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In
OPTIMOD-TV,
this
crossover
is
realized
as
a " distributed
crossover" ( U.S.
patent # 4,249,042).
This
means that
the first
6dB/octave section
is before the
VCA,
and
the
second
6dB/octave
section
is
after
the
VCA
and
the
control
voltage
rectifier.
The
control
voltage
circuitry
is
therefore
fed
from
a
6dB/octave crossover only.
The advantage of this configuration is that it permits insertion of a soft clipper
immediately after the " Bass" VCA to eliminate overshoots which would otherwise
intermodulate with the output from the " Master" VCA when the sum of " Bass"
and " Master"
is
preemphasized
and
clipped.
The
second
part
of
the " Bass"
crossover is after the " Bass" clipper, thus lowpass-filtering the clipper output and
rolling off harmonics and out- of- band IM introduced by the clipping process. In-
band IM is negligible because of the relatively narrow bandwidth processed by the
"Bass" channel.
The sum of the " Bass" and " Master" channels is applied to a deemphasis network
to " undo" the preemphasis introduced in the phase scrambler circuitry ( see 1.c)
2.c) Voltage-Controlled Amplifier (VCA) Operation:
The
voltage- controlled gain block used throughout OPTIMOD-TV
is a proprietary
Class- A VCA which operates as a two- quadrant analog divider with gain inversely
proportional to a current injected into the gain- control port. A specially- graded
Orban
IC contains two
matched non-linear
gain- control
blocks with differential
inputs and current outputs. The first of these is employed in the feedback loop
of
an
opamp to perform the gain control
function. The inputs of the first and
second
gain- control
blocks
are
connected
in
parallel,
and
the
output
of
the
second
block
is
a distortion- corrected
current
which
is
transformed
into
the
desired
gain- controlled
voltage
by
means
of
an
opamp
current- to- voltage
converter.
For
most
gains,
levels,
and
frequencies,
THD
is
well
under
0.1%.
Overload- to- noise ratio ( noise measured in a 20-20,000Hz band) is typically 90dB,
and is constant with respect to gain and level.
2.d) Compressor Control Circuitry: ( on Card # 5)
Each
compressor ( left
and right " Master"
and left
and
right " Bass")
feeds
its
own rectifier with threshold. The drive to the clippers following the compressors
and
preemphasis/high-frequency
limiters
is
determined
by
the
setting
of
the
CLIPPING control, which simultaneously adjusts all rectifier thresholds ( and thus
the average compressor output level). Left and right rectifier pairs ( which have
current- mode high- impedance outputs) are " OR"ed into individual timing circuitry
for " Master" and "Bass" channels.
This
timing circuitry is proprietary, and is
located within
sealed modules.
The
"Master" timing circuitry is most critical to achieving natural sound. It performs
the following functions:
1)
A
peak
limiting
function
with
very
fast
recovery
time
for
transient
material;
2)
A slower compression function whose recovery time is a function of gain
reduction; and,
3)
A
recovery- delay
function
which
provides
extra
smoothing
of
the
gain
control voltage to avoid low frequency distortion even with fast release times.
A-4
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