Studio Technologies 45A - Manual (Page 8)

Issue 2, November 2011 Model 45A User Guide
Page 8 Studio Technologies, Inc.
level also applies to their RVON-I/O VoIP
products. As such, reducing their nominal
input and output levels by 4 dB will ensure
optimal compatibility.
The Model 45A contains four 5-segment
LED level meters with two displaying the
level of the signals being received on the
4-wire inputs and two displaying the level
being sent out the 4-wire outputs. During
installation and setup the meters are in-
valuable in helping to confirm that proper
operation is taking place. The meters are
also useful during normal operation, allow-
ing confirmation of audio signal flow into
and out of the Model 45A.
Pro Audio Quality
The Model 45A’s audio circuitry was
designed in the spirit of professional
audio equipment, rather than that found
in typical party-line intercom gear. High-
performance components are used
throughout, providing low distortion, low
noise, and high headroom. Using passive
and active filters, the frequency response
is limited to nominally 100 Hz to 8 kHz.
This range was selected to provide excel-
lent performance for human speech, while
maximizing the ability of the hybrids to
create substantial “nulls.” When the Model
45A’s internal DC power source is se-
lected to provide 2-wire party-line intercom
power enhanced audio performance can
also be expected. The quality of the DC
supply circuit is very good, with very little
noise, hum, or “hiss” being added to the
2-wire connection. In addition, the imped-
ance characteristics of the interface’s DC
powered (“wet”) channel was tailored
to be essentially identical to that of the
unpowered (“dry”) channel. This unique
situation allows the automatic nulling
circuitry to provide excellent, consistent
results for both the powered and unpow-
ered channel.
Attention to detail is a hallmark of the
Model 45A’s design. For example, during
the brief auto nulling process the interface
channel’s 4-wire input and output signals
are normally muted, preventing unwanted
audio from reaching the connected equip-
ment. Associated with the 2-wire party-line
interface is circuitry that, under software
control, applies 200 ohm terminating
impedances to the 2-wire party-line circuit.
This, along with other circuitry that moni-
tors DC voltages present on pins 2 and
3 of the 2-wire circuit, ensures that audio
instability associated with unterminated
circuits will rarely occur.
Special Applications
While the Model 45A is designed to di-
rectly integrate into typical applications,
it’s ready to support the “one-in-a-million”
situations too. To accomplish this five DIP
switches, accessible on the front panel,
allow control over some of the unit’s
features. For example, one DIP switch
changes the way the auto null pushbutton
switch operates. Normally pressing the
button begins the process of auto nulling
both channels. The alternate switch mode
allows each auto null sequence to be
activated independently. One “tap” of the
auto null button will begin the auto null
process for channel 1. Two “taps” of the
button will begin the auto null process
for channel 2. Another DIP switch allows
the auto terminate feature to be disabled.
While this feature can help maintain audio
quality, advanced users might need
to disable it. This would allow full control
over the Model 45A’s two hybrid circuits,
enabling them to be used in a completely
independent manner.