Studio Technologies 47A Data Sheet

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

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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