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What it will and will not do.
The
Dolby
Noise
Reduction
circuits of
the 600 Ii reduce the hiss inherent in the
tape recording process. The Dolby system
cannot
reduce
the noise of your source
material. If your records, FM broadcasts,
microphones, and other sources are noisy,
the cassette deck's Dolby system will not
improve them. But the Dolby system will
reduce tape noise by as much as 10 dB.
The Dolby system has an encode (record)
and a decode (playback) process. Dolby-
encoded
cassettes
should
be labeled
as
such;
when
played
back
without
the
proper Dolby decoding, they will sound
unnaturally 'bright'. Conventional
(non-
Dolby)
cassettes
will
sound unnaturally
"dull''
when
played
back
with
Dolby
decoding.
This boosting
and cutting
of
the high frequencies has much to do with
how
Dolby
Noise
Reduction
is accom-
plished.
How the system works
The
Dolby
encoder
boosts
the
high-
frequency program content before it goes
onto the tape. The Dolby decoder gives
the high frequencies equal and opposite
treatment
during
playback.
While
the
decoder
returns the high-frequency
con-
tent to original levels, it simultaneously
reduces the high-frequency noise incurred
in the recording process.
Because
noise is more
noticeable during
quiet passages than during loud passages,
the Dolby
system does not treat all high
frequency signals equally. It gives more of
a
boost
to
low-level
high-frequency
signals than it does
to high-level
high-
frequency
signals. This variation
of the
Dolby system's effect across the dynamic
range
distinguishes
Dolby
Noise
Reduc-
tion from simply "turning up the treble"
on record and ''turning down the treble"
on playback.
"Turning
the treble' up and down is, in
The Dolby System
fact, the basic idea behind
tape record
and playback equalization,
RIAA phono
equalization,
and
FM_
pre-emphasis/de-
emphasis.
All
of these
systems
involve
boosting the high frequencies at one end
and equal reduction
of high frequencies
at the other.
All work to reduce
noise.
But the amount of boosting in each case
is limited by the headroom characteristics
of
the
medium.
In
the
case
of
tape
recording,
too
much
equalization
(too
strong
a
high-frequency
boost)
will
saturate the tape and distort the record-
ing. Hence there is alimit to the amount
of
noise
reduction
attainable
through
equalization alone. FM pre-emphasis and
phono
equalization
face
similar
limita-
tions.
The Dolby
Noise Reduction system does
not,
because
the
Dolby
system
only
boosts low-level signals. This provides an
additional
10 dB of noise reduction with-
out
threatening
the
headroom
of
the
medium.
Why is level calibration necessary?
The "'rec cal' controls of the Nakamichi
600 {I are used in conjunction
with the
400
Hz reference test tone. The adjust-
ment assures that a tone recorded at 0 dB
will play back at 0 dB. Although
tape
formulations
of the same category (Page
6 ) pose the same
equalization require-
ments,
variations
in tape sensitivity will
cause
one
formulation
to provide more
output
than
another
formulation.
The
controls
permit
you
to adjust the deck
for the sensitivity
of each formulation
you use.
Without
calibration,
the
Dolby
circuits
could
potentially
mis-track
on different
formulations. Recording on a more sensi-
tive
formulation
without
recalibrating
would mean that a test tone recorded at 0
dB would play back at higher than O dB.
The Dolby decoder would fail to provide
the full high frequency cut, and the tape
would
sound
slightly
'bright'
on play-
back.
Conversely,
recording
on
a
less
sensitive tape without recalibrating would
produce slightly "dull" playback.
Level
calibration
is essential
for Dolby system
compatibility from cassette to cassette.
Level
(dB)
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