Dual 2-Wire Analog Audio to 4-Wire Analog Audio Interface
Highlights
• Two 2-channel party-line intercom interfaces
• Four analog hybrids with auto null capability
• Input and output level metering
• Excellent audio quality
• 2-channel intercom power sources
• Direct support for broadcast SAP panel applications
• Standard audio connectors
• Universal input mains powered
The Model 47A is designed to interface 2-wire full-duplex party-
line (PL) intercom circuits with 4-wire audio circuits associated
with matrix intercom systems. Other specialized audio system
interfacing applications can also be supported. The Model 47A
provides two independent full-featured 2-channel interfaces.
Each interface contains two hybrid circuits which include au
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tomatic 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 to
devices such as user beltpacks and user stations. Independent
power sources are provided for both channels (pins 2 and 3)
of each interface, allowing direct support for broadcast source
assignment panel (SAP) applications. Configuration settings
allow the 2-wire and 4-wire interface circuitry to be compatible
with a range of nominal signal levels. Audio level meters provide
user confirmation of system performance during setup and
operation. Applications for the Model 47A include mobile and
fixed television facilities, theme park and theater installations,
and industrial testing environments. Standard audio connectors
are used for all input and output signals. The Model 47A mounts
in one space (1U) of a standard 19-inch rack enclosure and
requires 100-240 volts, 50/60 Hz for operation.
2-Wire Interfaces
The Model 47A’s 2-wire interfaces are optimized for direct
connection with 2-channel party-line intercom circuits. In
addition, single-channel party-line intercom circuits can also
be connected. Many broadcast applications use the 2-channel
TW-Series from RTS®, including their popular BP325 beltpack.
Other industry-standard single- and 2-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 47A’s 2-wire inter-
faces 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.
The Model 47A’s 2-wire interfaces can correctly function with
powered (“wet”) or unpowered (“dry”) intercom circuits. Pow
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ered circuits have a DC voltage present, typically provided by
power supplies such as the RTS PS20. This DC power, nor
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mally 30-32 volts, provides energy for connected devices such
as user stations or beltpacks. In this type of application the
Model 47A is configured to operate in its external 2-wire power
source mode. In this mode the Model 47A’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
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tion ensures that should a “wet” circuit not be connected, the
Model 47A’s interface circuitry will remain stable. This unique
feature makes certain that objectionable audio signals, includ
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ing oscillations and “squeals,” will infrequently be sent to the
connected 4-wire device.
A significant capability of the Model 47A’s 2-wire interfaces is
their ability to supply DC power and 200 ohm AC termination
to both channels (pins 2 and 3) of the connected intercom
circuits. Referred to as the Model 47A’s internal 2-wire power
source mode, the 30 volt outputs can power devices such as
50118-0914 Issue 1
user stations and beltpacks. With independent power sources
provided for both pins 2 and 3 of each interface, applications
that include flexible party-line intercom channel routing will be
directly supported. As such, in applications that include both
source assignment panel (SAP) and Model 47A units the need
for external intercom power supplies can be eliminated. Besides
reducing total system cost, this capability may also lower system
weight, reduce required mounting space, and decrease the mains
energy requirements.
With the Model 47A’s 2-wire interfaces able to supply up to 315
milliamperes (mA) of current on each channel, a typical broad
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cast application that uses four sets of three BP325 beltpacks
can easily be supported. And the sophisticated 30 volt DC
intercom power sources ensure that applications that require
long intercom cable runs will function correctly. The circuitry’s
output regulation is such that little change in the output voltage
will occur over its entire rated output current. Also, the unique
design of the power supply circuitry virtually eliminates the
noise and “hiss” associated with typical intercom power supply
performance. Under software control each of the four DC power
sources is monitored for over-current and short-circuit condi
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tions. This allows protective shut-down of individual DC outputs,
as well as providing an alert by way of a visual indicator.
Analog Hybrids with Auto Nulling
A key reason that the Model 47A’s interfaces achieve excellent
audio performance is the design of the 2-wire-to-4-wire hybrid
circuits. They provide low noise and distortion, 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 47A’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 circuits.
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 requirement. The Model 47A’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 series of adjustments to account
for the resistive, inductive, and capacitive conditions that are
present on the connected 2-wire party-line circuit. A party-
line’s condition is the sum of the impact made by the type and
quantity of cable, the connected user devices, and the intercom
power source.
Whenever a user presses one of the Model 47A’s auto null but
tons digital circuitry adjusts the analog hybrids to rapidly achieve
their maximum return-loss. The nulling process takes less than
ten seconds for each channel of an interface. However, it’s
important 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 hybrid is exposed to
the varying 2-wire conditions often found in broadcast applica-
tions. 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 automati
cally disabling “open” microphones the auto nulling process can
achieve better performance.
4-Wire Interfaces
Associated with the 4-wire portion of the Model 47A’s interfaces
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 transmission systems, and
other specialized audio equipment. The input and output circuitry
is transformer-coupled to minimize the chance of hum, noise,
or ground “loop” issues. A key characteristic of the Model
47A’s design 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 RTS. This setting also
applies to the related RVON-I/O VoIP product.
The Model 47A 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 helping to
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Studio Technologies, Inc. Model 47A Interface, Issue 1, Page 2
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 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.
Pro Audio Quality
The Model 47A’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 throughout, providing low-distortion, low-noise, and high
headroom. Using passive and active filters, the frequency re
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sponse is limited to approximately 100 Hz to 8 kHz. This range
was selected to provide excellent performance for human
speech, while maximizing the ability of the hybrids to create
substantial “nulls.” When the Model 47A’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 power sources is excellent, with very little noise, hum,
or “hiss” being added to the 2-wire connections.
Attention to detail is a hallmark of the Model 47A’s design.
For example, during the brief auto nulling process an interface
channel’s 4-wire input and output signal is muted, prevent
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ing unwanted audio from reaching the connected equipment.
Associated with the 2-wire interfaces is circuitry that, under
software control, applies 200 ohm terminating impedances to
the 2-wire party-line circuits. This, along with other circuitry that
monitors DC voltages present on the 2-wire circuits, ensures
that audio instability associated with unterminated circuits will
rarely occur. As previously mentioned the Model 47A’s 2-wire
DC power sources offer a unique level of performance. Their
ability to deliver power while maintaining audio quality is simply
unmatched.
Special Applications
While the Model 47A is designed to directly integrate into typical
applications, it’s ready to support the “one-in-a-million” situa
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tions, too. To accomplish this DIP switches, accessible on the
back panel, allow some of the automatic features to be disabled.
For example, one DIP switch allows the auto-terminate feature
to be disabled. While this feature can help maintain audio qual
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ity, advanced users might need to disable it. This would allow
full control over the Model 47A’s four hybrid circuits, enabling
them to be used completely independently. A second DIP switch
changes the way in which the front-panel auto null pushbut
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ton switches operate. Normally pressing one of the buttons
begins the process of auto nulling both channels associated
with an interface. The alternate switch mode allows each auto
null sequence to be activated independently. One tap (a quick
press and release) 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. For installer-selected applications,
three opto-coupled inputs allow selected Model 47A features
to be remotely controlled. These features include remote auto
nulling for each interface and remote “mic kill” activation.
Simple Installation
The Model 47A uses standard 3-pin XLR connectors to allow
convenient interconnection in most broadcast and general audio
environments. For flexibility, access to the 2-wire party-line
intercom interfaces can be made using the connectors provided
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 production, front-panel access allows
intercom beltpacks 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.
The Model 47A is housed in a rugged aluminum and steel
enclosure that is designed to be “road tough.” It mounts in one
space (1U) of a standard 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 47A’s enclosure,
provides conversion of the mains input power into the DC
voltage required by the party-line 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 supplies help to ensure cool, reliable operation
in a variety of settings.
Design Philosophy
While the “bits and pieces” that make up the Model 47A have
been described in conventional terms, the real strength of the
unit rests in how it integrates and performs in the “real world.”
Before beginning the Model 47A’s design process, conversations
with industry experts quickly made it apparent that installing
and configuring existing 2-wire-to-4-wire interface units was
invariably a time-consuming, aggravating 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 application
in a different way. This led to the over-riding Model 47A design
goal: create a “new breed of cat,” fundamentally changing how
broadcast 2-wire-to-4-wire interface equipment fit into actual
applications.
An important first step was to eliminate the requirement that a
senior technician, along with a screwdriver, be present during
every installation. (It was universally acknowledged that their
Studio Technologies, Inc. Model 47A Interface, Issue 1, Page 3
time can be better spent elsewhere!) The need to adjust trim
potentiometers, fabricate special cabling and connector straps,
use nulling earpieces, etc., had to be eliminated. For example,
in virtually all instances, input and output levels fall within just a
few dB of their nominal values and, as such, could be supported
with a limited number of configurable choices. In addition, it was
acknowledged that in this application analog audio circuitry was
capable of providing excellent audio performance, but that the
required manual nulling process was operationally taxing. By
adding digital control to the analog circuitry, automatic nulling
could be performed—the best of both worlds!
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
Model 47A Specifications
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
Signal-to-Noise Ratio: >55 dB, measured at 1 kHz, 4-wire input to
2-wire interface pin 2
Connectors:
Inputs from 4-Wire: 3-pin female XLR
Outputs to 4-Wire: 3-pin male XLR
2-Wire Party-Line Intercom: 3-pin male XLR
Remote Control: 10-pin header, requires user-provided 10-pin ribbon
connector assembly
AC Mains: 3-blade, IEC 320 C14 (mates with IEC 320 C13)
2-Wire Party-Line Intercom Interfaces:
Type: 2-channel party-line (PL), unbalanced (pin 1 common;
pin 2 channel 1 audio or DC with channel 1 audio; pin 3 channel 2
audio or DC with channel 2 audio)
Compatibility: single- and 2-channel intercom systems such as from
RTS® and Clear-Com®
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%
2-Wire Party-Line Power Sources:
Rating: 30 volts DC nominal, 315 mA maximum, software monitored
and controlled
Hybrids: 4, 2 per interface
Topology: 3-section analog circuitry compensates for resistive,
inductive, and capacitive 2-wire party-line loads
Nulling Method: automatic upon user initiation, processor
implements digital control of analog circuitry; null settings stored
in non-volatile memory
Nulling Line Impedance Range: 120-350 ohms
Nulling Cable Length Range: 0-3500 feet, typical
Trans-Hybrid Loss: >40 dB, typical at 800 Hz
2
4, 2 per interface
turned out that in many applications only a small number of user
devices, such as beltpacks, are typically connected to 2-wire
party-line intercom circuits. But power on both pins 2 and 3 of
each interface was necessary for compatibility with SAP panel
applications. By adding two DC power sources to each of the
Model 47A’s 2-wire interfaces, the need for external intercom
power supplies could often be eliminated.
The final step was to create a physical package that would
provide significant resources in a form that allowed simple and
reliable integration with other equipment. This was accomplished
by including two 2-channel interfaces in a 1U enclosure. Us
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ing standard 3-pin XLR audio connectors would enable rapid
installation and troubleshooting. And by making the Model
47A “universally” powered, installation in any locale could be
directly supported
4-Wire Inputs: 4, 2 per interface
Type: transformer-coupled, capacitor isolated
Impedance: 13 k ohms
Nominal Level: 0, +4, +6, or +8 dBu, selectable in tandem with
nominal output level
Maximum Level: +22 dBu
4-Wire Outputs:
Type: transformer-coupled, capacitor isolated
Impedance: 50 ohms nominal
Nominal Level: 0, +4, +6, or +8 dBu, selectable in tandem with
nominal input level
Maximum Level: +20 dBu into 2 k ohms
Meters: 8
Function: displays level of 4-wire inputs and outputs
Type: 5-segment LED, modified VU ballistics
Remote Control Inputs:
Type: optically coupled
Input: 3-32 volts DC; 2 mA minimum for operation, 20 mA maximum
Functions: auto null interface 1, auto null interface 2, and “mic kill”
AC Mains Requirement:
60 watts maximum
Dimensions (Overall):
19.0 inches wide (48.3 cm)
1.72 inches high (4.4 cm)
8.4 inches deep (21.3 cm)
Mounting: one space (1U) in a standard 19-inch rack
Weight:
Specifications subject to change without notice.
© by Studio Technologies, Inc.
4.7 pounds (2.1 kg)
4, 2 per interface
3
100-240 volts (–15/+10%), 50/60 Hz,
, September 2014
Studio Technologies, Inc.
Skokie, Illinois USA +1 847-676-9177 www.studio-tech.com
Studio Technologies, Inc. 50118-0914, Issue 1
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