Hitachi TRK-8800E Operating Manual page 12

HOW THE DOLBY NR SYSTEM WORKS
Making an ordinary recording
1. Music
Music is composed of sounds of different
loudness separated by intervals of silence.
Loud and soft sounds are here shown as long
and short lines. The music shown in this
diagram starts loud and gradually becomes
very quiet.
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 cassette tapes, tape
noise is much more noticeable than it is in
professional tape recordings, although even
there is a problem.
3. Music and Noise
When a tape recording is played, noise from
the tape obscures the quietest musical
sounds and fills the silence when no sound
should be heard at all. Only when the music
is foud the noise is not usually heard;
however, tape noise is so different from
wee that it sometimes can be heard even
then.
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Making Dolbyized recording
1. 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 disturb-
ed by hiss 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. The Recording
In a Dolby System recording the parts of the
music which have been made louder stand
out clearly from the noise. Thus recordings
sound brilliant and unusually 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 cir-
cuit, 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 and
this is by the same amount usually enough to
make it inaudible.