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6.3
CD-4 DEMODULATOR SECTION
Fig.
5
illustrates the composition of this section.
Sum Signal System
IC
HA1452
is an orthodox
2-channel
equalizer
amplifier.
In
CD-4 operation,
a
variable resistor
is
inserted
in
the NFB circuit
to
provide separation
control by
varying
the
main signal (sum
signal)
gain. Although
the final
objective
of
the
CD-4
demodulator is
to
matrix the
sum and difference
nN452-%
r.
P.
t.
Separati
on
control
RECORDING
AND PLAYBACK OF
CD.4
DISCS
The CD-4 disc
is a
recent development. Being
a
"Discrete"
4-channel
medium,
it
features
excellent
channel separation
when played over
suitable
4-
channel equipment,
but
can also be played
as
a
conventional 2-channel stereo record.
Fig.
6
shows
the configuration
of
signals present
in
a
CD-4 record.
Each
of
the two
sub-signals occupies
a frequency
modulated supersonic carrier
with
a
center
fre-
quency
of
30kHz.
The
sub-signal
conveys
the "Front-Rear"
differ-
ence
information.
The main
signals are recorded as
a
conventional
LEFT
CHANNEL
signals,
as
the
difference signal
is
demodulated
from a
frequency modulated 30kHz carrier
(sub
signal), and the sum signal varies according
to
the
cartridge output level (though indirectly related),
level
matching
becomes necessaq/.
In
other than
the
CD-4 mode,
a fixed
resistor
replaces
of
the variable resistor
to
provide a fixed
gain (35.6d8
at
lkHz)
equalizer amplifier.
The
inclusion
of
a
balanced
power supply
with
this
circuit
maintains input and output point potentials
at 0V,
preventing
click
noises
when switches
are
operated.
The
100kS2 impedance
of
this circuit
is
changed
to
50kA by
inserting
two 100k0
resistors
in parallel during other than
CD-4
operation.
A.
11.
R.
S.
Fig.
5
stereo
record, occupying
the
30Hz
-
15kHz
audio
band and conveying the
"Front+Rear"
sum
inform-
ation.
From
these
sum and difference
signals,
the original
4
channel signals
are
retrieved
in a
series of
algebraic operations
performed in the demodulatori
(Lf+Lr)+(Lf-Lr\=ZLf.
(Lf+Lr)-(Lf-Lr)=2Lr
(Rt+Rr)+(Rf-Rr)=2R,t
(Rf+Rr)-(Rf-Rr):2Rr
where
(6R"
stands
for
Right,
"L"
for
Left,
"f"
for
front,
"r"
for
rear.
DISC
SURFACE
R]GHT
CHANNEL
J
UJ
llJ
-J
t
J
lrJ
lrJ
J
t
HAI
335
Hl
Aoc,'p
r
i
[)e.emnhasis
llr-r
I
f-{-l
STYLUS
20kHz
20kHz
Fis.6
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