Datasheet

50219-0411, Issue 1
The Model 45A is designed to interface a 2-wire full-duplex
party-line intercom circuit with a 4-wire audio circuit associated
with a matrix intercom system. Other specialized audio system
interfacing applications can also be supported. Applications
for the Model 45A include television sports and special event
broadcasting, theme park and theater installations, corporate
events, and industrial testing environments.
Key Features:
• Portable “throw-down” or rack-mount use
• Analog hybrids with auto null capability
• Input and output level metering
• Excellent audio quality
• Flexible powering
• Standard audio connectors
• Transformer-coupled 4-wire inputs and outputs
• Integrated 2-wire intercom power source
Overview
The Model 45A provides a full-featured 2-channel interface which
includes two hybrid circuits with automatic nulling capability.
The analog hybrid circuitry provides excellent audio quality and
high return-loss. For flexibility, the Model 45A can be powered
by the connected 2-wire party-line circuit or by means of an
external source of 24 volts DC. When powered by an external
source the Model 45A is capable of supplying DC power to the
2-wire intercom circuit, allowing direct operation of devices such
as user belt packs. Audio level meters provide confirmation of
system performance during setup and operation. Standard audio
connectors are used for all input and output signals. The Model
45A’s enclosure has a “1/2-rack” 1U form factor and weighs
less than two pounds, making it well suited for use in portable
applications. Alternately, using one of the optional rack-mount
front panels, one or two Model 45A units can be mounted in a
single space of a standard 19-inch rack enclosure or one unit
can be mounted in a special 10.5-inch rack.
2-Wire Interface
The Model 45A’s 2-wire interface is optimized for direct con-
nection with dual-channel party-line (PL) intercom circuits such
as the TW-series of intercoms from RTS®. In addition, other
industry-standard single- and dual-channel PL intercom circuits,
including those from Clear-Com®, are compatible.
The Model 45A’s 2-wire interface can correctly function with
powered (“wet”) or unpowered (“dry”) intercom circuits. An
auto-terminate function ensures that should a “wet” circuit not
be connected, the Model 45A’s interface circuitry will remain
stable. This unique feature makes certain that objectionable
audio signals, including oscillations and “squeals,” won’t be
sent to the connected 4-wire device.
A significant capability of the Model 45A’s 2-wire interface is
its ability to supply DC power and 200 ohm AC terminations to
“create” an intercom circuit. The 30 volt output can power a lim
-
ited number of devices such as user stations and belt packs. With
up to 300 milliamperes of current available, a typical broadcast
application that uses two or three BP325 belt packs can easily
be supported. In many applications this can eliminate the need
for an external intercom power supply, reducing total system
cost, weight, and required mounting space.
Analog Hybrids with Auto Nulling
The 2-wire-to-4-wire hybrids provides 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 45A’s analog circuitry maintains 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, a 2-wire party-line circuit.
The Model 45A’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 software-directed adjustments to account for the resis
-
tive, inductive, and capacitive conditions that are present on the
connected 2-wire party-line circuit. Whenever a user presses the
Model 45A’s auto null button digital circuitry adjusts the analog
hybrids to achieve their maximum return-loss for both interface
channels in less than 15 seconds. While the nulling process is
automatic, it only takes place upon user request. The resulting
null parameters are stored in non-volatile memory.