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1.c) Allpass Phase Scrambler And Preemphasis/Deemphasis ( on
Cards # 3 and # 4)
The
TV
aural
carrier
has
symmetrical
positive
and
negative
overload
points
(+100%
modulation).
Some
music,
and
voice
in
particular,
have
highly
asymmetrical
waveforms.
Therefore,
maximum
loudness
and
minimum
distortion
consistent with the overload constraints of the TV aural carrier are achieved by
processing waveforms to make their peaks more symmetrical.
In OPTIMOD-TV this is achieved by a combination of the crossover network in
the
master/bass
multiband
compressor
and
a third- order
non- minimum- phase
filter.
This
crossover
is
12dB/octave,
and
when
its
outputs
are
summed,
it
provides
a single- order
phase
shift
to
complete
the
phase
scrambler
function.
The
frequency response
of the second stage
of
the phase scrambler
is slightly
peaked,
and provides preemphasis
into the multiband compressor to
improve
its
accuracy. A deemphasis stage after the bands are summed restores flat response.
The
phase
scrambler is a low " Q" circuit which does not introduce ringing. Its
audible effect is extremely subtle. It can be heard as a very slight change in the
"sound" of some voices. Music, in general, is audibly unaffected. Despite the fact
that square waves emerging from the scrambler no longer look like square waves,
the
purist
should
not
fear
that
it
is
degrading
audio
quality.
It
is
in
fact
significantly
improving
subjective
distortion
performance
of
the
system.
2.a) Dual-Band "Master/Bass" Compressor:
(audio on Cards # 3 and # 4; control on Card # 5)
The major
part
of the 8182A
compressor
is the " Master"
channel. This
carries
all
program material above 200Hz. It is a feedback compressor, and its control
voltage
can be summed in a dB- linear manner ( U.S. patent # 4,249,042) with the
control
voltage
developed by the " Bass" compressor to control
the gain
of
the
"Bass" VCA, which passes frequencies between 30 and 200Hz.
The
summation
is
variable
from
none
at
all ( in
which
case
the " Master"
and
"Bass"
bands operate independently, as in a conventional triband compressor) to
unity gain ( in which case the " Bass" channel always takes as much gain reduction
as
the " Master"
channel).
In
the
latter " quasi-wideband"
case,
the
feedback
compressor control
loop in the "Bass" channel
is still active, and causes further
gain
reduction
in
the " Bass" VCA
when
program
material
with
excessive
bass
energy is present. This avoids the pumping which would occur in a fully-wideband
system
if
excess
bass
were
to
force
gain
reduction
of
the
entire
program.
We
feel
that
operating
the
third
band
of
a conventional
triband
compressor
independently
of
the
rest
of
the
bands
yields
very
unnatural
high
frequency
response when auditioned on high quality receivers. In addition, operating the low
frequency
hand
independently
may
result
in
unnatural
frequency
balances
with
certain program material. For this reason, the multiband compressor in the 8182A
is quito dissimilar to other triband units, and offers unprecedented versatility in
combination with very natural, unfatiguing sound.
2.b) Crossover and Bass Clipper: ( on Card # 3 and # 4)
OPTIMOD-TV
employs
a 12dB/octave
crossover.
The
12dB/octave
configuration
is
simply
two
identical
6dB/octave
filters
in
series,
with the
polarity
of
the
"Bass"
band
inverted. It can be shown that the sum of the two outputs has a
perfectly flat magnitude response, but exhibits an overall phase shift. This phase
shift
is
purposely used as
part of
the " phase scrambler" to
make
peaks more
symmetrical.
A-3
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