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Issue 2, September 2007 Model 46 User Guide
Page 6 Studio Technologies, Inc.
The Model 46’s 2-wire interfaces can
correctly function with powered (“wet”)
or unpowered (“dry”) intercom circuits.
Powered circuits have a DC voltage pres-
ent, typically provided by power supplies
such as the RTS PS31 or Clear-Com
PS-232. This DC power, normally 30-32
volts, provides energy for connected
devices such as user stations or belt
packs. In this type of application the
Model 46 is configured to operate in its
external 2-wire power source mode. In this
mode the Model 46’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-
tion ensures that should a “wet” circuit not
be connected, the Model 46’s interface
circuitry will remain stable. This unique
feature makes certain that objectionable
audio signals, including oscillations and
“squeals,” will infrequently be sent to the
connected 4-wire device.
A significant capability of the Model 46’s
2-wire interfaces is their ability to supply
DC power and 200 ohm AC termination
to the connected intercom circuits. Re-
ferred to as the Model 46’s internal 2-wire
power source mode, the 30 volt output
can power devices such as user stations
and belt packs. In many applications this
will eliminate the need for external inter-
com 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 sup-
ply up to 300 milliamperes of current, a
typical broadcast application that uses two
sets of three BP-325 belt packs can easily
be supported. Each interface’s 30 volt DC
intercom power source ensures that sys-
tems requiring long intercom cable runs
will function correctly. The circuitry’s out-
put regulation is such that little change in
the output voltage will occur over its entire
rated output current. Also, the unique de-
sign of the power supply circuitry virtually
eliminates the noise and “hiss” associated
with typical intercom power supply perfor-
mance. Under software control the output
of the DC power supplies are monitored
for over-current and short-circuit condi-
tions. 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 46’s inter-
faces achieve excellent audio performance
is the design of the 2-wire-to-4-wire hybrid
circuits. They provide low noise and dis-
tortion, 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 46’s analog circuitry provides ex-
tended 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 mea-
sured in dB. A high value is important,
especially in applications where multiple
2-wire-to-4-wire interfaces are used togeth-
er. Remote sports broadcast applications