Datasheet

Studio Technologies, Inc. Model 46A Interface, Issue 1, Page 2
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 mil-
liamperes (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-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 46A’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. The party-
line’s conditions are 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 46A’s auto null but-
tons 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 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
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cally 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, 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
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. 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 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, offering a clear view
of the 2-wire DC power and auto null functions.