Manual

Issue 3, March 2016 Model 45DC User Guide
Page 20 Studio Technologies, Inc.
are actively using connected party-line
intercom devices. The tones sent to the
party-line circuit during the nulling process
are not excessively loud or obnoxious,
but most users might want to remove their
headsets during the process. In addition
to warning users, it’s a good idea to ask
them to mute any active microphones.
Muting the microphones can be significant
as obtaining a “deep” null requires that
no extraneous signals be present on the
intercom circuit. As previously discussed,
if the Model 45DC is supplying local inter-
com power the microphones will automati-
cally be muted. But for applications that
connect to an existing party-line intercom
circuit the user’s must manually mute the
microphones.
The Model 45DC provides a call light
support function, allowing call light sig-
nals associated with user devices on two
Model 45DC interfaces to work together.
The function also allows a Model 45DC
interconnected with a Model 45DR Dante
to 2-Channel Party-Line Intercom Inter-
face to have common call light activity. No
operator action is required for the call light
support function to perform its task. How-
ever, placing DIP switch SW1, located on
the back panel, to its on (up) position will
disable the function.
USB Interface
A USB type A connector and associated
status LED is located on the back panel
of the Model 45DC. This data interface is
used only for updating the unit’s applica-
tion firmware. No audio data of any kind
will pass through it. For details please
refer to the Technical Notes section of
this guide.
Technical Notes
Call Light Support
A “call” or “call light” indication on a Clear-
Com party-line intercom circuit is transmit-
ted by way of a DC voltage that is applied
to the audio path, which is typically pin 3
of the interconnecting cable. This DC
voltage is summed (added) to any audio
that is present. The Model 45DC detects
when a call light signal is active by moni-
toring the audio path for the presence of
a DC voltage. A signal of approximately
5 Vdc or greater is needed to indicate that
the call function is active. The Model 45DC
can also generate a call signal by applying
a DC voltage to audio path. The DC, ap-
proximately 16 V, is ramped up and down
to minimize the addition of clicks or pops
to the audio signal.
While the Model 45DC can detect and
generate a call signal, it’s not possible to
directly send and receive these DC signals
over a Dante interconnection that’s intend-
ed only for audio transport. The Model
45DC works around this issue by convert-
ing the DC call light signaling to one that’s
based on a 20 kHz audio tone. An astute
user will recognize this as the call method
used by the TW-series from RTS; instead
of signaling via DC in the audio path a
20 kHz signal is used. In the “telco” world
this would be referred to as in-band sig-
naling, not dissimilar to the touch-tone
dialing method that’s used on analog
telephone lines. But unlike touch-tone
signals, a 20 kHz signal has the advantage
of being above the hearing range of most
humans. This allows normal intercom
audio and a 20 kHz call signal to be ac-
tive simultaneously. And transporting this
combined talk/call signal over the Model