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4.3 - What is the difference between Dolby A, B, C, S, and SR? How do each of these systems work?


This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.

4.3 - What is the difference between Dolby A, B, C, S, and SR? How do each of these systems work?

The Dolby A, B, C, SR, and S noise reduction (NR) systems are
non-linear level-dependent companders (compressors/expanders). They
offer various amounts of noise reduction, as shown in the table

    Dolby   HF NR   LF NR  Number Of Active              Target
    System  Effect  Effect Frequency Bands               Market     Year
    ------  ------  ------ ----------------------------  ---------  ----
      A     10 dB   10 dB  4 fixed                       Pro audio  1967
      B     10 dB   --     1 sliding (HF)                Domestic   1970
      C     20 dB   --     1 sliding (HF)                Domestic   1981
      SR    24 dB   10 dB  1 sliding (HF), 1 fixed (LF)  Pro audio  1986
      S     24 dB   10 dB  1 sliding (HF), 1 fixed (LF)  Domestic   1990
    ------  ------  -----  ----------------------------  ---------  ----

The band-splitting system used with Dolby A NR is a relatively costly
technique, although it can deal with noise at all frequencies. The
single sliding band techniques used in Dolby B and C systems are less
costly, making them more suitable for consumer tape recording
applications where the dominant noise contribution occurs at high

The typical on-record frequency response curves for the Dolby B NR
system look something like those depicted below. The curves for Dolby C,
SR, and S are similar, but the actual response levels and behaviour at
high frequencies are modified to extract better performance form these
more advanced systems.

    0dB -|----------------------------------------------------
  -10dB -|
         |                    /-------------------------------
         |                   /
  -20dB -|------------------/
  -30dB -|                      /-----------------------------
         |                    /
         |                  /
  -40dB -|----------------/
         |                   |                                 |
        20Hz                1kHz                             20kHz

The above picture attempts to show that the encoding process provides
selective boost to high frequency signals (decoding is the exact
reciprocal), and the curves correspond to the results achieved when no
musical signal is applied. The amount of boost during the compansion
depends on the signal level and its spectral content. For a tone at
-40dB at 3 kHz, the boost applied to signals with frequencies above this
would probably be the full 10dB allowed by the system. If the same tone
were at a level of -20dB, then the boost would be less, maybe about 5dB.
If the tone was at 0dB, then no boost would be supplied, as tape
saturation would be increased (beyond it's normal amount).

The single band of compansion utilized with Dolby B NR reaches
sufficiently low in frequency to provide useful noise reduction when no
signal is present. Its width changes dynamically in response to the
spectral content of music signals. As an example, when used with a solo
drum note the companding system will slide up in frequency so that the
low frequency content of the drum will be passed through at its full
level. On replay, the playback of the bass drum is allowed to pass
through without modification to its level, while the expander lowers the
volume at high frequencies above those of the bass drum, thus providing
a reduction in tape hiss where there is no musical signal. If a guitar
is now added to the music signal, the companding band slides further up
in frequency allowing the bass drum and guitar signals through without
any compansion, while still producing a worthwhile noise reduction
effect at frequencies above those of the guitar.

The Dolby B NR system is designed to start taking effect from 300Hz, and
its action increases until it reaches a maximum of 10dB upwards of 4kHz.
Dolby C improves on this by taking effect from 100Hz and providing about
15dB of NR at 400Hz, increasing to a maximum of 20dB in the critical
hiss region from 2kHz to 10kHz. Dolby C also includes spectral skewing
networks which introduce a rolloff above 10kHz prior to the compander
when in encoding mode. This helps to reduce compander errors caused by
unpredictable cassette response above 10kHz, and an inverse boost is
added after the expander to compensate. Although this reduces the noise
reduction effect above 10kHz, the ear's sensitivity to noise in that
region is diminished, and the improved encode/decode tracking provides
important improvements in overall system performance. An anti-saturation
shelving network, beginning at about 2kHz, also acts on the high
frequencies but it only affects the high-level signals that would cause
tape saturation. A complementary network is provided in the decode chain
to provide overall flat response.

When the tape is played back, the inverse of the above process takes
place. For an accurate decoding to occur, it is necessary that playback
takes place with no offsets in levels between record and replay. i.e. If
a 400 Hz tone is recorded at 0dB (or -20dB), then it must play back at
0dB (or -20dB). This will help ensure correct Dolby "tracking".

Just think about it: if a -40dB tone at 8kHz was recorded with
Dolby B on, then it would actually have a level of -30dB on tape.
The same tone, if it were at a -20dB level, would have a level of
about -15dB on tape. If the sensitivity of the tape was such
that anything recorded at 0dB actually went on tape as -10dB,
then you can see that the Dolby encoded tones would actually be
at a lower level, and the system would have no way of determining
this. It assumes 0dB in = 0dB out. Hence the signal would be
decoded with the incorrect amount of de-boost.

The Dolby SR and S NR systems provide slightly more NR than Dolby C
at high frequencies, 24dB vs 20dB, but they also achieve a 10dB NR
effect at low frequencies below 200Hz as well. This is obtained
using a two-band approach, the low-frequencies being handled by a
fixed-band processor, while the high frequencies are tackled by a
sliding band processor. This reduces the potential for problems such
as "noise pumping", caused by high-level low frequency transient
signals (bass notes from drums, double basses, organs), raising the
sound level in a cyclic fashion. Dolby SR and S also contain the
spectral skewing and anti-saturation circuits for high-level
high-frequency signals that are implemented with Dolby C. The
performance of the sliding band is improved over that obtained with
Dolby B and C NR systems by reducing the degree of sliding that
occurs in the presence of high-frequency signals. This increases the
noise reduction effect available at frequencies below those
occurring in the music signal.

An additional benefit of the Dolby S NR system for consumers is that
the manufacturers of cassette decks who are licensed to use the
system must adhere to a range of strict performance standards. These
include an extended high frequency response, tighter overall
response tolerances, a new standard ensuring head height accuracy,
increased overload margin in the electronics, lower wow and flutter,
and a head azimuth standard. These benefit users by enhancing the
performance of cassette recorders as well as helping to ensure that
tapes recorded on one deck will play back accurately on any
other. [Witold Waldman - witold@aed.dsto.gov.au]


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