Studio Technologies 45A - Manual (Page 6)

Issue 2, November 2011 Model 45A User Guide
Page 6 Studio Technologies, Inc.
2-Wire Party-Line Interface
The Model 45A’s two-channel 2-wire party-
line interface is optimized for direct connec-
tion with a dual-channel party-line intercom
circuit. In addition, single-channel party-
line intercom circuits can also be connect-
ed. Many broadcast applications use the
dual-channel TW-series from RTS®, which
includes their popular BP325 belt pack.
The Model 45A’s 2-wire interface is config-
ured for a –10 dBu nominal level, exactly
matching the level of the TW-series. Other
industry-standard single- and dual-chan-
nel party-line intercom systems, including
those from Clear-Com®, are also directly
compatible. While their nominal level is
approximately 4 dB lower (–14 dBu) their
greater dynamic range tends to compen-
sate for any operational differences.
The Model 45A’s 2-wire party-line interface
is typically connected to a powered (“wet”)
unbalanced intercom circuit. Powered
party-line circuits have a DC voltage pres-
ent, typically provided by power supplies
from RTS or Clear-Com. This DC power,
normally 30-32 volts, provides energy for
connected devices such as user stations or
belt packs. A configuration setting is used
to select how the Model 45A will impact
the connected 2-wire circuit. In the default
mode the Model 45A’s circuitry maintains
a high-impedance load and draws no
power from the party-line circuit. An alter-
nate mode allows the power required by
the Model 45A’s circuitry to be supplied by
the connected party-line circuit.
A significant capability of the Model 45A’s
2-wire interface is its ability to create a
fully functioning 2-wire party-line intercom
circuit, supplying the required DC power
and 200 ohm AC terminations. Referred
to as the Model 45A’s 2-wire (PL) power
source mode the 30 volt, 300 milliampere
maximum output can power devices such
as user stations and belt packs. In many
applications this will eliminate the need for
an external intercom power supply. Besides
reducing total system cost, this feature can
also lower system weight, reduce required
mounting space, and decrease the mains
energy requirement.
With the 2-wire interface’s ability to supply
up to 300 milliamperes of current a typical
broadcast application which uses two or
three BP325 belt packs can easily be sup-
ported. The circuitry’s output regulation is
such that little change in the output voltage
will occur over its entire rated output cur-
rent. The power source is an “industry-
standard” 30 volts DC, helping to ensure
that applications requiring long intercom
cable runs will function correctly. Also,
the design of the power supply circuitry
helps to minimize the noise and “hiss”
often associated with intercom power
supplies. Under software control the DC
power supply’s output is 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.
An auto terminate function ensures that
should a party-line circuit not be connected,
the Model 45A’s interface circuitry will re-
main stable. This unique feature makes cer-
tain that objectionable audio signals, such
as oscillations and “squeals,” will rarely be
sent to the connected 4-wire device.
Analog Hybrids with Auto
Nulling
A key reason that the Model 45A achieves
excellent audio performance is the design
of its 2-wire-to-4-wire hybrid circuits. Each