14.4 What are Dolby B, C, and S, HX Pro, and DBX? Are they compatible?
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This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
14.4 What are Dolby B, C, and S, HX Pro, and DBX? Are they compatible?
Dolby B, C, S, and DBX are techniques for increasing the
signal/noise ratio of recordings. All work in similar ways:
they compress the dynamic range of the sound during recording,
then expand it back upon playback. As much as we would like
it to be otherwise, you only get correct reproduction if you
use Dolby B to play back a Dolby B tape. Same for Dolby C,
Dolby S, and DBX. Dolby HX Pro is the exception.
Dolby B works mostly with higher frequencies; it increases
their levels during recording and decreases their levels, and
the levels of high-frequency noise such as tape hiss, during
playback.
Dolby B tapes can be played back without Dolby B processing,
but high frequencies are over-emphasized and the sound will
be excessively bright. This can be compensated for to some
extent by turning down the treble control. Audio novices
often remark that commercially recorded tapes recorded using
Dolby B sound dull when played back with Dolby B; this is
because they are accustomed to the boosted high frequencies
they hear when playing these tapes without Dolby.
Dolby C achieves greater noise reduction (about 8-10 db) than
Dolby B by working with a greater range of frequencies and
altering relative levels more; this means that playing Dolby C
tapes back with no Dolby processing or with Dolby B, leads to
very bad frequency response and a sound that most people find
unpleasent. Dolby C may also be more sensitive to variations
among decks in exact frequency response, alignment, etc. Some
people find that tapes recorded using Dolby C sound best only
when played back on the deck on which they were recorded.
Dolby S works with an even broader range of frequencies than
Dolby C, and achieves slightly greater noise reduction. Its
has three advantages over Dolby C: (1) many people find that
tapes recorded and played back using Dolby S sound closer to
the original than tapes done using Dolby C; (2) tapes recorded
using Dolby S don't sound awful if played back on Dolby B decks,
and (3) Dolby S seems to be less sensitive to variations among
decks.
DBX is similar to Dolby B, C, and S, but uses the same compression
on all frequencies, high and low. However, DBX is mostly used
in the professional market. Very little home DBX equipment is
available, and some of that home equipment is no better than
comparable Dolby B home systems. All DBX systems are compatible
with all other DBX systems, but incompatible with Dolby. A DBX
tape will sound terrible without DBX processing during playback.
All compression/expansion systems suffer two problems. One is due
to the fact that compressors can't compress a loud signal before
they have heard a bit of it, so that little bit of loud signal
will get through uncompressed. Likewise, quiet passages will not
be expanded until after they are detected. These delays give rise
to an audible problem often called "breathing".
The other problem inherent in all compression/expansion systems
is that if there are any frequency response errors in the tape
recorder, they will be made worse by the compression/expansion.
For example, if there is a 2dB dip in frequency response at 1kHz
in the tape recorder, this will be accentuated to a 4dB dip if
the compressor is using a 2:1 ratio. So compression/expansion
trades noise for frequency response error. For that reason and
the previously mentioned breathing, some people prefer to use
their recorder without any noise reduction at all. They prefer
a bit of noise to the other errors.
Dolby HX Pro is not noise reduction and does not use
compression or expansion. HX Pro is a technique developed by
Dolby Labs to increase tape headroom by decreasing the bias
when recording signals with a large high frequency component.
This allows better transient response, particularly on less
expensive tapes, and requires no processing when the tape is
played back. Dolby HX tapes can be played back on any system
with no decrease in quality.
Dolby Corporation has developed other techniques and other
acronyms for products related to surround sound. The phrase
"contains Dolby" isn't as meaningful today as it used to be.
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