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- From: jeh@cmkrnl.com
- Newsgroups: sci.electronics
- Subject: Re: Dolby B/C? (A??)
- Message-ID: <1992Jul28.113002.607@cmkrnl.com>
- Date: 28 Jul 92 18:30:01 GMT
- References: <1992Jul22.130639.933@wpi.WPI.EDU> <1992Jul23.113150.7221@ohm.york.ac.uk>
- Organization: Kernel Mode Consulting, San Diego, CA
- Lines: 48
-
- In article <1992Jul23.113150.7221@ohm.york.ac.uk>, u9dmlb@ohm.york.ac.uk
- (Duncan McL Barclay) writes:
- >
- > Well in a nutshell Dolby A was developed for the recording industry
-
- right so far.
-
- > and used about more than 3 bands
-
- "about more than three"???
-
- > which were equalised
- > depending on signal level. When Dolby wanted to produce a consumer
- > version they produced Dolby B which has 2 bands to which EQ is
- > applied coz it was cheaper than doing the full Dolby A.
-
- No. No. No. Dolby noise reduction is not an EQ system. It is a dynamic
- range compander (compression on recording, expansion on playback) system. Since
- Dolby nr affects different audio frequency bands differently (ie Dolby B or C
- tapes played on a non-Dolby deck sound overly "bright", and you can correct
- this to some extent by turning down the treble), you may think of it as
- "equalization".
-
- But the key difference here is that EQ is a static thing, ie the same EQ curve
- is used throughout a recording, whereas the action of a compander depends on
- the signal level being recorded or played back.
-
- Full-bandwidth companders are fairly old hat. What Dolby A nr did that was new
- was to derive four different control signals from four different audio bands,
- and use them to control companders which operate only in those respective
- bands. The result was a marked reduction in "pumping" and "breathing" and
- "noise tails" which were associated with simple full-bandwidth companders.
-
- dbx, incidentally, is a sort of a cross-breed. Its compander operates on the
- entire audio band, but its control signal is derived not from a full-bandwidth
- "level" but from a proprietary filter network that samples just a few key
- frequencies.
-
- Dolby B has just one band. (If you like you could think of Dolby B as having
- two bands, one which it processes and one which it leaves strictly alone, but
- this is REALLY stretching a point.)
-
- Dolby C similarly has just one band, but does about twice as much compression
- as Dolby B.
-
- --- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
- Internet: jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
- Uucp: ...{crash,eisner,uunet}!cmkrnl!jeh
-