Data Sheet

Studio Technologies, Inc. Model 47 Interface, Issue 2, Page 2
user stations and belt packs. 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 47 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 47’s 2-wire interfaces able to supply up
to 300 milliamperes of current on each channel, a typical
broadcast application that uses four sets of three BP325 belt
packs 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 conditions. 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 47’s interfaces achieve excel-
lent 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 47’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 47’s
sophisticated auto nulling function uses analog circuitry under
micro-processor control to achieve significant trans-hybrid loss.
This return-loss “null” is achieved by making a series of adjust
-
ments 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 47’s “auto null”
buttons 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. How
-
ever, 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 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 47’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 47’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 47 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