Datasheet

Studio Technologies, Inc. Model 47A Interface, Issue 1, Page 2
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
-
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
-
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