Studio Technologies M78 SV1 - Manual (Page 3)

Model 78 Special Version 1 User Guide Issue 2, December 2002
Studio Technologies, Inc. Page 3
for Surround
An isolated contact is connected in a
similar fashion. First jumper pins 2 and 3
together. Then connect pin 4 to one side
of the contact and pin 1 to the other side
of the contact. Pin 1 on the Model 78 is
connected to ground, allowing the current
into the GPI to complete. Note that pin 1
is connected to the Model 78’s chassis,
as well as the mains connector ground
lead and the power supply commons.
A +5V logic signal is also simple to con-
nect. The + connection of the logic signal
would connect to pin 3, the GPI’s + input.
The – connection of the logic signal would
connect to pin 4, the GPI’s – input. As the
Model 78’s GPI input is fully isolated, no
ground loops or other issues should arise.
Mains Power
Mains power should be connected accord-
ing to the details provided in the user guide.
Operation
Upon application of AC mains power,
the Model 78 will go through a power-up
sequence then begin operation. Depending
on the state of the GPI input, input A or
input B will be selected. As a confirmation,
the data LED will display which input is
active. The LED will flash once per second
if input A is selected and twice per second
if input B is selected.
Technical Notes
Operating Levels
The Model 78 Special Version 1 is config-
ured for unity gain, with a +4dBu nominal
output level. The input trim potentiometers
allow signals with nominal levels of –12dBV
to +6dBu to be correctly interfaced. Cali-
bration is very straight forward. Essentially
it requires application of a steady signal at
the input sources’ reference level. Then,
while the output channels are monitored, the
input trim pots are adjusted to give +4dBu.
The procedure listed in the Model 78 User
Guide should be reviewed for details.
Switching Characteristics
To meet the requirements of post-require-
ment audio applications, a special switching
characteristic was implemented in software.
A 10mSec make-before-break sequence is
always active when switching between
inputs. This ensures that no audio dropouts
will be introduced. To highlight this feature,
take the situation where input B is active
and the GPI indicates that input A is de-
sired. While input B is still active, input A will
also be selected. Then after a 10mSec
pause input B will disconnect. This overlap
will have both inputs active for 10mSec.
Note that the GPI has a 20mSec
“debounce” associated with detecting a
state change. This is implemented to ensure
that false GPI signals will not be detected.
So to change between inputs requires a
total of 30mSec. The debounce takes
20mSec and the make-before-breaking
function takes 10mSec.
Hardware and Software Modifications
At the factory, three changes are made
to a standard Model 78 Central Controller
to become a “Special Version 1.” As previ-
ously mentioned, the control console output
voltage is modified from +16V to +5V. This
is accomplished by changing one resistor
on the circuit board. To implement the GPI
one jumper strap is added to pins on the
Model 78’s circuit board. This routes the
signal that normally connected MIDI data
to the processor to a pin that can “read”
the state of the optical coupler. The third
change is the use of a special version of