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DOLBY NOISE REDUCTION SYSTEM—1
GENERAL DESCRIPTION
The Dolby B-Type Noise Reduction System is used in
this equipment.
It reduces
the level of background
noise,
normally
introduced
during
tape
recordings
without changing the frequency response of the audio
signal.
METHOD
OF DOLBY SYSTEM
Signal deviation of Dolby System is shown in Fig. 1.
Fig. 1 shows
the Record
(Encoding)
and
Playback
(Decoding) characteristics for a signal of about 5 kHz at
different signal levels.
For example, following a level in Fig. 1.
1. When
—30 dB REC signal is applied.
It is boosted
to
~—20
dB
and
recorded
on
a
magnetic tape.
[ENCODED or COMPRESSED]
2. Playing back this recorded tape (at —20 dB).
The output signal is attenuated by the Dolby System
and
reproduced
at —30
dB, the same
level as
recorded.
[DECODED
or EXPANDED]
0 dB Dolby Level
(a =
el
Z.
aah
Sae Ze
OUTPUT
LEVEL
(dB)
20
10
0
+10
INPUT
LEVEL
(dB)
FIG. 1
Tape
DOLBY
ENCODER
REC
PRE AMP
(Rec)
Rec Signal Input
RPH
(Play)
REC
RPH
PRE AMP
(Red)
Fig. 2 shows
the boosting characteristics
of the fre-
quencies for different signal levels.
GAIN
(dB)
nm
is}
TEN
DUAL
HEED
G
0.1
FREQUENCY
(kHz)
FIG. 2
Fig. 3 shows
the
general
boosting
vs.
attenuation
Characteristics
for
record
and
playback
with
the
resulting overall Record
Playback response.
The point at which the Record
and Playback curve
flattens out the 0 dB level and it is called the Dolby
Level.
The Dolby level corresponds
to a level at which the
magnetic flux density of the signal recorded on tape is
200 pwb/mm
and is indicated by +3 dB on the VU
meter of this unit.
GAIN
(dB)
FREQUENCY
(kHz)
FIG. 3
DOLBY
PLAYBACK],
InECODER
MAIN
eee
PRE AMP
(Play)
AMP
DOLBY NOISE REDUCTION SYSTEM—2
PRINCIPLE OF NOISE REDUCTION
Much of the audible noise is a hiss noise in the medium
and
high frequencies
created
during the recording
process on magnetic tape, while some of the noise is
playback amplifier noise.
At high input levels the noise is suppressed, however, at
low input signal levels the Signal to Noise Ratio (S/N)
decreases and the noise is heard.
The Dolby System in this unit is designed to solve this
problem by boosting the low level mid and high fre-
quencies
by about
10 dB
during
recording
(com-
pressing) and attenuating (expanding) the same signals
during
playback
by an equal amount,
as shown
in
Fig. 4.
The amount of boost/attenuation depends on the level
and the frequency of the signal.
Fig. 5 shows the noise reduction with Dolby "IN" vs.
Dolby "OUT".
DOLBY
OUT
Approx
10 dB
—
DOLBY
IN
NOISE
LEVEL
100
1k
10k
FIG. 5
CIRCUIT OPERATION
The operation of the Noise Reduction System is shown
in the Block Diagram of Fig. 6. By turning the Rec/Play
switch to Record or Play and the Dolby In/Out switch to
IN this circuit operates either in the Encode or Decode
mode, as follows:
[ENCODE]:
ST3, ST7, ST11
and ST9 in RECORD
position.
The recording signal is applied to the REC AMP and
passes through AMP "A" of the Dolby IC and the low-
pass-filter
(LPF),
and
enters
AMP
"B"
the
LPF
attenuates unwanted frequencies such as tape recorder
bias of FM MPX sub. carrier signals to the level which
will not influence the Dolby circuit operation.
The out-
put signal of AMP "B" is divided into two paths; one is
directly supplied to ADDER
"E", and is called a direct
signal, while the other controls the noise
reduction
signal. This signal, passing through ST9, S801-3, high-
pass-filter (HPF), variable resistance '"F" and AMP "C",
enters ADDER
"E" and increases the direct signal.
As a result the REC
signal from
INVERTER
"K" is
boosted.
[DECODE]:
Essentially the same circuitry.
ST3, ST7, ST11 and ST9 in PLAY posi-
tion.
The playback signal from the playback head (RPH) is
applied to PLAYBACK
PRE AMP and passes through
ST7, AMP "A" of the Dolby IC, "LPF" and AMP "B",
and then it enters ADDER "E" as the direct signal. One
part of output signal from
INVERTER
"kK", passing
through ST8, is supplied to the MAIN AMP, and the
other,
as
Noise
Reduction
Signal,
passes
through
$801-3, "HPF".
Variable Resistance "F" and AMP "C", are applied to
ADDER "E" in opposite phase. As a result, the level of
the playback signal is attenuated.
This circuit operates
to boost at REC position [ENCODE] and to attenuate at
PLAY position [DECODE].
"G" is the rectifying and smoothing
circuit of Noise
Reduction
Signal
and
controls
Variable
Resistance
"F',
And "F" and "HPF" compose the dynamic high
pass filter, and control the boosting and attenuating
degree of high and medium frequency in response to
the signal level (Fig. 2 and Fig. 3). '"T" is the circuit set-
ting the response time of Dolby.
—20 dB
WU =
Wu"
4
Low Level Signal —— VL
Wu
0 dB———=""_ fj
High Level Signal
——~ 30 dB ——=
"| fP Low Level Signal
—40 dB
Tape Noise
=
www
—e
www
=
—50 dB
wewew
Tape Noise (DOLBY
OUT)
www:
—. — 60 dB
———a=» Wawwww
Tape Noise (DOLBY IN)
FIG. 4
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