Studio Technologies 45DR - Manual (Page 18)

Issue 2, January 2015 Model 45DR User Guide
Page 18 Studio Technologies, Inc.
“create” the party-line, with the Model 45DR
simply serving as a user device. When
connected to an external party-line circuit
the active LED will light when the voltage
on pin 2 is equal to or greater than approxi-
mately 18. If this condition is present then
normal Model 45DR operation will be able
to take place.
Auto Null
The Model 45DR contains circuitry to au-
tomatically null the hybrid networks as-
sociated with the two party-line interface
channels. This procedure minimizes the
mixing of the audio signals being received
by and sent to the party-line circuit. Nor-
mally the nulling process is performed at
the time of initial Model 45DR configuration
but there’s no reason why it can’t be initi-
ated any time one desires. The only time
that auto null must be performed is if condi-
tions have changed with the party-line user
devices and wiring connected to a Model
45DR’s party-line connectors. Even a small
change to a party-line intercom circuit,
such as adding or removing a section of
cable, is often enough to require that the
auto null process be performed.
A pushbutton switch is provided to acti-
vate the auto null process. To initiate auto
null first requires that the active LED be lit.
When the operating mode is set for lo-
cal power the active LED will light when
current is flowing from the internal power
supply. When the local power LED is not lit
the active LED must be lit, indicating that
sufficient DC voltage is present on pin 2 of
the connected party-line circuit. Once the
active LED is lit initiating the auto null func-
tion only requires pressing and releasing
(“tapping”) the auto null button. The auto
null process takes place on both channels
at the same time. Two LEDs provide a visual
indication of the auto null process, flashing
when the auto null process for its respec-
tive channel is active. If the auto null button
is pressed when the active LED is not lit the
auto null process will not start. The auto
null LEDs will quickly flash on and off a few
times to indicate this condition.
An auto null sequence begins with the mut-
ing of the audio input and output signal
paths associated with the Dante input and
output channels. This is followed by a short
period of 24 kHz signal that is sent to both
channels of the party-line intercom inter-
face. This will turn off microphones on those
connected user devices that are compatible
with the RTS TW-series “mic kill” protocol.
The actual auto nulling process is per-
formed next. A series of tones will be sent
to each of the party-line interface channels.
Other Model 45DR circuitry, under firmware
control, will rapidly perform adjustments
to achieve the best null possible. After the
adjustments have been made the results
are stored in non-volatile memory. Once the
process is complete the Dante audio input
and output paths are again activated.
If possible, prior to performing an auto null
it’s polite to warn all personnel who are
actively using the connected party-line in-
tercom 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 might be a good time to ask them
to mute any active microphones. While the
automatic “mic kill” signal will apply to many
user devices it may not apply to all. Muting
microphones is significant as obtaining a
“deep” null requires that no extraneous sig-
nals be present on the intercom circuit.