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MAKING AN ORDINARY RECORDING
1 . Music. Music is made of sounds of dif-
ferent, loudness separated by intervals of
silence. Loud and soft sounds are shown
here as long and short lines. The music re-
presented by this diagram starts loud and
gradually becomes very quiet.
MAKING A DOLBY SYSTEM RECORDING
1 .
What the Dolby System does first.Be-
fore the recording is made, the Dolby Sys-
tem "listens" to the music to find the places
where a listener might later be able to hear
the noise of the tape. This happens mainly
duhng the quietest parts of the music.
When it finds such a place, the Dolby Sys-
tem automatically increases the volume so
that the music is recorded louder than it
would be normally.
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2. Noise. Any recording tape, even the best
kind, makes a constant hissing noise when
played. At the very slow speeds and nar-
row track- widths used in tape cassettes,
tape noise is much more noticeable than
it is in professional tape recordings, al-
though even there it is a problem.
2. The Recording. In a Dolby System re-
cording, the parts of the music which have
been made louder stand out clearly from
the noise. As a result, Dolby System record-
ings sound brilliant and unusually clear even
when played back without the special Dolby
System circuit.
3
.
Music and Noise. When a tape record-
ing is played, the noise of the tape con-
ceals the quietest musical sounds and fills
the silence when no sound should be heard
at all. Only when the music is loud will the
noise be masked and usually not heard.
However, tape noise is so different from
musical sounds that it sometimes can be
heard even then.
3. What the Dolby System does during
playback. When the tapes are played on a
high fidelity tape recorder equipped with the
Dolby System circuit, the loudness is auto-
matically reduced in all of the places at
which it was increased before recording.
This restores the music to its original loud-
ness again. At the same time, the noise
which has been mixed with the music is
reduced in loudness by the same amount—
usually enough to make it inaudible.
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