DOLBY S NOISE REDUCTION - PART 2
Low Frequency Noise Reduction
In addition to the HIGH FREQUENCY noise reduction discussed above, Dolby
S-type Noise Reduction also has a LOW FREQUENCY FIXED BAND PROCESSOR,
which provides 10 dB of low frequency noise reduction below 200 Hz (in
addition to the 24 dB in the high frequency stages). Because of: (1) the
nature of cassette noise, (2) the ear's reduced ability to hear low
frequency noise, and (3) the low frequency that the high frequency bands
start at, the low-frequency fixed band processor needs to work only
below 200 Hz and does not have to exceed 10 dB in action. This amount of
low frequency noise reduction provides the smooth noise spectrum, with
no one area dominating, that is desired. An additional benefit of this
low frequency band processor is that it keeps the music balanced when
listening to a Dolby S-type encoded tape on a machine using Dolby B-type
noise reduction.
Spectral Skewing
Like Dolby C-type above, Dolby S-type has Spectral Skewing too; but in
addition to the rolled off high end used in Dolby C-type, Dolby S-type
has added networks in the encoder which precisely roll off (lower the
volume of) the extreme low end. Of course, mirror-image networks in the
decoder restore flat response. The purpose of Spectral Skewing is to
reduce decoder mistracking due to tape recorder generated errors. Some
of these recorder generated errors are caused:
At low frequencies, by head bumps.
At high frequencies, by variations among tape formulations, even within
the same tape type.
At high frequencies, by head-azimuth variations between the machine on
which a tape is recorded and those on which it is played back.
While Spectral Skewing causes some loss of noise reduction effect at the
frequency extremes, the ear does not sense noise at the extremes of the
frequency range well, so the benefits of Spectral Skewing far outweigh
the potential noise reduction loss.
Anti saturation
Dolby S-type includes Anti saturation high frequency shelving networks
in the encoder which operate only at (loud) high signal levels, just
like Dolby C-type. However, in Dolby S-type, the Spectral Skewing
networks also contribute to Anti saturation as well. Compact cassettes
use a standard equalization (3180 microseconds) which boosts low
frequencies during recording and cuts them equally during playback.
However, when attempting to record loud bass sounds, the tape can
become saturated (over loaded) due to this equalization boost. Because
Dolby S-type provides noise reduction at low frequencies, in Dolby
S-type encoders, the standard cassette equalization is eliminated. This
results in a noticeable reduction in distortion on recordings of strong
(loud) low frequency signals.
The combined effects of Spectral Skewing and Anti saturation techniques
at both low and high frequencies can be seen in Figure 13, which
illustrates Dolby S-type's overall encode characteristics. Figure 14 is
a Block Diagram of the Dolby S-type Noise Reduction system.
Figure 13. Dolby S-type encode characteristics, showing the effects of
spectral skewing and antisaturation at various input levels. The decode
characteristics are the inverse of these curves.
Dolby S-Type Performance
"We have found that, at the highest playback levels likely to be
encountered in the home, sophisticated listeners subjected to A/B
comparisons of CDs and Dolby S-type cassettes are unable to identify
which is which with any regularity."
Although the benefits of a Dolby S-type recording will only be fully
realized with S-type decoding, Dolby S-type has been designed so that it
can be played quite acceptably via B-type decoder equipment such as car
stereo players, battery operated portable players, or existing Hi Fi
decks, or even via equipment without decoding capability. This makes
Dolby S-type ideal for pre-recorded tape use. In fact, RCA Victor, BMG
Classics, Warner Brothers Records, and Atlantic Records have already
produced over 30 million pre-recorded tapes encoded in Dolby S-type.
Dolby S-type encoding reduces the dynamic range (quieter sounds are
brought up in level, while loud ones are left alone). In noisy
surroundings, such as cars, trains, or planes, it is no longer
necessary to keep turning the volume up and down as the level of the
music changes. This benefit is enjoyed even if a Dolby S-type decoder is
not used.
         SUMMARY OF DOLBY S-TYPE NOISE REDUCTION BENEFITS
 Dolby S-type Cassette Decks are the highest quality designs.
     Dolby S-type Cassette Decks provide performance comparable to that
of a Compact Disc, utilizing standard (Type I) blank cassette tapes.
     Sonically, Dolby S-type noise reduction gives cleaner transient
response, provides lower noise levels, both at low and high frequencies,
and allows higher signal levels to be recorded safely; in short, it
permits more accurate recording. Over 30 million pre-recorded cassettes
have been produced using Dolby S-type noise reduction.
     In noisy environments, when tapes encoded with Dolby S-type noise
reduction, whether pre-recorded or recorded in the home, are played
back with Dolby B-type noise reduction, or even with no noise reduction,
they will sound better than they would if Dolby S-type noise reduction
had not been used.
TECHNICALLY SPEAKING
Dolby S-type Noise Reduction utilizes combined fixed and sliding type
compression-expansion bands in two different high frequency bands as
well as a fixed type band for low frequencies. It provides 24 dB of High
Frequency Noise Reduction plus an additional 10 dB of Low Frequency
Noise Reduction.
Dolby S-type's MODULATION CONTROL helps to keep all low-level signals in
a more constantly boosted state which makes noise reduction more
effective.
SPECTRAL SKEWING and ANTI SATURATION networks have been used to provide
significantly increased headroom and to reduce decoder mistracking due
to recorder generated errors.
Copyright © 1996 Dolby Laboratories, Inc.
--- QScan/PCB v1.18b / 01-0249
---------------