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1.c)
Allpass Phase Scrambler and Preemphasis/Deemphasis: ( on Cards # 3 and # 4)
This filter contains a single- order allpass filter IC303A, R320, R321, R322, C306
followed by
a second- order non- minimum- phase peaking equalizer 1C30313,
R323,
R324,
R325, R326,
R327, C307, C308. The phase response of
the first
section
varies
from
0 to
180
degrees
as
a function
of
frequency,
while
the
phase
response
of
the second section varies from
0 to
360 degrees
as
a function of
frequency.
The
amplitude response
of
the
first
section
is
flat;
the
amplitude
response
of
the
second
section
is
broadly
peaked
at
approximately
200Hz.
To
restore
flat
response, a complementary
dipping- equalizer
section is
inserted
after
the
two
bands
of
the
dual- band compressor
have
been
recombined.
This
circuit consists of IC307A, R359, R360, R361, R362, R363, C319, C320. Its gain
far from 200Hz is - 1.76dB, and it exhibits a second- order dip centered at 200Hz.
2.a)
Dual-Band "Master/Bass"
Compressor (General):
( on
Cards # 3, # 4,
and # 5)
The
dual- band
compressor
consists of
an
audio
path
and
control
circuitry.
We
will
first
discuss the
audio
path
generally.
Details
of
the
VCA
operation
and
control circuitry operation are found immediately below in sections 2.c, 2.d, and
2.e.
2.b) Crossover and Bass Clipper ( on Cards # 3 and #4)
The
crossover
consists
of
12dB/octave
sections.
The
first
6dB/octave
filter
is
located
before
the
VCA,
the
second
after.
Since
the
input
to
the
control
rectifiers
is
taken
from
the
VCA
outputs,
the
control- voltage
crossover
is
6dB/octave.
The
first
200Hz
highpass section
for the " Master"
compressor
is
filter
C309,
R328. The second 200Hz highpass section is C318, R357. The first 200Hz lowpass
section for the " Bass" channel is R342, R343, C314. The second lowpass section
for the " Bass" channel is R367, R366, R365, C321.
A clipper, consisting of biased diodes CR303, CR304, and resistors R367, R366,
is
located before
the
second lowpass section.
Thus
the second lowpass section
rolls
off harmonics created by clipping. ( NOTE: This " distributed crossover with
embedded clipper" system is protected by U.S. patent # 4,249,042.)
In order to force the " Master" and "Bass" channels to add correctly, the polarity
of the " Bass" VCA is inverted by using the appropriate inputs of IC309B ( compare
with IC305B).
2.c) Voltage-Controlled Amplifier (VCA) Operation:
NOTE
This
section
contains
a general
description
of
the
voltage-
controlled amplifier circuitry used through OPTIMOD-TV. The
"Master" VCA will be specifically described.
The
basic operation of the VCA
depends on
a precisely- matched
pair
of
gain-
control
blocks with
differential
voltage
inputs
and
current- source
outputs.
The
gain of each block is controlled by means of a control current.
If
used
alone,
one
such
gain- control
block
would
introduce
considerable
distortion. Therefore, the first of the two matched blocks IC305A is used as the
feedback element
in a high- quality
operational
amplifier, IC304.
The second
of
B-2
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