Sierra Video Manzanita 44VSCP - Manual (Page 5)

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Sierra Video Systems • P.O. Box 2462 • Grass Valley, CA 95945 • (530) 478-1000 • Fax (530) 478-1105
MODEL 44 INSTRUCTION MANUAL
The Model 44’s each have a separate “SYNC”
input. The signal connected to this input can either
be composite sync, composite video or composite
black. The signal is AC coupled to op-amp U7A,
a unity gain buffer. The low impedance output of
U7A drives Q2, a sync separator. The separated
sync at the collector of Q2 connects to two invert-
ers in U9. These inverters are configured as a low
pass filter which separates vertical sync from the
composite sync at the collector of Q2. The output
of the inverters drives the IRQ input of the control
processor, U8.
Audio circuits. The 503146 has two identical
audio channels. One channel has its input and
output connections on edge connector J2. The
other channel uses exactly the same pin numbers
on J3. Only channel one is discussed here. Refer
to the 503146 schematic diagram. Each audio
input is configured for a balanced signal with two
signal connections. R54/R53 and R55/R56 divide
the signal level to one third level and provide a
30K ohm input impedance to the audio signal.
U10 is a dual op-amp with each op-amp con-
nected to one of the input divider outputs. U10’s
only function is to reduce the impedance from
3.3K ohms (the parallel equivalent of each input
divider) to less than a few ohms. The low imped-
ance is needed to keep the unselected audio inputs
from crosstalking into the desired channel. The
outputs of the input buffer op-amps are connected
to four four-input dual channel analog switches,
U11. The entire audio processing path of the
Model 44 is balanced. Balanced switching tech-
nique has a number of advantages:
• Double the signal level for 6dB more dynamic
range.
• Common mode crosstalk is completely canceled
in the output amplifier.
• Using balanced audio and unbalanced video
prevents their interacting when they must exist in
the same environment.
• Switching transients are rejected by the output
amplifier resulting in a silent switch.
The output of the analog switches feeds U6 which
is both a common mode rejection circuit and a
6dB amplifier to make up for the loss of the input
attenuators.
Control circuits. Control of the Model 44’s is
done by a MC68HC711D3 processor, U8. The U8
is a OTP (one time programmable) part. The
software that runs the Model 44 is ROM in U8.
As such U8 can only be replaced with a pro-
grammed part from SVS. Each of the four buses
of the Model 44 has its control in the form of a
two bit input / output of U8. U8 waits for vertical
interrupt at its IRQ input before switching cross-
point. The RS-232 receive input is conditioned by
Q1. The RS-232 transmit output of U8 is inverted
and level shifted by U7B. The Model 44 has a
diagnostic LED connected to an output of U8.
After a reset sequence the LED remains on. If it
does not light there may either be no power to the
Model 44 or a failure of the control program. If
after reset the LED flashes five times then goes
out, it means there is a check sum failure of the
internal ROM. If there are ten flashes, it means
the internal RAM check failed.
Power supply. 24VAC center tapped is produced
by an external power transformer. The low volt-
age AC enters the module on J3 pins 27 and 28. It
is rectified by D1 through D4. C26 and C25 are the
positive and negative primary filters respectively.
U5 produces +5V for the video circuits and control
logic. U6 produces -5 for the video circuits. U14
produces +12V for the audio op-amps. U15
produces -12V for the audio circuits. U12 and
U13 produce + and - 5V for the audio crosspoints
+5V for the MC68HC711D3 processor and
control logic is produced by U4.