Manual

Model 45DC User Guide Issue 3, March 2016
Studio Technologies, Inc. Page 19
External Party-Line Circuit
Operation
When an interface channel’s local power
LED is not lit that interface is intended to be
connected to an external party-line circuit.
The external circuit must provide power
and termination impedances to “create”
the party-line, with the Model 45DC simply
serving as a user device. When connected
to an external party-line circuit the active
LED associated with that interface channel
will light when the voltage on pin 2 is equal
to or greater than approximately 18. If this
condition is present then normal Model
45DC operation will be able to take place.
Auto Null
The Model 45DC contains circuitry to auto-
matically null the hybrid networks associ-
ated with the two party-line interfaces.
This procedure minimizes the mixing of the
audio signals being received by and sent
to each party-line interface circuit. Normally
the nulling processes are performed at the
time of initial Model 45DC configuration but
there’s no reason why either or both can’t
be initiated any time one desires. The only
time that an auto null must be performed is
if conditions have changed with the party-
line user devices and wiring connected to
a Model 45DC’s party-line connector. Even
a small change to a party-line intercom cir-
cuit, such as adding or removing a section
of cable, is often enough to require that the
auto null process be performed.
Two pushbutton switches are provided
to activate the auto null process for each
interface. To initiate auto null first requires
that an interface’s active LED be lit. When
an interface’s operating mode is set for
local 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 an auto null func-
tion only requires pressing and releasing
(“tapping”) the appropriate auto null but-
ton. An LED provides a visual indication of
the status of the auto null process, flashing
when the auto null process is active. If the
auto null button is pressed when its as-
sociated active LED is not lit the auto null
process will not start. The auto null LED
will quickly flash on and off a few times to
indicate this condition. Only one auto null
process can be active at any one time.
Nothing will happen if either auto null but-
ton is pressed when the auto null process is
active for an interface. (An interface’s mode
can be changed when the auto null is active
on the other interface.)
An auto null sequence begins in this man-
ner: If local power is being provided to the
party-line interface by the Model 45DC it will
momentarily turn off and then turn on again.
This will act as a “mic kill” signal, ensuring
that microphones associated with the con-
nected user devices will be placed in their
off state. Then the audio input and output
signal paths associated with the Dante
input and output channels will mute. Next a
series of tones will be sent to the party-line
interface’s audio path. Other Model 45DC
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 nonvol-
atile 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