowered 2-wire party-line  
circuits. They are capable of supplying DC  
power, allowing direct operation of devices  
such as user belt packs. Configuration set-  
tings allow the 2-wire and 4-wire interface  
circuitry to be compatible with a range  
of nominal signal levels. Audio level me-  
ters provide user confirmation of system  
performance during setup and operation.  
Applications for the Model 46 include tele-  
vision sports and special event broadcast-  
ing, theme park and theater installations,  
corporate events, and industrial testing  
environments. Standard audio connectors  
are used for all input and output signals.  
2-Wire Interfaces  
The Model 46’s 2-wire interfaces are  
optimized for direct connection with dual-  
channel party-line (PL) intercom circuits.  
In addition, single-channel party-line  
intercom circuits can also be connected.  
Many broadcast applications use the dual-  
channel TW-series from Telex®/RTS®,  
including their popular BP325 belt pack.  
Other industry-standard single- and dual-  
channel PL intercom systems, including  
those from Clear-Com®, are also directly  
compatible. To provide optimal signal  
matching and interface performance the  
nominal level of the Model 46’s 2-wire  
interfaces can be selected. Using DIP  
switches, accessible on the front panel,  
the nominal levels can be selected to be  
either –10 or –14 dBu, settings that should  
allow compatibility with virtually all party-  
line systems. For example, for applications  
that use equipment from RTS the –10 dBu  
setting is appropriate. For other equip-  
ment, such as that from Clear-Com, the  
–14 setting is correct.  
Model 46 Front Panel  
Model 46 Back Panel  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 5  
 
The Model 46’s 2-wire interfaces can  
correctly function with powered (“wet”)  
or unpowered (“dry”) intercom circuits.  
Powered circuits have a DC voltage pres-  
ent, typically provided by power supplies  
such as the RTS PS31 or Clear-Com  
PS-232. This DC power, normally 30-32  
volts, provides energy for connected  
devices such as user stations or belt  
packs. In this type of application the  
Model 46 is configured to operate in its  
external 2-wire power source mode. In this  
mode the Model 46’s circuitry maintains  
the required high-impedance load and,  
as in all cases, draws no power from the  
party-line circuit. An auto-terminate func-  
tion ensures that should a “wet” circuit not  
be connected, the Model 46’s interface  
circuitry will remain stable. This unique  
feature makes certain that objectionable  
audio signals, including oscillations and  
“squeals,” will infrequently be sent to the  
connected 4-wire device.  
intercom power source ensures that sys-  
tems requiring long intercom cable runs  
will function correctly. The circuitry’s out-  
put regulation is such that little change in  
the output voltage will occur over its entire  
rated output current. Also, the unique de-  
sign of the power supply circuitry virtually  
eliminates the noise and “hiss” associated  
with typical intercom power supply perfor-  
mance. Under software control the output  
of the DC power supplies are monitored  
for over-current and short-circuit condi-  
tions. This allows protection shut-down  
of the output DC, as well as providing an  
alert by way of a visual indicator.  
Analog Hybrids with Auto  
Nulling  
A key reason that the Model 46’s inter-  
faces achieve excellent audio performance  
is the design of the 2-wire-to-4-wire hybrid  
circuits. They provide low noise and dis-  
tortion, good frequency response, and  
high return-loss (“nulling”), even when  
presented with a wide range of 2-wire  
conditions. Unlike telephone-line (“POTS”)  
oriented DSP-based hybrid circuits, the  
Model 46’s analog circuitry provides ex-  
tended 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 circuits.  
A significant capability of the Model 46’s  
2-wire interfaces is their ability to supply  
DC power and 200 ohm AC termination  
to the connected intercom circuits. Re-  
ferred to as the Model 46’s internal 2-wire  
power source mode, the 30 volt output  
can power devices such as user stations  
and belt packs. In many applications this  
will eliminate the need for external inter-  
com power supplies. Besides reducing  
total system cost, this feature can also  
lower system weight, reduce required  
mounting space, and decrease the mains  
energy requirement.  
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 mea-  
sured in dB. A high value is important,  
especially in applications where multiple  
2-wire-to-4-wire interfaces are used togeth-  
er. Remote sports broadcast applications  
With each 2-wire interface’s ability to sup-  
ply up to 300 milliamperes of current, a  
typical broadcast application that uses two  
sets of three BP-325 belt packs can easily  
be supported. Each interface’s 30 volt DC  
Issue 2, September 2007  
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Model 46 User Guide  
Studio Technologies, Inc.  
 
are especially sensitive to this requirement.  
The Model 46’s sophisticated auto nulling  
function uses analog circuitry under mi-  
croprocessor control to achieve significant  
trans-hybrid loss. This return-loss “null” is  
achieved by making a series 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 46’s interfaces are analog line-level  
inputs and outputs. These are intended  
to interconnect with a variety of 4-wire de-  
vices, including matrix intercom systems,  
audio-over-fiber transmission systems,  
and other specialized audio equipment.  
The input and output circuitry is transform-  
er-coupled to minimize the chance of  
hum, noise, or ground “loop” issues.  
A key characteristic of the Model 46’s de-  
sign is the ability to select the 4-wire input  
and output nominal levels. This helps to  
ensure compatibility with virtually all audio  
equipment. Front-panel-accessible DIP  
switches allow the nominal levels to be  
configured from among four choices: 0,  
+4, +6, or +8 dBu. The 0 dBu setting was  
specifically provided for compatibility with  
Clear-Com matrix intercom systems. The  
+4 dBu setting allows “standard” audio  
signals to be directly connected. Digital  
matrix intercom systems from Riedel® can  
be effectively connected using the +6 dBu  
setting. And the +8 dBu setting allows  
proper level matching with the popular  
ADAM™ series of matrix intercom systems  
from Telex/RTS. This setting also applies  
to the related RVON-I/O VoIP product.  
Whenever a user presses one of the Model  
46’s “auto null” buttons digital circuitry ad-  
justs the analog hybrids to rapidly achieve  
their maximum return-loss. The nulling pro-  
cess takes less than 10 seconds for each  
channel of an interface. However, it’s impor-  
tant to note that while the nulling process  
is automatic, it only takes place upon user  
request. This can lead to more stable and  
consistent audio performance when a hy-  
brid is exposed to the varying 2-wire condi-  
tions often found in broadcast applications.  
The parameters obtained during the nulling  
process are stored in non-volatile memory;  
mains power interruptions won’t require the  
auto nulling function to again be performed.  
A sine-wave audio tone is generated for  
use during the auto nulling process. The  
frequency is software-controlled to maxi-  
mize the ability of the hybrid circuits to  
reach a “deep” null. In addition, at the  
beginning of each auto null sequence a  
short period of 24 kHz tone is sent to the  
associated 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 better performance.  
The Model 46 contains eight 5-segment  
LED level meters. Four of the meters  
are provided for each interface, with two  
displaying the level of the signal being  
received from the 4-wire source and  
two displaying the level being sent to  
the 4-wire output. During installation and  
setup the meters are invaluable in help-  
ing to confirm that the nominal level DIP  
switch settings have been properly made.  
During normal operation the meters offer  
rapid confirmation of audio signal flow in  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 7  
 
and out of the unit. Additional LED status  
indicators are also provided, offering a clear  
view of the 2-wire DC power and auto null  
functions.  
circuits. This, along with other circuitry that  
monitors DC voltages present on the  
2-wire circuits, ensures that audio instabil-  
ity associated with unterminated circuits  
will rarely occur. As previously mentioned  
the Model 46’s 2-wire DC power sources  
offer a unique level of performance. Their  
ability to deliver power while maintaining  
audio quality is simply unmatched.  
Pro Audio Quality  
The Model 46’s audio circuitry was de-  
signed in the spirit of professional audio  
equipment, rather than that found in typical  
party-line intercom gear. High-performance  
components are used throughout, providing  
low-distortion, low-noise, and high head-  
room. Using passive and active filters, the  
frequency response is limited to approxi-  
mately 100 Hz to 8 kHz. This range was se-  
lected to provide excellent performance for  
human speech, while maximizing the ability  
of the hybrids to create substantial “nulls.”  
When the Model 46’s internal DC power  
sources are selected to provide 2-wire  
party-line intercom power, enhanced audio  
performance can also be expected. The  
quality of the DC supply circuits is excellent,  
with very little noise, hum, or “hiss” being  
added to the 2-wire connections. In addi-  
tion, the impedance characteristics of each  
interface’s DC powered (“wet”) channels  
was tailored to be essentially identical to  
that of the unpowered (“dry”) channel. This  
unique situation allows the automatic null-  
ing circuitry to provide excellent, consistent  
results for both the powered and unpow-  
ered channels.  
Special Applications  
While the Model 46 is designed to directly  
integrate into typical applications, it’s  
ready to support the “one-in-a-million”  
situations too. To accomplish this DIP  
switches, accessible on the back panel,  
allow some of the automatic features to be  
disabled. For example, one switch allows  
the auto-terminate feature to be disabled.  
While this feature can help maintain audio  
quality, advanced users might need to dis-  
able it. This would allow full control over  
the Model 46’s four hybrid circuits, en-  
abling them to be used completely inde-  
pendently. A second DIP switch changes  
the way in which the front-panel auto null  
push button switches operate. Normally  
pressing one of the buttons begins the  
process of auto nulling both channels  
associated with an interface. The alter-  
nate 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 interface one.  
Two taps of the button will begin the auto  
null process for channel two. For installer-  
selected applications, three opto-coupled  
inputs allow selected Model 46 features  
to be remotely controlled. These features  
include remote auto nulling for each inter-  
face and remote “mic kill” activation.  
Attention to detail is a hallmark of the Model  
46’s design. For example, during the brief  
auto nulling process an interface channel’s  
4-wire input and output signal is muted,  
preventing unwanted audio from reaching  
the connected equipment. Associated with  
the 2-wire interfaces is circuitry that, under  
software control, applies 200 ohm termi-  
nating impedances to the 2-wire party-line  
Issue 2, September 2007  
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Model 46 User Guide  
Studio Technologies, Inc.  
 
apparent that installing and configuring  
existing 2-wire-to-4-wire interface units  
was invariably a time-consuming, aggravat-  
ing process that required the talent of an  
expert to achieve reasonable results. And  
even under those constraints the resulting  
audio performance was often mediocre.  
Any new design had to look at the applica-  
tion in a different way. This led to the over-  
riding Model 46 design goal: create a “new  
breed of cat,” fundamentally changing how  
broadcast 2-wire-to-4-wire interface equip-  
ment fit into actual applications.  
Simple Installation  
The Model 46 uses standard 3-pin XLR-  
type connectors to allow convenient inter-  
connection in most broadcast and general  
audio environments. For flexibility, access  
to the 2-wire party-line intercom interfaces  
can be made using the connectors pro-  
vided on both the front and back panels.  
In permanent installations the back-panel  
connectors will typically be utilized. In  
other settings, such as field television pro-  
duction, front-panel-access allows inter-  
com belt packs to be rapidly connected,  
without requiring access to the inside of a  
rack enclosure or necessitating the use of  
a secondary input/output (I/O) panel.  
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 earpiec-  
es, etc. had to be eliminated. For example,  
in virtually all instances, input and output  
levels fall within just a few dB of their nomi-  
nal values and, as such, could be support-  
ed with a limited number of configurable  
choices. In addition, it was acknowledged  
that in this application analog audio cir-  
cuitry 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 couldbe  
performed—the best of both worlds!  
The Model 46 is housed in a rugged steel  
enclosure that is designed to be “road  
tough.” It mounts in one space of a stan-  
dard 19-inch rack enclosure. The mains  
input source can range from 100 to 240  
volts, 50/60 Hz allowing direct operation  
virtually anywhere in the world. A switch-  
mode power supply, contained within the  
Model 46’s enclosure, provides conver-  
sion of the mains input power into the DC  
voltage required by the intercom power  
supply circuits. Also contained within the  
unit is a DC-to-DC converter that creates  
the voltages required by the analog and  
digital circuitry. These efficient power sup-  
plies help to ensure cool, reliable opera-  
tion in a variety of settings.  
The next step was to identify resources that  
would improve the installation process and  
make operation more reliable. This led to  
the inclusion of multiple LED level meters,  
allowing continuous monitoring of the input  
and output signals. It also turned out that  
in many applications only a small number  
of user devices, such as belt packs, are  
Design Philosophy  
While the “bits and pieces” that make  
up the Model 46 have been described in  
conventional terms, the real strength of  
the unit rests in how it integrates and per-  
forms in the “real world.” Before beginning  
the Model 46’s design process, conversa-  
tions with industry experts quickly made it  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 9  
 
typically connected to 2-wire party-line  
circuits. By adding DC power sources to  
the Model 46’s 2-wire interfaces, the need  
for external intercom power supplies could  
often be eliminated.  
link the unit with the connected devices.  
This is really only a factor in relation to the  
2-wire party-line intercom circuits. These  
circuits carry unbalanced audio which  
can be susceptible to interference and  
crosstalk issues. Intercom circuits also  
carry DC power which may make resistive  
losses come into the picture. In general,  
shorter intercom cable lengths will as-  
sure more reliable and consistent system  
performance. Another location criterion is  
access to the Model 46’s front panel. An  
optimal location will allow convenient use  
of the auto null push buttons and easy  
observation of the status and level meter  
LEDs.  
The final step was to create a physical  
package that would provide significant re-  
sources in a form that allowed simple and  
reliable integration with other equipment.  
This was accomplished by including two  
2-channel interfaces in a one-rack-space  
enclosure. Using standard 3-pin XLR-type  
audio connectors would enable rapid  
installation and troubleshooting. And by  
making the Model 46 “universally” pow-  
ered, installation in any locale could be  
directly supported.  
Mounting the Model 46  
Once a mounting location has been se-  
lected installation can begin. The Model  
46 requires one space (1.75 vertical  
inches) in a standard 19-inch (48.3 cm)  
equipment rack. Secure the unit into  
the equipment rack using two mounting  
screws per side.  
Installation  
In this section you will be installing and  
interconnecting the Model 46. The one-  
rack-space unit will be mounted in an  
equipment rack. Audio input, audio out-  
put, and party-line intercom connections  
will be made by way of 3-pin XLR-type  
connectors. AC mains power will be con-  
nected by means of a detachable cord  
set that is compatible with the Model 46’s  
3-pin IEC 320 C14-type inlet connector.  
4-Wire Audio Inputs and  
Outputs  
Two audio line input and two audio line  
output signals are associated with the  
4-wire section of each of the Model 46’s  
two interface circuits. Connections are  
made using standard 3-pin male and  
female XLR connectors. Refer to Ap-  
pendices A, B, and C for details on inter-  
connecting with Telex/RTS, Riedel, and  
Clear-Com matrix intercom systems.  
System Components  
The shipping carton contains the Model  
46 Interface and associated user guide.  
Units destined for North America and  
Japan also include an AC mains cord.  
Your dealer or distributor should provide  
an AC mains cord for other destinations.  
4-Wire Line Inputs  
As previously mentioned, each of the  
Model 46’s two interfaces allows two  
analog line-level audio sources to be  
Locating the Model 46  
The Model 46’s mounting location will dic-  
tate the length of the cable runs needed to  
Issue 2, September 2007  
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Model 46 User Guide  
Studio Technologies, Inc.  
 
 
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 trans-  
mission system. The 4-wire input circuitry  
is balanced, capacitor coupled, transform-  
er isolated, and has an impedance of 13 k  
ohms. The line inputs are compatible with  
signals that have a nominal level of 0, +4,  
+6, or +8 dBu. Configuration switches,  
located on the Model 46’s front panel, al-  
low the circuitry to be adjusted to match  
the connected nominal level. The switches  
work in tandem to control both the input  
and output nominal levels.  
remove a potential ground current path  
from the Model 46’s earth/chassis ground  
point through the shield of a cable. Also, if  
a hum or noise issue does arise be certain  
to confirm that, unless absolutely neces-  
sary, 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 connector is often the cause of seem-  
ingly inexplicable noise issues.  
With an unbalanced source connect  
pin 2 to signal high (+ or hot) and both  
pins 1 and 3 to shield. If connecting an  
unbalanced source in this manner results  
in hum or noise, connect pin 2 to high  
(+ or hot) and pin 3 to shield; leave pin 1  
unterminated.  
Audio sources are connected to the line  
inputs by way of 3-pin female XLR-type  
connectors which are located on the unit’s  
back panel. Refer to Figure 1 for a detailed  
view. Note that pin 1 of these connectors  
is internally connected to the Model 46’s  
earth/chassis ground point.  
4-Wire Line Outputs  
Each of the Model 46’s two interfaces  
provides two analog line-level audio out-  
puts. These outputs are intended to be  
connected to inputs on the devices asso-  
ciated with the 4-wire audio signals. The  
outputs are capacitor coupled, transform-  
er balanced with a nominal level of 0, +4,  
+6, or +8 dBu. As previously mentioned,  
the 4-wire nominal level configuration  
switches, located on the Model 46’s front  
panel, control the level of both the line  
outputs and the line inputs. The 4-wire line  
Prepare the mating connectors (males)  
so that 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 remov-  
ing the shield connection from pin 1 can  
eliminate the issue; “floating” pin 1 will  
Figure 1. Detail of back panel showing line inputs and outputs  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 11  
 
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 46’s back panel. Refer to Figure 1  
for a detailed view.  
and two on the front. They are organized  
in groups of two connectors per interface,  
one on the back and one on the front.  
They are wired in parallel (“multed”) and  
provide access to the identical signals.  
Dual-Channel Intercom Systems  
If compatibility with RTS TW systems is  
desired the mating connectors (females)  
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 is correct whether the con-  
nected devices are to be powered by an  
external source or by means of the Model  
46’s internal power sources.  
The mating connectors (females) should  
be prepared so that signal high (+ or hot)  
is expected on pin 2. Signal low (– or cold)  
should be expected on pin 3. The cables’  
shields can be connected to pin 1. How-  
ever, in order to minimize the chance that  
ground-interaction problems will arise,  
pin 1 on each of the line output connec-  
tors is isolated from all Model 46 circuitry  
and ground points. “Floating” pin 1 virtu-  
ally eliminates the chance that a “ground  
loop” problem will occur.  
Single-Channel Intercom Systems  
There are two ways of connecting to the  
Model 46’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 the female 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 chan-  
nel two, associated with pin 3 of the Model  
46’s 2-wire PL intercom interface connec-  
tors will be utilized. Pin 2, the Model 46’s  
audio channel one, will only be used for  
connecting DC power to the connected  
devices. While the audio resources pro-  
vided by channel one will not be used, the  
Model 46 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. Alternative-  
ly, an external intercom power supply can  
support the connected devices.  
2-Wire Party-Line Intercom  
Connections  
The Model 46’s 2-wire party-line (PL)  
intercom interfaces are designed to di-  
rectly connect with standard single- and  
dual-channel party-line intercom devices.  
Each of the two 2-channel PL interfaces is  
compatible with intercom circuits that have  
their own DC power sources. Alternately,  
the Model 46 can provide the DC power  
required by connected intercom user  
devices. The internal 30 volt DC intercom  
power source is limited to 300 milliam-  
peres of current. This moderate amount  
of power requires that the type and num-  
ber of connected user devices be selected  
appropriately.  
For convenience, the 2-wire PL intercom  
circuits can be connected to the Model 46  
by way of four 3-pin male XLR-type con-  
nectors; two are located on the back panel  
In some single-channel PL intercom sys-  
tem applications it may be desirable to  
take full advantage of the two channels  
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Model 46 User Guide  
Studio Technologies, Inc.  
 
associated with each Model 46 interface.  
In these applications one might want  
to view the Model 46 as providing four  
2-wire-to-4-wire interface circuits. The  
Model 46 can certainly be used in this  
fashion, but adapter cables will have to be  
prepared. These adapters will “split” the  
Model 46’s 2-wire PL intercom connectors  
into two 3-pin male XLR connectors, one  
for each audio channel. Pin 1 of the female  
3-pin XLR intended to mate with the Model  
46 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 designated  
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 2 for details. Us-  
ing two adapter cables the Model 46 can  
be directly interconnected with four Clear-  
Com intercom circuits. However, power for  
the connected devices must be provided  
by external power sources. The Model  
46’s ability to supply intercom power will  
not be utilized.  
Refer to the Configuration and Advanced  
Configuration sections of this user guide  
for details.  
Remote Control Inputs  
The Model 46 allows connection of three  
externally provided DC signals. These  
signals can provide remote control opera-  
tion of three functions: auto nulling for  
interface 1, auto nulling for interface 2, and  
a special “mic kill” function. Remote con-  
trol of the auto nulling functions provides a  
resource identical to that of the front-panel  
pushbutton switches. The exact manner in  
which the buttons and the remote control  
inputs operate depends on the setting of  
auto null button mode configuration DIP  
switch. The “mic kill” function is unique,  
only being available using the remote  
control input. It causes a 500 millisecond  
“burst” of 24 kHz signal to be sent se-  
quentially to both of the 2-wire partly-line  
interface channels associated with each  
of the Model 46’s two interfaces. To clarify,  
a “mic kill” signal is sent to a total of four  
intercom channels whenever the func-  
tion is activated. Independent control of  
sending “mic kill” signals to interface 1 or  
interface 2 is not supported. User intercom  
devices compatible with this 24 kHz “mic  
kill” signal include RTS TW-series belt-  
packs such as the BP325.  
Figure 2. Adapter cable wiring diagram  
The opto-coupled remote control inputs  
are designed for direct connection with  
3.3 and 5 volt DC logic signals. An internal  
475 ohm resistor, in series with each opto-  
coupler’s photodiode, acts to limit the  
current flow. Signals of up to 32 volts DC  
can be safely connected as long as the  
current is limited to 20 milliamperes maxi-  
mum. If necessary, an external resistor  
can serve to limit the current. For example,  
Note: It’s critical that the correct con-  
figuration settings be made when using  
the Model 46’s interfaces to support four  
independent intercom circuits. Specifically,  
the 2-wire power source configuration DIP  
switches must be set for external. In addi-  
tion the auto terminate disable DIP switch  
must be placed in its on (up) position.  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 13  
 
with a 12 volt DC signal using a 560 ohm,  
¼-watt resistor in series with the connec-  
tion would be appropriate. With a 24 volt  
DC control signal a series resistor of 1.8 k  
(1800) ohm is recommended. For correct  
operation a minimum current of 2 milliam-  
peres is recommended.  
onto a piece of 10-conductor flat ribbon  
cable. This ribbon cable can safely exit  
the Model 46 by laying flat between the  
chassis and the cover. The ribbon cable  
can then “fan out” to loose wires or, better  
yet, be terminated onto another connector  
such as a 9-pin D-subminiature type.  
Access to the remote control inputs  
is provided by means of a 10-pin male  
“header” connector which is located on  
the Model 46’s circuit board. Refer to  
Figure 3 for a view of the connector’s  
location. The “keyed” and “shrouded”  
header follows a common industry-  
standard specification: 2 rows of five pins  
each with 0.1 inch between rows and pins.  
The mating connector is intended to be  
an insulation-displacement (IDC) socket  
connector such as the AMP 746290-1. The  
connector would be “crimp” terminated  
There are several ways of preparing to  
connect to the remote control inputs. The  
easiest means is to purchase an inter-  
connecting cable assembly from Studio  
Technologies, Inc. The cable assembly  
consists of a length of ribbon cable with  
a 10-pin socket on one end and a 9-pin  
D-sub female on the other. Contact the  
factory or check the website for details.  
Alternately, a competent technician can  
easy fabricate an appropriate remote con-  
trol input interconnect wiring assembly. An  
example of a partially pre-made assembly  
Figure 3. Location of 10-Pin male header connector on the Model 46 printed circuit board  
Issue 2, September 2007  
Page 14  
Model 46 User Guide  
Studio Technologies, Inc.  
 
is part number A1AXH-1036G-ND which is  
available from Digi-Key (www.digikey.com).  
It’s important that a competent techni-  
cian be available to perform the actual  
remote control input connection process.  
As expected, mating the interconnecting  
cable assembly with the Model 46’s 10-  
pin “header” requires removing the unit’s  
cover. As such, this must be done only  
after confirming that all sources of mains  
power have been disconnected from the  
unit.  
It’s important to ensure that the source  
of the remote control signals can meet  
the Model 46’s logic-state and timing re-  
quirements. The Model 46 recognizes an  
idle remote control input as one that has  
no current flowing through it. An active  
remote control input is one that has current  
flowing through it for a minimum of 30  
milliseconds. A special case arises when  
the auto null button operating mode has  
been set to independent. This requires  
one front-panel button “tap” to auto null  
channel 1 and two button “taps” to auto  
null channel 2. The remote control equiva-  
lent for one “tap” is current flowing for a  
minimum of 30 milliseconds. The equiva-  
lent for two “taps” is current that flows  
for 30 milliseconds, then a minimum no-  
current period of 30 milliseconds, followed  
by current flow for a minimum of 30 milli-  
seconds. The second period of current flow  
must come within one second of the start  
of the first.  
AC Mains Power  
The Model 46 operates directly from AC  
mains power of 100 to 240 V, 50/60 Hz.  
As a “universal mains input” device, there  
are no switches to set or jumpers to install.  
A 3-pin IEC 320 C14-type inlet connector  
on the Model 46 mates with a detach-  
able mains cord set. For units shipped to  
North America and Japan a cord is sup-  
plied that has a North-American (NEMA  
15L) standard plug on one end and an  
IEC 320 C13-type connector on the other.  
Units bound for other destinations require  
that the appropriate cord set be obtained.  
The wire colors in the mains cord should  
conform to the internationally recognized  
color code and should be terminated  
according to the following chart:  
Once the interconnect method and the  
source of the control signals have been  
selected, refer to Figure 4 for connection  
details. Ensure that the connected signals  
follow the specified polarity or damage to  
the opto-coupled inputs may occur.  
10-Pin  
9-Pin D-Sub  
Function  
Header (P9) Interface Cable  
Remote Auto Null Interface 1 +  
Remote Auto Null Interface 1 –  
Remote Auto Null Interface 2 +  
Remote Auto Null Interface 1 –  
Remote “Mic Kill” +  
7
9
2
4
6
8
4
5
6
7
8
9
Figure 5. Detail of back panel showing AC  
mains power connector  
Remote “Mic Kill” –  
Figure 4. Remote control input pin-out chart  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 15  
 
Connection  
Neutral (N)  
Line (L)  
Wire Color  
Light Blue  
Brown  
Configuration – Interface 1  
and 2  
The DIP switch assemblies associated with  
interface 1 and 2 are accessible on the  
Model 46’s front panel. They provide identi-  
cal capability for their respective interfaces.  
Refer to Figure 6 for a detailed view. In this  
section the four switches associated with  
interface 1 will be covered in detail. This  
information applies to interface 2 as well.  
The four switches allow selection of the  
2-wire party-line power source, the nominal  
level for the 2-wire party-line channels, and  
the nominal level of the 4-wire inputs and  
outputs.  
Earth/Ground (E) Green/Yellow  
Safety Warning: The Model 46 does  
not contain an AC mains disconnect  
switch. As such, the AC mains cord  
plug serves as the disconnection de-  
vice. Safety considerations require that  
the plug and associated inlet be easily  
accessible to allow rapid disconnec-  
tion of AC mains power should it prove  
necessary.  
As soon as AC mains power is applied  
the Model 46 will begin its power-up  
sequence. As a “boot up” indication the  
power LED and each of the status LEDs  
will momentarily light. After the sequence  
has completed the power LED will again  
light and remain lit. The unit is now fully  
functional.  
Configuration  
Figure 6. Detail of front panel showing four DIP  
switches  
For the Model 46 to support the needs of  
specific applications a number of operat-  
ing parameters must be configured. These  
include the 2-wire party-line power source,  
the nominal 2-wire level, and the nominal  
4-wire level. Three 4-position DIP-type  
switch assemblies are used to establish  
the desired configuration. One DIP switch  
assembly is associated with interface 1, a  
second is associated with interface 2, and  
a third associated with advanced operat-  
ing features that apply to both interfaces.  
2-Wire Party-Line Power Source  
DIP switch 1 is used to select whether or  
not the Model 46 provides DC power to  
pin 2 of connectors associated with the  
2-wire party-line intercom channels. Two  
3-pin male XLR-type connectors, one  
located on the Model 46’s front panel  
and one on the back panel, are used to  
interconnect the Model 46 with the inter-  
com user devices. When switch 1 is in its  
off (down) position the Model 46 will not  
provide DC power. This switch setting is  
appropriate when an external intercom  
Issue 2, September 2007  
Page 16  
Model 46 User Guide  
Studio Technologies, Inc.  
 
 
power supply is providing power to the  
intercom circuit. In addition to power, it’s  
expected that the external power supply  
will also provide the required 200 ohm ter-  
minating impedance to both pins 2 and 3.  
4-Wire Nominal Audio Level  
DIP switches 3 and 4 are used to config-  
ure the nominal level of the input and out-  
put circuitry associated with interface 1’s  
two 4-wire input and output connections.  
Four level settings are available: 0, +4,  
+6, and +8 dBu. Prior to setting these DIP  
switches it’s important to determine the  
nominal audio level of the equipment con-  
nected to the Model 46’s 4-wire input and  
outputs. This may take a little investigation  
but is important for obtaining the best pos-  
sible Model 46 performance.  
When switch 1 is in its on (up) position the  
Model 46 will provide a 30 volt, 300 mil-  
liamperes maximum source of DC power  
to pin 2 of the 2-wire PL intercom connec-  
tors. The Model 46 will also provide 200  
ohm terminating impedances for both pin  
2 and pin 3 of the party-line connectors.  
The DC power and termination capabili-  
ties allow direct powering of dual-chan-  
nel intercom belt packs, such as the RTS  
BP325. In addition, listen-only belt packs,  
such as the Model 34 from Studio Tech-  
nologies, Inc. can be directly connected.  
There’s also no problem connecting  
single-channel intercom belt packs or user  
stations.  
The 0 dBu setting is provided specifically  
for use when a Model 46 is interconnected  
with a Clear-Com matrix intercom system.  
The +4 dBu setting is appropriate when  
standard audio lines with “+4” nominal  
levels are interconnected. The +6 dBu  
setting can be appropriate when intercon-  
necting with equipment manufactured in  
Europe, such as the digital matrix intercom  
systems from Riedel. The +8 dBu setting  
is provided specifically when interconnect-  
ing the Model 46 with audio signals asso-  
ciated with an RTS ADAM-series of matrix  
intercom system. This includes the related  
RVON-I/O unit.  
2-Wire Nominal Audio Level  
DIP switch 2 allows adjustment of the  
nominal audio level for the two 2-wire  
party-line channels associated with inter-  
face 1. When the switch is in its off (down)  
position the nominal level is –14 dBu.  
This level is compatible with intercom belt  
packs from manufacturers such as Clear-  
Com. When the switch is in its on (up)  
position the nominal level is –10 dBu. This  
setting is appropriate when belt packs  
from RTS, such as the BP325, or listen-  
only talent amplifier units from Studio  
Technologies, Inc. are connected. For best  
Model 46 performance it’s important that  
the 2-wire nominal level selection be made  
correctly.  
When DIP switches 3 and 4 are in their  
off (down) position the nominal level is  
set for 0 dBu. When switch 3 is on (up)  
and switch 4 is off (down) the nominal  
level is +4 dBu. When switch 3 is off  
(down) and switch 4 is on (up) the nomi-  
nal level is +6 dBu. When both switches  
3 and 4 are in their on (up) position the  
nominal level is configured for +8 dBu.  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 17  
 
are connected to the Model 46. In this lat-  
ter case only one of the Model 46’s audio  
channels will be used.  
Advanced  
Configuration  
There may be situations where it’s nec-  
essary for the two channels associated  
with each Model 46 interface be used  
with separate 2-wire party-line circuits.  
An example of this might be where two  
“loops” of single-channel belt packs,  
such as units from Clear-Com, need to be  
connected. Adapter cables as shown in  
Figure 2 would be used. In this case the  
intercom circuit’s DC power will not be  
connected to the Model 46, but only to the  
user devices. In this scenario, a DC volt-  
age 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 46. In both these cases a DC volt-  
age won’t be present on either pin 2 or 3  
and the Model 46 will automatically apply  
200 ohm terminations. In both these cases  
the termination will be in error and will re-  
sult in incorrect audio levels due a double-  
termination condition.  
Four DIP switches are provided on the  
Model 46’s back panel. They allow the two  
interfaces to be configured for several ad-  
vanced operating modes. Refer to Figure  
7 for a detailed view of these DIP switches.  
The available modes include auto termi-  
nate disable, auto null button operation,  
refresh disable, and factory test.  
Figure 7. Detail of back panel showing the  
Advanced Mode DIP switches  
Auto Terminate Disable  
To prevent this problem the auto termi-  
nate function can be disabled. When DIP  
switch 1 is in its off (down) position the  
auto terminate function is active. When  
switch 1 is in its on (up) position the auto  
terminate function associated with each  
interface is disabled. Repeating for clarity,  
the auto terminate disable function applies  
to both interfaces 1 and 2. With the auto  
terminate function disabled the two 46  
interfaces, when set for external power,  
will operate normally with the exception  
that pins 2 and 3 will never be terminated  
by the Model 46’s circuitry.  
The auto terminate function is designed to  
ensure that each 2-wire-to-4-wire interface  
circuit remains stable under most oper-  
ating conditions. Specifically, 200 ohm  
impedances are automatically applied to  
both pins 2 and 3 of a Model 46 2-wire  
party-line interface when that interface is  
configured for external power and no ex-  
ternal source of intercom power is detect-  
ed on pins 2 or 3. This function is directly  
compatible with RTS TW-series intercom  
applications in which the three conductors  
of a cable support both DC power and two  
channels of audio. It’s also compatible in  
situations where all three conductors of a  
Clear-Com single-channel intercom circuit  
Issue 2, September 2007  
Page 18  
Model 46 User Guide  
Studio Technologies, Inc.  
 
 
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 ter-  
minated 2-wire party line-intercom circuits  
be connected to pins 2 or 3 on the Model  
46’s 2-wire PL connectors. If they are not  
connected, it’s likely that audio oscilla-  
tions, noise, and distortion will be gener-  
ated in the Model 46’s 2-wire-to-4-wire  
converter circuitry. These audio artifacts  
will not cause any damage, but will be  
sent out the 4-wire line output connectors.  
Users of the 4-wire equipment may be less  
than pleased with what they hear!  
This is provided specifically for cases  
where the Model 46’s 2-wire party line  
interfaces 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 interface to  
be auto nulled at approximately the same  
time. The dual auto null mode allows a  
single “tap” of the button to initiate nulling  
of both hybrid circuits.  
When DIP switch 2 is in its on (up) posi-  
tion the independent auto null mode is  
selected. This would be appropriate for  
applications in which the two audio paths  
associated with each interface are used  
with independent party-line intercom  
circuits. This situation might arise when  
two Clear-Com single-channel intercom  
circuits are connected to one of the Model  
46’s dual-channel interfaces. The hybrid  
circuit associated with each channel can  
be auto nulled as desired. Another exam-  
ple would be in an RTS TW-series applica-  
tion 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 each  
Model 46 interface will often end up not  
routed to the same user device; indepen-  
dent auto nulling is certainly desired.  
Auto Null Button Operation  
One pushbutton switch is associated  
with each of the Model 46’s two interface  
circuits. An advanced configuration pa-  
rameter 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) of one of  
the two front-panel pushbutton switches  
initiates a routine that begins with chan-  
nel 1’s auto null sequence taking place  
followed immediately by channel 2’s auto  
null sequence being performed. Note that  
the mode selected for auto null button op-  
eration will also apply to the remote auto  
null inputs.  
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.  
Refresh Disable  
While testing the Model 46 as part of the  
development process it was found that  
in cases of extreme ESD (electro-static-  
discharge or “static”) an integrated circuit  
in the audio signal path could “latch up.”  
This would result in the audio signal no  
When DIP switch 2 is in its off (down) posi-  
tion the dual auto null mode is selected.  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 19  
 
longer passing through this component  
and on to the 4-wire audio output. When-  
ever this situation arose the component  
was not damaged, but restoring the flow of  
audio required either a power down/power  
up sequence, an auto null operation, or a  
change to one of the level configuration  
DIP switches to take place. While it is very  
unlikely that this type of ESD event would  
occur during normal field operation it is of  
concern for critical Model 46 applications.  
To minimize the chance that the audio  
path could experience this problem a  
“refresh” routine was added to the Model  
46’s operating software (“firmware”). To  
ensure that the ESD-sensitive component  
always remains functioning correctly the  
software resends its operating instructions  
four times each second. With this imple-  
mentation the worst case scenario would  
result in an audio loss of 250 milliseconds  
or less in the event of an extreme ESD  
“hit” being experienced.  
ESD-induced audio path problem could  
occur. A good “rule of thumb” is that if a  
Model 46 is mounted in an equipment rack  
with a known-good ground connection,  
disabling refresh shouldn’t pose any risk  
to reliable operation. But if a unit is mount-  
ed in a portable rack, or used loose as a  
single device, refresh should always remain  
enabled. The reasoning is quite simple:  
the latter condition is much more likely to  
allow an ESD “hit” to dissipate into the  
Model 46’s enclosure, while a substantial  
rack enclosure will offer a low-impedance  
path for ESD energy to dissipate into.  
Factory Test  
Back-panel DIP switch 4 is used to select  
between normal mode and factory test  
mode. When switch 4 is in its off (down)  
position the Model 46 operates in its stan-  
dard fashion. When switch 4 is in its on (up)  
position the factory test mode is selected.  
As expected, when the Model 46 is de-  
ployed in the field switch 4 should remain  
in its off (down) position. No damage to  
the Model 46 or connected equipment will  
occur when factory test mode is active.  
But nothing comes without a price and in  
this case the refresh routine adds a very  
slight audio “tick” to the 4-wire output and  
2-wire party-line signals. During typical  
applications these refresh “ticks” will not  
be noticeable. But there may be situa-  
tions where any added audio artifact may  
simply not be acceptable. To address this  
possible, but highly unlikely, case back-  
panel DIP switch 3 can be used to disable  
the refresh routine. When switch 3 is in its  
off (down) position the Model 46 operates  
in its standard fashion. When switch 3 is  
in its on (up) position the refresh routine  
is disabled. The Model 46 will continue to  
function normally with the exception that  
the four-times-per-second component  
refresh routine will not occur. Goodbye  
“click” but hello to the possibility that an  
Operation  
Technician intervention is typically not  
required during normal Model 46 operation.  
The unit is designed for continuous opera-  
tion with no routine maintenance necessary.  
Activating the auto null functions may be  
warranted should connected user devices  
or wiring associated with the 2-wire party-  
line intercom be changed. Upon power-up  
the Model 46 will go through a short initial-  
ization sequence before normal operation  
will begin. The power and status LEDs  
will each light sequentially and, upon  
Issue 2, September 2007  
Page 20  
Model 46 User Guide  
Studio Technologies, Inc.  
 
 
completion, the power LED will light  
steadily. The settings for the four hybrid  
circuits are stored in non-volatile memory  
and will be recalled during the power-up  
process.  
an excessive level condition, but it does  
provide a warning that at some stage  
reducing the signal level may be prudent.  
Normal 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.  
Level Meters  
The Model 46 contains eight 5-segment  
LED level meters. These meters are pro-  
vided as a support aid during installation,  
configuration, operation, and trouble-  
shooting. The meters represent the sig-  
nals coming in from, and going out to,  
the 4-wire connections. The meters are  
organized in four groups each represent-  
ing one input and one output. They are  
calibrated to reflect the level in dB rela-  
tive to the configured nominal operating  
level. This is similar to the way in which  
now-legacy VU meters functioned. As an  
example let’s take the situation where the  
nominal operating level of an interface is  
configured for +4 dBu and an output (“to  
4-wire”) LED labeled “0 dB” is lit. (For this  
example, the level on the 2-wire PL circuit  
is just large enough to light the “0” LED.)  
This indicates that a signal with an approx-  
imate level of +4 dBu is present on the as-  
sociated 4-wire output connector. Another  
example would be a Model 46 interface  
configured for +8 dBu operation and a  
–12 dB LED is lit. This indicates that a  
signal with audio level of –4 dBu is  
If the “from 4-wire” meters display consis-  
tency lower or higher levels than their 0 dB  
points it’s possible that a configuration is-  
sue exists. One potential problem is incor-  
rectly set 4-wire nominal audio level DIP  
switches for one or both of the Model 46’s  
interfaces. If the nominal level is set too  
low then sufficient audio headroom might  
not be available. It can also result in ex-  
cessively-high audio signals being sent to  
2-wire party-line circuits. Setting the nomi-  
nal level for too high a value will reduce  
the signal-to-noise performance. And, as  
expected, it may also result in audio sig-  
nals being sent to the 2-wire circuits at  
too low a level. Begin a review by confirm-  
ing that the two 4-wire level DIP switches  
associated with each interface are set as  
required.  
If the Model 46’s 4-wire nominal levels  
are set correctly but the meters still reflect  
sub-optimal levels, the issue may be relat-  
ed to incorrect settings on the equipment  
connected to the 4-wire inputs and out-  
puts. It’s possible that although a connect-  
ed 4-wire device’s nominal level matches  
the Model 46’s level setting, its actual  
nominal level may be significantly differ-  
ent. With a digital matrix intercom system  
this problem could be due to an incorrect  
configuration having been made to a spe-  
cific channel or port. For example, the RTS  
present on its associated output.  
Each level meter contains four green  
LEDs and one yellow LED. The four green  
LEDs indicate signal levels at or below the  
configured nominal level. The top LED is  
yellow in color and indicates a signal that  
is 6 dB or greater than the nominal level.  
An audio signal that causes the yellow  
LED to light doesn’t necessarily indicate  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 21  
 
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 inter-  
com system so that its levels comply with  
its own published nominal. This should,  
at least in theory, lead to the best system  
performance, including correct interfacing  
with the Model 46. But often it’s impos-  
sible to “clean up” the levels of a existing  
system. In this case it may be necessary  
to revise the Model 46’s 4-wire nominal  
level settings to match the reality of the  
connected signals.  
of at least 6 dB. Removing the unwanted  
termination is really the only valid means  
of correcting the problem.  
Connecting Party-Line  
Devices  
The Model 46’s two dual-channel interface  
circuits allow two sets of user devices to  
be connected. Depending on the setting  
of the configuration DIP switches, either  
an external intercom power source or the  
Model 46’s internal 2-wire power source  
has been designated for supporting the  
connected user devices.  
External Power Source  
The “to 4-wire” meters display the level  
of signals that come from the 2-wire party-  
line circuits and go out the 4-wire inter-  
faces. An issue may arise if the signals  
coming from the connected party-line  
user devices aren’t at a sufficient level  
so that a normal meter display can be  
reached. This could be the result of an  
incorrectly configured 2-wire nominal  
audio level. Confirm that the DIP switch,  
associated with each interface, is set to  
match the nominal level of the connected  
devices. RTS user devices have a nominal  
level of –10 dBu; Clear-Com devices typi-  
cally have a –14 dBu nominal level.  
When set for an external 2-wire power  
source, the Model 46 will not provide  
power on pin 2 of the 2-wire connectors,  
nor will the Model 46 use any power from  
a connected intercom circuit. As expected,  
audio signals will be sent from, and re-  
ceived by, each interface’s two audio cir-  
cuits. In this mode of operation the Model  
46 acts as a typical user station on the  
connected intercom circuit.  
As a connection confirmation, hardware  
and software in the Model 46 monitors  
pins 2 and 3 of the 2-wire PL intercom  
connectors for the presence of DC volt-  
age. 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  
pin 3 status LED to light.  
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  
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  
Issue 2, September 2007  
Page 22  
Model 46 User Guide  
Studio Technologies, Inc.  
 
external 2-wire power mode the Model 46  
is simply an observer; whether intercom  
power is present on pin 2, pin 3, or both  
pins 2 and 3 is not significant.  
mount component technology and have  
a lower maximum current draw of 65 mil-  
liamperes. Four of these “modern” BP325  
units can easily be supported.  
So that the Model 46’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  
so long as neither of the 2-wire power sta-  
tus 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 connect-  
ed. In this case the 200 ohm termination is  
expected to be provided elsewhere, typi-  
cally as part of the external power supply,  
and the Model 46’s termination is “lifted.”  
The Model 46’s intercom power supply  
circuits operate under software control.  
This allows detection of fault conditions  
and protection of both the Model 46’s  
circuitry and connected intercom user  
devices. Upon initial Model 46 power  
up no monitoring of the intercom power  
output takes place for a period of five  
seconds. This allows the Model 46’s  
circuitry and the connected intercom user  
devices to stabilize. The LED associated  
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 pin  
2 falls below 24 volts for a continuous one  
second interval. The hardware and soft-  
ware responds to this condition by turning  
off the power source to pin 2 and flashing  
the pin 2 LED as a warning. After a five  
second “cool down” interval the output re-  
turns to the same condition as upon initial  
power up: power is again applied to pin 2,  
the pin 2 status LED will light, and moni-  
toring won’t begin for another five sec-  
onds. A full short-circuit condition applied  
to the Model 46’s 2-wire connectors will  
result in a continuous cycle of 6 seconds  
on (5 seconds for start up and one second  
for detection) and 5 seconds off.  
Internal Power Source  
When an interface’s front-panel DIP switch  
is set for internal 2-wire power the Model  
46’s 2-wire party-line interface supplies  
30 volt DC on pin 2 of the output con-  
nector. 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 easiest  
figure to calculate but a web search will  
generally find specifications for all com-  
monly used devices. For example,  
As a diagnostic aid the 2-wire power  
status LED associated with pin 3 remains  
active in the internal power mode. When-  
ever DC in excess of approximately  
18 volts is present on pin 3 the LED will  
light. This condition will normally never  
exist but could prove useful in special  
circumstances.  
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 46 interface.  
Newer versions of the BP325 use surface  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 23  
 
An actual auto null sequence starts by  
muting the 4-wire input and output signal  
paths associated with the specific channel  
to be nulled. Then a short period of 24 kHz  
signal is sent out the 2-wire PL intercom  
interface channel. This will turn off micro-  
phones on those connected user devices  
that are compatible with the RTS TW-  
series “mic kill” protocol. The actual auto  
nulling process will next be performed. A  
series of tones will be sent out the 2-wire  
interface. Other Model 46 circuitry, un-  
der software control, will rapidly perform  
adjustments to achieve the best null pos-  
sible. After the adjustments are made the  
results are stored in non-volatile memory.  
Once the process has completed the  
4-wire input and output paths are again  
activated.  
Auto Null  
Each of the Model 46’s dual-channel inter-  
faces has circuitry to automatically null the  
two 2-wire-to-4-wire interfaces. Normally  
this process is performed at the time of  
initial Model 46 configuration but there’s  
no reason why “auto nulling” can’t be initi-  
ated anytime one desires. The only time  
that auto null must be performed is if con-  
ditions have changed vis-à-vis the inter-  
com user devices and wiring connected to  
a Model 46 2-wire PL interface connector.  
Even a slight change to an intercom cir-  
cuit, such as adding or removing a section  
of cable, is sufficient to require that the  
auto null process be performed.  
Two buttons, one associated with each  
interface, are provided to activate the  
auto null process. Refer to Figure 8 for a  
detailed view. To initiate auto null simply  
requires pressing and releasing (“tap-  
ping”) a button. The process begins by  
nulling channel one of an interface and,  
when completed, moves on to channel  
two. Two LEDs provide a visual indication  
of the auto null process, flashing when the  
auto null process for its respective chan-  
nel is active.  
Advanced configuration DIP switch 2,  
located on the back panel, allows an  
independent auto null button mode to  
be selected. If switch 2 is in its on (up)  
position, the front-panel pushbuttons 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.  
If possible, prior to performing an auto  
null it’s polite to warn all personnel who  
are actively using the connected intercom  
devices. The tones sent to the 2-wire inter-  
com 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  
Figure 8. Detail of front panel showing auto null  
section  
Issue 2, September 2007  
Page 24  
Model 46 User Guide  
Studio Technologies, Inc.  
 
active microphones. While the automatic  
“mic kill” signal will apply 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 intercom circuit.  
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 the LEDs will no longer  
indicate the intercom circuit’s termination  
status.  
For special externally powered 2-wire ap-  
plications the auto terminate function can  
be disabled. This primarily will come into  
play when “splitting” the Model 46’s re-  
sources into four separate single-channel  
interfaces. Moving advanced configuration  
DIP switch 1, located on the Model 46’s  
back panel, to its on (up) position disables  
the auto terminate function. To someone  
observing only the Model 46’s front panel  
this change would not be readily appar-  
ent. 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 46 will never apply 200  
ohm terminations to pins 2 or 3. For the  
hybrid circuits to remain stable termina-  
tion impedances must be provided by the  
connected circuits. If these are not pres-  
ent one should expect the hybrid circuits  
to generate a very impolite noise. This  
condition, caused by the 2-wire output  
circuitry being in an “unloaded” state, will  
not damage the Model 46’s circuitry. But  
errors in cable “patching,” a disconnected  
cable, or other real-world issues, can lead  
users listening to signals that originate in  
the Model 46’s 4-wire outputs from being  
presented with a rude surprise!  
Advanced  
Operation  
The Model 46 allows several of the operat-  
ing parameters to be configured to meet  
the needs of specific applications. The  
following paragraphs provide details about  
the auto terminate disable and factory  
test modes.  
Auto Terminate Disable  
As previously discussed in this user guide,  
the auto terminate function can come into  
play when a 2-wire interface is configured  
for external power. Using a combination  
of hardware and software, pins 2 and 3  
of each 2-wire PL intercom connector are  
monitored for the presence of DC voltage.  
If a level of greater than approximately  
18 volts DC is not detected on either pin,  
200 ohm terminating networks are applied  
to those same pins. This ensures that the  
Model 46’s hybrid circuitry remains stable,  
preventing objectionable audio signals  
from being sent to the 4-wire output con-  
nectors. One caveat does apply; 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.”  
In conclusion, it’s important that technical  
personnel working with the Model 46 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.  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 25  
 
 
Factory Test Mode  
Technical Notes  
Back-panel DIP switch 4 allows a factory  
test mode to be enabled. During normal  
operation switch 4 should remain in its off  
(down) position. When switch 4 is in its on  
(up) position factory mode is active. En-  
abling this mode will result in the following  
operating condition: during an auto null  
sequence the associated 4-wire output  
channel will remain active. This will allow  
the tones associated with the nulling pro-  
cess to be present on the 4-wire output.  
While not appropriate during actual Model  
46 use, it is interesting to “hear” the null-  
ing process take place. But unless direct-  
ed by factory personnel switch 4 should  
remain in its off (down) position.  
Software Version Display  
A special Model 46 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 four status LEDs associated with  
interface 1 are used to display the major  
release number with a range of 1 through  
4. The four status LEDs associated with  
interface 2 are used to display the release  
sub-number which again ranges from 1  
through 4. Refer to Figure 9 for a detailed  
view of the status LEDs and the corre-  
sponding software version numbering  
scheme. The Model 46’s initial software  
release is version 1.1 which is represented  
by the bottom LEDs of each status column  
being lit.  
Interface 1 LED Section  
(Major Release Number)  
Interface 2 LED Section  
(Release Sub-Number)  
O 4  
O 3  
O 2  
1  
.4 O  
.3 ●  
.2 O  
.1 O  
Figure 9. Detail of front panel showing the status LEDs that display the software version. In this  
example, the software version is 1.3.  
Issue 2, September 2007  
Page 26  
Model 46 User Guide  
Studio Technologies, Inc.  
 
 
To display the Model 46’s software version  
is very simple. From the powered-down  
state, press and hold the auto null button  
associated with interface 1. Apply mains  
power while continuing to press the but-  
ton. The normal power-up sequence will  
not occur but instead one LED will be lit in  
the column associated with interface 1 and  
one LED will be lit in the column associated  
with interface 2. As previously described,  
these two LEDs represent the unit’s current  
software version. While the software version  
number is being displayed the LED level  
meters will likely display random readings.  
This is not a problem and can simply be  
ignored. 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.  
tasks. But it’s not intended to act as a  
“universal” 2-wire-to-2-wire interface such  
as is provided by the venerable Clear-Com  
TW-12B. The Model 46’s 2-wire inter-  
faces are not isolated from each other;  
they share the Model 46’s internal power  
supply’s common connection. (The power  
supply common point is DC isolated from  
chassis and earth ground.) Specifically,  
pin 1 on the XLR connector associated  
with interface 1’s 2-wire party-line inter-  
face is electrically connected to the pin 1  
connection on interface 2. This is due to  
the fact that both 2-wire party line inter-  
faces are capable of supplying DC power  
to connected devices. They also use the  
resources of a single set of logic circuitry.  
And as such they each require access to  
the Model 46’s common power supply  
circuitry. There’s no doubt that a Model 46  
could be used to create a 2-wire-to-2-wire  
party line “bridge” by cross-linking the  
4-wire inputs and outputs using standard  
XLR-type audio cables. But it’s likely that  
a significant “ground loop” would be cre-  
ated through pin 1 of each of the Model  
46’s 2-wire interfaces. While it’s possible  
to use a set of special isolation transform-  
ers to eliminate the ground loop, that’s  
best left to the brave of heart!  
Note that while it’s easy to determine which  
software version is loaded into the Model  
46 a trip back to the factory is required to  
update it. The 8-bit microcontroller that  
provides the unit’s logic “horsepower” also  
includes internal FLASH memory. This non-  
volatile memory is used to store the operat-  
ing software (“firmware”). Re-programming  
this memory requires using a specialized  
programming unit. While not outrageous in  
price, it still costs in the range of US$500.  
The “programmer” uses a ribbon cable  
and socket to interface with a 6-pin “head-  
er” on the Model 46’s printed circuit board.  
And, as you would guess, once connected,  
reprogramming takes only a matter of sec-  
onds. But unfortunately the programmer  
is not something that would be found in  
a typical “field shop” or repair facility.  
Model 72 Interface  
And now for a shameless “product plug.”  
Studio Technologies, Inc. manufactures  
a nifty product called the Model 72 Level  
Meter/Interface. A compact, standalone  
unit, it can prove very useful when setting  
up, maintaining, and troubleshooting inter-  
com circuits. It will give a direct indication  
of signal levels at any point in a single-  
or dual-channel intercom circuit. Check  
it out!  
Not a TW-12B Replacement!  
The Model 46 will provide a high level of  
performance over its range of intended  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 27  
 
Specifications  
4-Wire Inputs: 4, 2 per interface  
Type: transformer-coupled, capacitor isolated  
Impedance: 13 k ohms  
General Audio:  
Frequency Response: 2 dB 100 Hz to 8 kHz  
Distortion (THD+N): <0.5%, measured at 1 kHz,  
4-wire input to 2-wire interface pin 2  
Nominal Level: 0, +4, +6, or +8 dBu, selectable  
in tandem with nominal output level  
Signal-to-Noise Ratio: >55 dB, measured at 1 kHz,  
4-wire input to 2-wire interface pin 2  
Maximum Level: +22 dBu  
Connectors:  
4-Wire Outputs: 4, 2 per interface  
Type: transformer-coupled, capacitor isolated  
Impedance: 50 ohms nominal  
Inputs from 4-Wire: 3-pin female XLR-type  
Outputs to 4-Wire: 3-pin male XLR-type  
2-Wire Party-Line Intercom: 3-pin male XLR-type  
Nominal Level: 0, +4, +6, or +8 dBu, selectable  
in tandem with nominal input level  
Remote Control: 10-pin header, requires optional  
10-pin ribbon connector assembly  
Maximum Level: +20 dBu into 2 k ohms  
AC Mains: 3-blade, IEC 320 C14-type (mates with  
IEC 320 C13)  
Meters: 8  
Function: displays level of 4-wire inputs and  
outputs  
2-Wire Party-Line Intercom Interfaces: 2  
Type: 2-channel party-line (PL), unbalanced  
(pin 1 common; pin 2 DC with channel 1 audio;  
pin 3 channel 2 audio)  
Type: 5-segment LED, modified VU ballistics  
Remote Control Inputs: 3  
Type: optically coupled  
Compatibility: single- and dual-channel intercom  
systems such as from Telex®/RTS® and  
Clear-Com®  
Input: 3-32 volts DC; 2 milliamperes minimum for  
operation, 20 milliamperes maximum  
Impedance, External Power Mode: >10 k ohms  
Impedance, Internal Power Mode: 200 ohms  
Nominal Level: –10 or –14 dBu, selectable  
“Mic Kill” Signal: square wave, 24 kHz, 1%  
Functions: auto null interface 1, auto null interface  
2, and “mic kill”  
AC Mains Requirement: 100-240 volts  
(–15/+10%), 50/60 Hz, 0.7 amperes maximum @  
100 volts, 0.4 amperes maximum @ 240 volts  
Internal Power Source: 30 volts DC nominal, 300  
milliamperes maximum  
Dimensions (Overall):  
19.0 inches wide (48.3 cm)  
1.72 inches high (4.4 cm)  
8.5 inches deep (21.6 cm)  
Hybrids: 4, 2 per interface  
Topology: 3-section analog circuitry compensates  
for resistive, inductive, and capacitive 2-wire party-  
line loads  
Mounting: one space in a standard 19-inch rack  
Nulling Method: automatic upon user initiation,  
processor implements digital control of analog cir-  
cuitry; null settings stored in non-volatile memory  
Weight: 7.75 pounds (3.5 kg)  
Nulling Line Impedance Range: 120-350 ohms  
Nulling Cable Length Range: 0-3500 feet, typical  
Trans-Hybrid Loss: >40 dB, typical at 800 Hz  
Specifications and information contained in this  
User Guide subject to change without notice.  
Issue 2, September 2007  
Page 28  
Model 46 User Guide  
Studio Technologies, Inc.  
 
 
Appendix A  
Interfacing Telex®/RTS® Matrix Intercom Systems with the Model 46 Interface  
ADAMMatrix Intercom System Analog Ports to  
Model 46 Interface  
RVON-I/O I/O Connections to Model 46 Interface  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 29  
 
 
Appendix B  
Interfacing Riedel® Artist™ Matrix Intercom System Analog Ports with the  
Model 46 Interface  
Issue 2, September 2007  
Page 30  
Model 46 User Guide  
Studio Technologies, Inc.  
 
 
Appendix C  
Interfacing Clear-Com® Matrix Intercom System Analog Ports with the  
Model 46 Interface  
Model 46 User Guide  
Studio Technologies, Inc.  
Issue 2, September 2007  
Page 31  
 
 
 
 
 

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