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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
15kHz
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
100%
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
fiat
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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