Sharp RT-1157H - Circuit Description

DOLBY
NOISE
REDUCTION
CIRCUIT
CHECK
(Refer
to
Figure
13—1)
1.
Connect
the
V.T.V.M.
to
the
test
point
(TP102,
TP202}
5.
Give
the
unit
1
kHz
signal,
10
kHz
signal
and
12.5
kHz
and
earth.
signal
in
the
order
and
if
the
V.T.V.M.
reads
as
tabulated
2,
Set
Dolby
NR
switch
(SW5)
in
the
“ON”
position,
and
below
with
each
input
signal,
it is
normal.
input
selector
switch
(SW4)
in
the
“LINE”
position.
3.
Insert
the
tape.
4.
With
the
set
at
record
mode,
give
it
signal
of
—35
dB,
(17.8
mV)
100
Hz
from
line
input
jacks
(J3A,
J@B)
and
adjust
the
record
level
controls
({VR102,
VR202)
so
that
the
V.T.V.M,
reads
32.6
mV.
10 kHz
V.T.V.M.
reads
45.7
—
81
mV
445
—
70mV
6.
Give
the
unit
signal
of
—50
dB
(3.16
mV},
100
Hz
and
7.
In
the
same
manner
as
in
the
step
5
above,
give
the
unit
adjust
the
record
level
controls
(VR102,
VR202)
so
that
1
kHz
signal,
10
kHz
signal
and
12.5
kHz
signa!
in
the
the
V.T.V.M.
reads
5.8
mV.
order
and
if
the
V.T.V.M.
reads
as
tabulated
below
with
each input signal, it is normal.
12.5 kHz
42.6 — 68.2 mV
V.T.V.M. reads 8.2 — 16.4mV
13.7 — 249mvV
TP102
V.T.V.M
Oscilloscope
CR Oscillator
lOOHz, | kHz, 10 kHz, 12.5 kHz
LINE
IN
Jacks
(J3A,
J3B)
Figure 13-1
~13-—
subtracted
from
the
each
other
in
play
mode
showing
an
CIRCUIT DESCRIPTION
DOLBY NOISE REDUCTION SYSTEM
The
construction
of
Dalby
NR
circuit
is
as
shown
below.
DOLSY €8—TYPE INTEGRATED CIRCUIT
SIGNETICS Pegs
9 1) 2 3 4 18 16
SWOE, SWSC
1 NOISE
REDUCTION
Figure 14—1
The
amplifier
circuits
(A)
and
(B)
are
ordinary
type.
But
the
Dolby
Summing
(E)
is
an
amplifier
circuit
that
inverses
only
the
polarity,
having
the
gain
of
1.
Dolby
Side
Chain
(F,
C,
D,
G)
is
high-speed
limiter
that
operate
on
high-
or
medium
frequency.
The
Fig.
14—2
shows
the
input/output
characteristic
curve
of
the
Dolby
Side
Chain
circuit
and
the
curve
varies
according
to
the
change
of
frequency.
The
frequency
characteristic
is
as
shown
in
Fig.
14—3
and
it is
the
same
as
that
of
the
high-pass
filter
of
6
dB/octave.
The
turnover
frequency
of
the
curve
also
varies
according
to
the
change
of
input
level.
If
the
output
signal
from
Dolby
Side
Chain
circuit
shown
in
Fig.
14—2
or
14-3
and
ordinary
signal
from
the
amplifier
circuit
{(B)
are
supplied
to
the
Dolby
Summing
(E)
circuit,
these
two
signals
are
added
to
the
each
other
in
the
record
mode
showing
a
compressor
characteristic
while
they
are
Dolby Side Chain
Input-Output Characteristic
Frequency: SkHz
expander characteristic — the input/output characteristic
shown in Fig. 15-1 is obtained at Rec. Amp. terminal.
Dolby Side Chain Circuit
The
characteristic
of
the
high-pass
filter
of
6
dB/octave
shown
in
Fig.
14—3
is
obtained
by
that
the
composite
imped-
Output
ances
with
C114
(C214),
C115
(C215),
C116
(C216),
R122
(R222),
R145
(R245)
and
R114
(R214),
and
the
filter
impedance
are
added
to
the
each
other
to
form
an
attenuator.
The
input/output
characteristic
in
Fig.
14—2
is
obtained
by
that
DC
voltage
in
proportion
to
the
input
is
fed
back
to
the
attenuator
(F).
The
circuit
(C}
increases
the
output
of
the
attenuator
to
the
specified
voltage
to
supply
the
output
to
the Dolby Summing Circuit. The circuit (D, G} generates
DC
voltage
and
it
also
controls
attack
time
and
recovery
time
of the limiter.
Simultaneously,
AC
signal
is
applied
to
DC
voltage
to
become
NF
signal
being
fed
to
the
circuit
(F},
in
order
to
reduce
the
distortion.
The
Fig.
1S—2
represents
all
the
blocks
oinpur
composing
of
the
Dolby
Side
Chain
circuit
and
shows
how
they are operating.
Described
as
above,
the
Dolby
NR
circuit
compounds
linear
signal
and
non-linear
signal
to
produce
compressor
charac-
teristic
in
record
mode
and
expander
characteristic
in
play-
back
mode
—
if
these
two
characteristics
are
mixed,
an
100
IK
lOoK
original
linear
characteristic
is
restored.
Thus,
noise
is
FREQUENCY
(Hz)
reduced
because
high-frequency
signal
is
attenuated
when
it
passes
through
the
exparider
circuit.
Input ———————
Figure 14—2
INPUT LEVEL —40dB at Dolby Level
TURNOVER FREQUENCY
ABOUT 53 kHz
Figure 14—3
—14—