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6)
Frequency-Contoured Sidechain (FCS) Overshoot Compensator:
(on Cards # 8 and # 9)
Overshoots are derived from
the
input
signal
by center clipper IC804A,
R823,
R824, R825, R826, CR803, CR804. This circuit is a differential amplifier which
subtracts the output of a clipper from the clipper's input. This clipper consists of
Schottky
diodes
biased
with
approximately + 4.2v,
and
is
therefore
substantially
"harder" than the first clipper ( associated with IC801A).
If
the
output
of IC804A were simply added to
its
input, the sum would be
a
clipped signal; a " differential clipper" would be created. However, the output of
IC804A contains clipper- induced high frequencies which could cause " aliasing" in a
stereo generator or possible
therefore lowpass-filtered by
C818,
L804,
L805,
before
overshoots. This filter has a
the loss of high frequencies
overshoots
emerging
from
around 15kHz are increased by the frequency- contouring.
interference to the video. The output
of IC804A is
passive 15kHz ladder filter R828, R829, C816, C817,
being
added
back
into
the
input
signal
to
cancel
response that rises 4dB at 15kHz. This makes up for
which would otherwise reduce the peak level of the
the
filter.
To
compensate,
the
fundamental
levels
(NOTE:
Patent
claims
are
pending
on
this "Frequency- Contoured
Sidechain"
overshoot compensation scheme.)
The
filter has phase shift. To assure correct
addition of the filtered overshoot,
the
input
signal
is
delayed
in
a modular
phase
shift
network,
Al,
whose
amplitude response is flat, but which accurately matches the phase response of
the sidechain filter throughout its passband. Al is also equipped with a summing
input
for
the
overshoot
signal,
which
appears
at
the
output
of
IC8048.
The time delay of this network is not constant at all frequencies. The output of
Al
is
thus
passed
through
allpass
network
IC805A,
R831,
R832,
R833,
R834,
C819,
C820
to
create
constant
time
delay
from
the
input
of
the
overshoot
compensator
system
to
its
output.
The
allpass
network
has
a flat
amplitude
response, but
frequency- dependent
phase response. ( This network is
designed to
also
compensate
for
the
non- constant
group
delay
of
the
following
third- order
lowpass filter).
The output of IC805A is passed through a composite capacitor ( consisting of two
aluminum
electrolytics
back-to-back,
bypassed
by
a polycarbonate)
to
remove
accumulated
DC
offsets.
Recent
research
has
suggested
that
this
sort
of
composite
structure
minimizes
the
audible
degradation
caused
by
passing
audio
signals through polar capacitors with high dielectric absorption.
The signal then passes through a third- order active 15kHz lowpass filter IC805B,
R836,
R837, R838, R839, C822, C823, C824 to provide further reduction of any
remaining out- of- band energy above 19kHz.
Finally,
to
catch
any
slight
errors
made
by
the
overshoot
compensator,
the
signal
is
applied
to
safety
clipper
R840,
R841,
CR805,
CR806.
The
basic
overshoot compensator is extremely effective; thus, the safety clipper is virtually
never active and no additional filtering is included after its output. The output of
the safety clipper is buffered by IC806A.
7) Balanced Line Amplifier With Deemphasis: ( on Card # 7)
Card # 7 contains
two
identical
balanced
line
amplifiers
with
provision
for
jumpering
deemphasis
in
or
out.
The
operation
of
this
circuit
in
completely
straightforward, and no further information beyond that
found in Appendix A is
necessary for understanding.
B-12
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