Dolby Noise Reduction System - Nakamichi 350 Operating Instructions Manual

make sure no microphones are plugged
into the left and right inputs (this does
not apply to the Blend Mic). .
3) In order to prevent the accidental
erasure of a recording at some future
date, break off the tabs on the top
edge of the cassette as shown in the
photograph below. This will prevent
the recorder from going into the record
mode. Break off both tabs only if both
sides A and B are to be protected. The
tab for any given side of a cassette is
located on its upper left.
The Dolby Noise Reduction System
The Nakamichi 350 incorporates the ''B-Type"'
Dolby Noise Reduction circuitry under license
from Dolby Laboratories, Inc. It is a system
that significantly reduces the '"'hiss'' noise,
which, although inherent in magnetic tape
Dolby Noise Reduction System
recording, is particularly noticeable in cassette
recording because of the narrow track width
and slow tape speed.
Such tape noise is generally distributed over a
frequency range of 2 kHz to 10 kHz. Very
basically, the Dolby System reduces the noise
by amplifying the signals in this range during
record and attenuating them during playback as
shown in Fig. 1.
INPUT — PREAMP. —-| DOLBY |— REC.AMP.
am cg ee {- _, PLAYBACK __
EQ. AMP.
DOLBY | —- PREAMP. -+ OUTPUT
In actuality, however, the system is quite a bit
more complicated because it is clear that if all
high frequency signals were boosted during
record, the tape would become grossly satu-
rated and the sound would be very distorted.
The Dolby System, therefore, is selective in its
boosting: a special threshold sensing circuit
makes sure that only the low level high fre-
quency signals are amplified (the lower the
level, the greater the boost). This same circuit is
applied in reverse during the playback process
to restore flat frequency response. In attenuat-
ing the boosted signals it also reduces the noise.
If high frequency signals (at 4 kHz) of —40dB,
for example, enter the Dolby circuit during
record, they will be boosted to a level of
—30dB before being recorded onto the tape.
Then, during playback, high frequency signals
entering the Dolby circuit at a level of —30dB
will be attenuated to a level of —40dB. The
accompanying reduction in the recorded noise
will be 10dB. The circuit, however, will not
affect signals greater than —5dB (see Fig. 2).
Input Signals
—40 -30 -20 -10 (dB)
'o)
c
ot
n°]
c
--
2 So
3
ry
a
Amplification & Attenuation at 4kHz
Fig. 2 Working Principle of Dolby NR
Circuit
As is clear from the foregoing explanation, it is
possible to gain an overall improvement in
signal-to-noise ratio of approximately 10dB if a
tape is recorded and played with the Dolby
System. The system, furthermore, is interna-
tionally standardized so that any tape recorded
on any machine with Dolby NR can be played
on any other machine with Dolby NR.
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