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Briefly,
the
FCS
Overshoot
Compensator
functions
by
deriving
the
overshoots
from
its
input
with
a " center
clipper"
circuit,
lowpass-filtering
the
overshoots
with a fifth- order passive LC filter to remove out- of- band frequency components,
and then
mixing the filtered overshoots out- of- phase with a delayed version
of
the input signal. This delay, created by an encapsulated active allpass network, is
identical to the delay in the overshoot filter, thus assuring that the input signal
and filtered overshoots arrive in the same place at the same time.
If
no
filter
were
used,
this
process would
be
identical
to
clipping
the
input
signal, and would create a " differential clipper". However, the overshoot filtering
process reduces the peak level of the high frequency components of the overshoot
by removing harmonics. To compensate for this loss of peak level ( which would
cause
less than
full
cancellation
of
overshoot),
the
frequency
response
of
the
overshoot filter rises at 15kHz -- thus increasing the level of the high- frequency
fundamental to compensate for the loss of harmonics. This is why the system is
called "Frequency- Contoured Sidechain".
The final
sum of
input
and out- of- phase filtered overshoot
is passed through
a
third- order
lowpass
filter
to
provide
further
attenuation
of
unwanted
high
frequency energy. Phase
correction is applied to the combination of
this
filter
and the overshoot filter. ( The phase response of the overshoot filter is identical
to
its
matched
main- path
delay
network --
thus
the
phase
correction
also
corrects
the
response
of
the
main- path
delay
network.)
This
phase
correction
makes
the
overall
time
delay
through
the
entire FCS
Overshoot
Compensator
approximately
constant,
and
assures
that
the
various
filters
within
the
compensator do not upset carefully controlled peak levels in unpredictable ways.
If any very unusual waveforms cause residual overshoots, these are dealt with in
a safety
clipper
at
the
output
of
the
FCS
Overshoot
Compensator
system.
However, the basic circuit is so effective that this safety clipper is rarely active.
7) Balanced Line Amplifier and Deemphasis: ( on Card # 7)
The
line
amplifier
is totally straightforward. It
consists of
a pair
of
inverting
opamps.
These
are
5532
devices
which can
drive
600
ohm
loads
directly.
The
feedback resistor of the first opamp can be shunted by a capacitor ( selected by
a plug-in jumper) to effect deemphasis if desired.
The
second
opamp
is
a unity- gain
inverter
driven
by
the
first
opamp.
The
outputs
of
the
two
opamps
thus
provide
an
output
balanced
to
ground
which
drives
a non- overshooting
EMI
filter
to
interface
to
the
outside
world.
The
balanced driving capability of the circuit is approximately + 20dBm into 600 ohms.
8) Power Supplie=
Primary
power
for
the OPTIMOD-TV
circuitry
comes
from
a highly
regulated
+15 volt power supply. The main supply is + 15 volts. This is controlled by means
of
a 723C
IC
regulator
with
current- boosted
output,
current
limiting,
and
overvoltage protection using a zener diode and fast-blo fuse.
The - 15
volt
supply
is
essentially
a current- boosted
opamp
in
a unity- gain
inverting
configuration
which " amplifies"
and
inverts
the + 15
V supply,
thus
"tracking"
it.
The - 15
volt
supply
is
also
current- limited
and
overvoltage
protected. Both + 15 and - 15 supplies are located on a non- plug-in card mounted
on the inside of the rear chassis apron. This apron is also used as a heat sink for
the regulator power transistors.
A-10
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