Studio Technologies 47 - Manual (Page 6)

Issue 1, August 2010 Model 47 User Guide
Page 6 Studio Technologies, Inc.
The Model 47’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 PS20. 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 47 is configured to operate in
its external 2-wire power source mode.
In this mode the Model 47’s circuitry
maintains the required high-impedance
load and, as in all cases, draws no power
from the party-line circuit. An auto-termi-
nate function ensures that should a “wet”
circuit not be connected, the Model 47’s
interface circuitry will remain stable. This
unique feature makes certain that objec-
tionable audio signals, including oscilla-
tions and “squeals,” will infrequently be
sent to the connected 4-wire device.
A significant capability of the Model 47’s
2-wire interfaces is their ability to supply
DC power and 200 ohm AC termination
to both channels (pins 2 and 3) of the
connected intercom circuits. Referred to
as the Model 47’s internal 2-wire power
source mode, the 30 volt outputs can
power devices such as 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 ap-
plications that include both source assign-
ment 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 broad-
cast 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 virtu-
ally eliminates the noise and “hiss” associ-
ated 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 47’s interfaces
achieve excellent audio performance is
the design of the 2-wire-to-4-wire hybrid
circuits. They provide low noise and distor-
tion, good frequency response, and high
return-loss (“nulling”), even when present-
ed 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 frequen-
cy 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