MAKING CASSETTES SOUND BETTER
DOLBY S-TYPE NOISE REDUCTION
Dolby S-type noise reduction was the culmination of over 20 years
experience in the design of noise reduction systems and the licensing of
audio cassette decks. Just as Dolby's noise reduction technology made
dramatic advances in two decades, so did the capability to manufacture
quality audio cassette decks. The drive mechanisms, motors, tape heads,
tape formulations and electronics had all progressed significantly over
that time. Dolby took the position that if S-type were to be its
premiere noise reduction technology, there should be specifications on
the rest of the cassette deck to ensure the overall result would be
consistent with that level of performance.
Dolby Laboratories requires that cassette decks that are to include
Dolby S-type Noise Reduction meet new, higher performance standards,
including:
* EXTENDED HIGH FREQUENCY RESPONSE
* TIGHTER OVERALL RESPONSE TOLERANCES
* A NEW STANDARD ENSURING HEAD-HEIGHT ACCURACY
* INCREASED OVERLOAD MARGIN IN THE DECK'S ELECTRONICS
* REDUCED WOW AND FLUTTER
* A NEW HEAD AZIMUTH ALIGNMENT STANDARD
Cassette decks meeting these new Dolby S-type standards will provide
improved cassette performance, and tapes recorded on one machine
including prerecorded cassetteswill play back with unprecedented
accuracy on any other machine.
Dolby Laboratories tests and approves prototype samples of all cassette
decks that include Dolby Noise Reduction systems to insure that these
standards are met in the design, before the product may be manufactured.
In general, if a cassette deck has Dolby S-type Noise Reduction, it is
the consumer's assurance that it was designed to meet a higher standard
of performance, and will produce better recordings, even if Dolby S-type
Noise Reduction is not used.
ACTION SUBSTITUTION
Dolby S-type Noise Reduction applies the principle of least treatment in
some powerful new ways. Even though it provides even more effect than
ever before (up to 24 dB), the Dolby S-type Noise Reduction processor
devotes much of its energy to not processing the audio signal. That's
right, more effect with less visible effort.
This economy of movement is achieved through the use of a new kind of
filter design combined with a sophisticated method of controlling
filters' actions and interactions. The goal of this exercise is to
keep the encoded signal as steady as possible. If the encoded signal
does not sound like it is changing drastically, and exhibits no
"pumping" types of modulation sounds, then that signal is inherently
easier to decode flawlessly.
Like Dolby C-type, Dolby S-type uses TWO HIGH FREQUENCY SLIDING
COMPRESSION-EXPANSION BANDS. But, in addition, EACH SLIDING
COMPRESSION-EXPANSION BAND IS COUPLED WITH A SEPARATE HIGH
FREQUENCY FIXED BAND PROCESSOR. By using these coupled processors,
called Action Substitution, Dolby S-type noise reduction is able to
provide 24 dB of HIGH FREQUENCY noise reduction.
When loud (high level) signals are allowed to effect soft (low level)
signals, the ear hears a "pumping" sound. The coupling of a sliding band
processor with a fixed band processor (Action Substitution) reduces to a
minimum the effect of loud signals on soft ones, eliminating "pumping."
TECHNICALLY SPEAKING
Figure 11a shows the operation of a Sliding Compression-Expansion Band
in the presence of a single "dominant frequency." Figure 11b shows the
operation of a Fixed Compression-Expansion Band in the presence of the
same single "dominant frequency." Figure llc shows the benefit of a
combination (Action Substitution) approach: the action of the fixed band
works on the frequencies below the "dominant frequency, while the action
ofthe sliding band works on the frequencies above the "dominant
frequency." With Action Substitution, the soft (low-level) signals are
more constantly boosted, and noise reduction is more effective.
Figure 11. Effects of dominant signals within the NR band also vary with
the system. To effect a given decrease in gain (2 dB shown) at the
dominant frequency, the NR band of a sliding-band system (B), boost (and
thus NR) is reduced throughout the band by the amount of gain decrease
required at the dominant frequency. Combining fixed and sliding
bands (the "action substitution" system used in Dolby S-type NR)
confines the gain decrease more closely to the dominant frequency,
resulting in less loss of NR (C).
Action Substitution has an additional benefit. With an encode-decode
noise reduction system like Dolby's, changes in level occurring after
the signal has been encoded can cause the playback decoder to mistrack,
that is, it may not act as a precise mirror-image of the encoder. Using
only a sliding band system, a small level-change, introduced by the tape
recorder, after the encoder, at the "dominant frequency," can cause
decoder mistracking at lower frequencies large enough to be heard. Dolby
S-type Noise Reduction, with Action Substitution, uses the action of the
fixed band, working on the frequencies below the "dominant frequency,"
to eliminate this problem.
Modulation Control
Dolby S-type uses a special technique, called Modulation Control, which
is applied to both the sliding and the fixed bands. It reduces the
tendency of a sliding band to move further away from high-level (loud)
signals than is necessary (See Figure 12A). It also reduces a fixed
band's tendency to react to high-level (loud) signals outside, but close
to, the band (See Figure 12B). Like Action Substitution, Modulation
Control helps to keep ALL low-level (soft) signals more constantly
boosted, and noise reduction more effective.
Figure 12. Modulation control reduces the tendency of a sliding band to
move further away from a high-level signal than necessary (A) and of a
fixed-band system to reduce gain in the presense of a high-level signal
at a nearby frequency (B).
Copyright © 1996 Dolby Laboratories, Inc.
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