Studio Technologies 45DC - Manual (Page 19)

Model 45DC User Guide Issue 1, January 2015
Studio Technologies, Inc. Page 19
If possible, prior to performing an auto null
it’s polite to warn all personnel who are ac-
tively 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 mi-
crophones 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 intercom power the mi-
crophones will automatically 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 sup-
port function, allowing call light signals
associated with user devices on two Model
45DC interfaces to work together. The
function also allows a Model 45DC inter-
connected with a Model 45DR Dante to
2-Channel Party-Line Intercom Interface to
have common call light activity. No opera-
tor action is required for the call light sup-
port function to perform its task. However,
placing DIP switch 1, 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 application firm-
ware. 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 volt-
age is summed (added) to any audio that
is present. The Model 45DC detects when
a call light signal is active by monitoring 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 gener-
ate a call signal by applying a DC voltage to
audio path. The DC, approximately 16 V, is
ramped up and down to minimize the addi-
tion of clicks or pops to the audio signal.
While the Model 45DC can detect and gen-
erate a call signal, it’s not possible to direct-
ly send and receive these DC signals over a
Dante interconnection that’s intended only
for audio transport. The Model 45DC works
around this issue by converting the DC call
light signaling to one that’s based on a 20
kHz audio tone. An astute user will recog-
nize 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 signaling, 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 advan-
tage of being above the hearing range of
most humans. This allows normal intercom
audio and a 20 kHz call signal to be active
simultaneously. And transporting this com-
bined talk/call signal over the Model 45DC’s
Dante connection shouldn’t be a problem
as a digital audio path that has a 48 kHz