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The Dolby System Explained
To understand how the Dolby System works and
how it
achieves
noise reduction without altering the
music, let's first look at
conventional
recording,
that is,
recording without the Dolby
System.
Ii
11,
I
I
Conventional
Recording
,
Music. Music consists of
sounds
of different loudness
separated
by intervals of
silence.
Loud and
soft sounds
are
shown
here
as
long and
short
lines. The music repre-
sented
here
starts
loud and gradually becomes
very
quiet.
oi e. Any recording tape
(even
the best
"low
noise"
kind)
contains inherent noise,
much
as even
the best
photographic film has
a
discernible
"grain."
Music and oise. When
a conventional
tape recording
is
made,
the noise
of
the tape
conceal
the quietest
musical
sound
and fills the ilence when no
sound
should
be heard
at
all. Only when the
music
i loud is
the noi
e
not usually heard.
~
Recording with
the
Dolby System
The
conventional signal
to
be
recorded passes through
the record Dolby proces
or
. Loud parts
of
the
music
pass through unchanged. The quieter high frequency
portions of
the mu
ic are automatically
made louder, to
a
degree dependent upon how quiet they are. The quietest
high frequencies
are
increased the
most,
up to 10
dB.
This processed
signal is appropriately calledDolby-
processed, or
"Dolbyized."
The
recording:
When the Dolby-processed
signal is
recorded
onto tape,
the parts of the music that have been
made louder stand out
clearly
from
the inevitable noi
e.
uch recordings are called Dolby-processed tapes.
When
a
Dolby-processed
tape is
passed through the
Dolby playback
decoders,
the loudne
s
is
automatically
reduced
in all of
the
places at
which
it
was increased
before recording. Thi
restores
the music to its
original
loudness and
frequency response.
At the
same
time, the
tape noise which has
become mixed
with the music is
r duced by the
ame amount-u
ually
enough
to make it
inaudible.
The signal
from
a
Dolby-processed tape, after
it has passed through the decoding circuitry, is referred
to as a
decoded signal.
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