Studio Technologies 45A - Manual (Page 19)

Model 45A User Guide Issue 2, November 2011
Studio Technologies, Inc. Page 19
will not be connected to the Model 45A,
but only to the user devices. In this sce-
nario, a DC voltage won’t be present on
pin 2 or pin 3. Another example of this
might be where adapter boxes with DC
blocking are used to interconnect inter-
com circuits with the Model 45A. In both
these cases a DC voltage won’t be pres-
ent on either pin 2 or 3 and the Model 45A
will automatically apply 200 ohm termina-
tions. These terminations would be in
error, resulting in incorrect audio levels
due to a double-termination condition.
To prevent this problem the auto terminate
function can be disabled. When configura-
tion DIP switch 4 is in its off (down) posi-
tion the auto terminate function is active.
When switch 4 is in its on (up) position the
auto terminate function associated with
each interface is disabled.
The auto terminate disable function should
be used only when absolutely necessary;
it’s possible that a significant downside
could be experienced. With auto terminate
disabled it’s important that properly ter-
minated 2-wire party-line intercom circuits
be connected to pins 2 or 3 of the Model
45A’s 2-wire (PL) interface. If they are not
connected, it’s likely that audio oscilla-
tions, noise, and distortion will be gener-
ated in the Model 45A’s 2-wire-to-4-wire
converter circuitry. These audio artifacts
will not cause any damage but will be
sent out of the 4-wire line outputs. Users
of the 4-wire equipment may be less than
pleased with what they hear!
4-Wire Mute during Auto Null
As part of the auto nulling process audio
tones are generated and sent out the
2-wire intercom interfaces. These tones,
depending on the nulled state of the
hybrid circuits, will also appear on the
outputs of the 4-wire interface circuits. To
prevent these unwanted tones from being
sent to the inputs of the connected 4-wire
devices by default the 4-wire outputs mute
when the auto nulling process is active.
There may be cases, primarily at the fac-
tory, where it’s desirable to have the null-
ing tones appear on the 4-wire outputs.
Configuration DIP switch 5 allows the mut-
ing of the 4-wire outputs to be disabled.
When switch 5 is in its off (down) position
the Model 45A operates in its standard
fashion. When switch 5 is in its on (up)
position the 4-wire outputs will not mute
during the auto nulling process. As ex-
pected, when the Model 45A is deployed
in the field switch 5 should remain in its off
(down) position. However, no damage to
the Model 45A or connected equipment
will occur when muting during auto null is
disabled.
Operation
Upon power-up the Model 45A will always
go through a short initialization sequence
before normal operation will begin. The
LEDs will light sequentially and, upon
completion, the applicable input power
LED will light. The null settings for the
two hybrid circuits are recalled and used,
having been previously stored in nonvola-
tile memory.
Significant user intervention is typically not
required during normal Model 45A opera-
tion. The unit is designed for continuous
operation with no routine adjustment or
maintenance necessary. Assuming that
the desired configuration has been select-
ed using the DIP switches, only occasion-
ally activating the auto null function should