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HOW THE DOLBY NR SYSTEM WORKS
7. Music
Music
is composed
of sounds
of different
-
loudness separated by intervals of silence.
Lound and soft sounds are here shown as
long and short lines.
The music shown by
this
diagram
starts
loud
and
gradually
becomes very quiet.
7. What the Dolby System does first
Before
the recording
is made,
the Dolby
system
"'listens'' to the music to find the
places where a listener might later be able to
hear the noise from the tape.
This happens
mainly
during
the
quietest
parts
of the
music.
When
it finds such a passage,
the
system automatically increases the volume so
that music is recorded at a higher level than it
would be normally.
2. Noise
Any recording tape, even of the best kind,
makes
a
constant
hissing
noise
when
played.
At the very slow speeds and narrow
track widths
used
in tape cassettes,
tape
noise is much more noticeable than it is in
professional tape recordings, although even
there it is a problem.
2. The Recording
In a Dolby system recording, the parts of the
music which have been made louder stand
out Clearly from the noise.
Asa result, Dolby
system
recordings
sound
brilliant and un-
usually clear even when played back without
the special Dolby system circuit.
3.
What the Dolby System does during
playback
When tapes are played on a high-fidelity tape
recorder
equipped
with the Dolby system
circuit, the loudness is automatically reduced
in all of the places at which it was increased
before recording.
This restores the music to
its original level again.
At the same time, the noise which has been
mixed with the music is reduced in level by
the same amount, usually enough to make it
inaudible.
3. Music and Noise
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