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5. " Hilbert-Transform Clipper": The Hilbert- Transform Clipper provides the peak
limiting
function,
and
contains
filters
to
assure
that
the
clipping
does
not
introduce out- of- band frequency components above 19kHz.
The
output
of
the
high
frequency
limiter
is
applied
to
a feedforward
analog
computation circuit which develops control voltages for a pair of VCA's, one of
which
handles low
frequencies and one of which handles high frequencies. The
"clipping", which is characterized by essentially
instantaneous attack and release
times,
is
effected
by
the
VCA's
as
controlled
by
the
output
of
the
analog
computer.
The
control
voltage
on
the
VCA's
is
conditioned
by
means
of
a
patented
algorithm
which
permits
no
harmonic
distortion
to
be
introduced
on
frequencies below 4kHz, yet permits frequencies above 4kHz to produce harmonic
distortion. Simultaneously, IM distortion below 2.2kHz is sharply cancelled by an
adaptation of the patented Orban feedforward distortion- cancelling filter to this
circuit topology.
The
result
is
very
low perceived distortion
on both voice and music. Voice is
most severely degraded by harmonic, not IM, distortion. No harmonic distortion is
produced in the voice frequency range, keeping voice clean. Sibilance distortion is
eliminated
by
the
distortion- cancelling
filter.
In
the
frequency
range
in
which
music
has
substantial
energy ( particularly
after
preemphasis),
IM
distortion
is
minimized, optimizing music reproduction as well.
Since the operation of this circuit is complex, further details will be postponed
until Appendix A: System Description.
6. Frequency-Contoured Sidechain (FCS) Overshoot Corrector: The output of the
Hilbert- Transform
Clipper
contains
overshoots
due
to
the
addition
of
the
distortion- cancelling
signal,
and
to unavoidable
overshoots
in
its
integral
1 5kHz
lowpass filter. These
overshoots must be
eliminated
without
adding
out- of- band
frequency components. This is done in the FCS Overshoot Corrector.
The
FCS
circuit
first
derives
that
part
of
the
signal
exceeding
the
1130%
modulation point by means of a " center- clipper". If these overshoots were then
subtracted from the input signal,
the overshoots would be cancelled -- in fact,
doing so would be equivalent to simple clipping. Unfortunately, this can't be done
because the overshoots contain out- of- band frequency components.
The
overshoots
are
therefore
lowpass
filtered
to
eliminate
out- of- band
components. If the overshoot filter had a flat response to its cutoff frequency,
this filtering action would reduce the amplitude of high- frequency overshoots ( by
removing out- of- band harmonics which make the overshoots "spikey"). This would
result
in
incomplete
cancellation
of
the
overshoots
after
subtraction.
The
overshoot
filter
is
therefore
designed
to
have
a rising
response
at
15kHz,
effectively
increasing
the
gain
of
the
fundamentals
of
the
higher- frequency
overshoots
and
compensating
for
the
fact
that
their
harmonics
have
been
removed. The overshoot extractor and
this
filter
are
the "Frequency- Contoured
Sidechain".
The overshoot filter has phase shift. Phase shift networks are therefore included
in
the
main
path
to
make
sure
that
the
overshoot
subtraction
process
works
correctly, and that the overall FCS system has constant time delay.
The
rising
response
of
the
overshoot
filter
means
that
essentially
no
extra
subtraction gain ( compared to the system operated without the filter as a simple
differential
clipper)
is
required.
Any
low
frequency
IM
introduced
by
the FCS
circuit
is
therefore
no
worse
than
the
low- frequency
IM
caused
by
a simple
clipper.
Because
the FCS circuit is an instantaneous system and uses no gain reduction
or dynamic filtering,
it causes neither pumping nor dulling of program material.
1-6
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