The following 4 messages are ASCII translations of HTML from dolby.com
operated by Dolby Laboratories. Many excellent diagrams of the
principles involved could not be placed into these messages.
Here's the first:
MAKING CASSETTES SOUND BETTER
COMMON TO ALL THREE SYSTEMS
Dolby noise reduction is a two-step process:
Step 1.
     When music is being recorded, it is encoded just before reaching
the tape. The purpose of encoding is to raise the level of soft,
high-frequency passages so they become louder than the tape's
noise. During the trip through the Dolby encoder, loud passages (that
hide tape hiss) are not altered. Soft, high-frequency passages (that
tape hiss affects) are made louder than normal as they are recorded on
the tape.
Step 2.
     When playing back the tape, the sound is decoded by a precise
mirror-image process of the encoding in Step 1. The loud sounds are left
unaltered, while the soft, high-frequency sounds are lowered back down
to their original levels. (You may have noticed that Dolby B tapes sound
brighter when played without any Noise Reduction decoding. Now you know
why! You are hearing the encoded sound, not the original.
NOISE REDUCTION TAKES PLACE DURING DECODING. Tape Hiss is added to the
recording during the recording process. In step 1 we learned that the
Dolby encoder boosted (made louder) the soft, high-frequency passages
before the signal reached the tape and before tape hiss was mixed in.
During Step 2, the Dolby decoder doesn't "know," as it scans the signal
coming off the tape, that tape noise has been added to the music it just
goes about the business of reducing the encoded sounds to their original
levels, with the noise automatically getting the same treatment. The
result? Completely restored musical balance but with less hiss in the
reproduced sound (see figure 1).
Such encode-decode systems are generally called "companders." They
compress the range between loud and soft when recording and expand the
range back again on playback, and reduce noise in the process. While
Dolby B-type, C-type, and S-type noise reduction systems all operate
as companders, there are many differences in the amount of noise
reduction, the methods used to achieve it, and the level of technology
used in each.
Copyright© 1996 Dolby Laboratories, Inc.
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