Studio Technologies 46A User Manual

User Guide

Issue 1, September 2014

This User Guide is applicable for serial numbers:

M46A-01151 and later

Copyright © 2014 by Studio Technologies, Inc., all rights reserved

www.studio-tech.com

50115-0914, Issue 1

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Table of Contents

Introduction ................................................................... 5

Installation ..................................................................... 10

Configuration ................................................................ 16

Advanced Configuration ............................................... 17

Operation ...................................................................... 20

Advanced Operation ..................................................... 24

Technical Notes ............................................................. 25

Specifications ................................................................ 27

Appendix A—Interfacing with RTS® Matrix

Intercom Systems ......................................................... 28

Appendix B—Interfacing with Riedel® Artist™

Matrix Intercom Systems .............................................. 29

Appendix C—Interfacing with Clear-Com®

Matrix Intercom Systems .............................................. 30

Block Diagram

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Issue 1, September 2014 Model 46A User Guide
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Introduction

The Model 46A 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
46A provides two independent full-fea­tured 2-channel interfaces. Each interface
contains 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 beltpacks. 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 46A include television sports and
special event broadcasting, theme park and
theater installations, corporate events, and
industrial testing environments. Standard
audio connectors are used for all input and
output signals. The Model 46A mounts in

one space of a standard 19-inch rack
enclosure and requires 100-240 volts,
50/60 Hz for operation.

2-Wire Interfaces

The Model 46A’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 46A’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 equipment, such as that
from Clear-Com, the –14 setting is correct.

The Model 46A’s 2-wire interfaces can
correctly function with powered (“wet”)
or unpowered (“dry”) intercom circuits.

Model 46A Front Panel

Model 46A Back Panel

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Powered circuits have a DC voltage pres­ent, 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 beltpacks. In this
type of application the Model 46A is config­ured to operate in its external 2-wire power
source mode. In this mode the Model
46A’s circuitry maintains the required
high-impedance load and, as in all cases,
draws no power from the party-line circuit.
An auto-terminate function ensures that
should a “wet” circuit not be connected,
the Model 46A’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.

A significant capability of the Model 46A’s
2-wire interfaces is their ability to supply
DC power and 200 ohm AC termination
to the connected intercom circuits. Re­ferred to as the Model 46A’s internal 2-wire
power source mode, the 30 volt output can
power devices such as user stations and
beltpacks. In many applications this will
eliminate the need for external intercom
power supplies. Besides reducing total
system cost, this feature can also lower
system weight, reduce required mounting
space, and decrease the mains energy
requirement.

With each 2-wire interface’s ability to sup­ply up to 315 milliamperes (mA) of current,
a typical broadcast application that uses
two sets of three BP325 beltpacks can eas­ily be supported. Each interface’s 30 volt
DC intercom power source ensures that
systems requiring 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 the output
of the DC power supplies are monitored
for over-current and short-circuit condi­tions. This allows protection shut-down
of the output DC, as well as providing an
alert by way of a visual indicator.

Analog Hybrids with Auto
Nulling

A key reason that the Model 46A’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 46A’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-wire­to-4-wire interface is critical. The quality of
this isolation, technically known as return­loss or trans-hybrid loss, is measured in
dB. A high value is important, especially
in applications where multiple 2-wire-to­4-wire interfaces are used together. Re­mote sports broadcast applications are
especially sensitive to this requirement.
The Model 46A’s sophisticated auto null­ing function uses analog circuitry under

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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 party­line circuit. The party-line’s conditions are
the sum of the impact made by the type
and quantity of cable, the connected user­devices, and the intercom power source.

Whenever a user presses one of the
Model 46A’s auto null buttons digital
circuitry adjusts the analog hybrids to
rapidly achieve their maximum return-loss.
The nulling process takes less than 10
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 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 maxi­mize 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.

4-Wire Interfaces

Associated with the 4-wire portion of the
Model 46A’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, audio­over-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 46A’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. Front­panel-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 46A 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 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,

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offering a clear view of the 2-wire DC power
and auto null functions.

Pro Audio Quality

The Model 46A’s audio circuitry was
designed in the spirit of professional
audio equipment, rather than that found
in typical party-line intercom gear. High­performance components are used
throughout, providing low-distortion, low­noise, and high headroom. Using passive
and active filters, the frequency response
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
46A’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 addition, 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 nulling circuitry
to provide excellent, consistent results
for both the powered and unpowered
channels.

Attention to detail is a hallmark of the
Model 46A’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 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 46A’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 46A 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 dis­able it. This would allow full control over
the Model 46A’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
push button 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 46A features to be remotely
controlled. These features include remote
auto nulling for each interface and remote
“mic kill” activation.

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Simple Installation

The Model 46A 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 46A is housed in a rugged alu­minum and steel enclosure that is designed
to be “road tough.” It mounts in one space
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 46A’s enclosure, provides
conversion 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
supplies help to ensure cool, reliable op­eration in a variety of settings.

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 46A 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 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!

Design Philosophy

While the “bits and pieces” that make up
the Model 46A 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 46A’s design process, conversa­tions with industry experts quickly made

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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 beltpacks, are

typically connected to 2-wire party-line cir­cuits. By adding DC power sources to the
Model 46A’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 one-rack-space
enclosure. Using standard 3-pin XLR au­dio connectors would enable rapid instal­lation and troubleshooting. And by making
the Model 46A “universally” powered,
installation in any locale could be directly
supported.

Installation

In this section you will be installing and
interconnecting the Model 46A. The one­rack-space unit will be mounted in an
equipment rack. Audio input, audio out­put, and party-line intercom connections
will be made by way of 3-pin XLR connec­tors. AC mains power will be connected
by means of a detachable cord set that is
compatible with the Model 46A’s 3-pin IEC
320 C14 inlet connector.

Locating the Model 46A

The Model 46A’s mounting location will
dictate the length of the cable runs need­ed to 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 assure more reliable and consistent
system performance. Another location
criterion is access to the Model 46A’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.

Mounting the Model 46A

Once a mounting location has been se­lected installation can begin. The Model
46A 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.

4-Wire Audio Inputs and
Outputs

System Components

The shipping carton contains the Model
46A 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.

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Two audio line input and two audio line
output signals are associated with the
4-wire section of each of the Model 46A’s
two interface circuits. Connections are
made using standard 3-pin male and
female XLR connectors. Refer to Appen­dices A, B, and C for details on intercon­necting with RTS, Riedel, and Clear-Com
matrix intercom systems.