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- Path: sparky!uunet!sun-barr!ames!agate!dog.ee.lbl.gov!nosc!crash!cmkrnl!jeh
- From: jeh@cmkrnl.com
- Newsgroups: sci.electronics
- Subject: Re: Dolby B/C? (A??)
- Message-ID: <1992Jul26.112933.601@cmkrnl.com>
- Date: 26 Jul 92 18:29:32 GMT
- References: <1992Jul22.130639.933@wpi.WPI.EDU>
- Organization: Kernel Mode Consulting, San Diego, CA
- Lines: 103
-
- In article <1992Jul22.130639.933@wpi.WPI.EDU>, cubanski@tensor4.WPI.EDU (David J Cubanski) writes:
- > Here is a question for you netfolk. I'm not sure if this is the right
- > group to ask this in, but I can't seem to find a better one, so here
- > goes.
-
- rec.audio might be better. But then again....
-
- > What, exactly, is the difference between the Dolby B and the Dolby C
- > settings on my tape deck? What ever happened to Dolby A, if indeed
- > there ever was such a thing? And how, exactly, does dolby work,
- > anyways? From my observations of its performance, it seems to be some
- > kind of adaptive filtering scheme, but I would like to have a
- > specific, in-depth, engineering-type explanation.
-
- Dolby tape noise reduction systems (as opposed to other Dolby systems, such as
- "Dolby stereo", which is a surround-sound system for movie soundtracks) are
- essentially band-limited dynamic range companders. Dolby B is the simplest. It
- provides up to 10 dB of compression on recording, and similar (you hope!)
- expansion on playback, limited to the high frequencies.
-
- Put simply, it is as if you "turned up the treble" during recording, during
- passages when the treble content of the program material is at a low level, and
- correspondingly "turned down the treble" during the same passages on playback.
- The net effect on the treble part of your program material is (hopefully)
- zero, but the hiss that comes off the cassette tape gets reduced by up to 10
- dB. (If the treble content is high, Dolby B does nothing, but due to the
- psycho-acoustic effect known as "masking", you aren't supposed to be able to
- notice the hiss at those times anyway, since you're paying attention to the
- program material.)
-
- Note the "hopefully" in the above 'graf. It's possible for the expansion
- that's done during playback to not quite match up with the compression that was
- done during recording. This is called "tracking error", and some degree of it
- is quite common in Dolby n.r. systems (and in all other compander systems, such
- as dbx). It's usually acceptable for consumer-grade uses and is far better than
- what it replaces (the hiss from a non-noise-reduced cassette playback). It
- wouldn't be acceptable in a studio.
-
- Dolby C is similar but the compander limits are about 20 dB, so noise is
- reduced even more. The specs that came with your deck will show this in their
- S/N figures for "no noise reduction", "Dolby B", and "Dolby C". Dolby C was
- adopted quickly by the equipment manufacturers because the circuitry is much
- the same as that for B. Unfortunately, because of the increased compression,
- the effects of tracking error on Dolby C are far more noticeable (and
- objectionable). The slight machine-to-machine variances in head alignment, and
- even the differences in tape tracking from one "play" of a cassette to another
- *on the same machine*, are often enough to cause unacceptable levels of
- tracking error in Dolby C. If you have a good cassette deck that controls the
- tape well (dual capstans, that sort of thing), Dolby C generally works well for
- record and playback on the same machine. It isn't so good for recording tapes
- for recording tapes on one deck and playing them on another. It proved useless
- for mass-duplicated tapes.
-
- Dolby A was the first Dolby noise reduction system. It has never been seen on
- consumer gear, being sold exclusively to studios. It is a four-band system,
- rather than operating exclusively on the high frequencies. (In fact, Dolby B
- is pretty much just the high band of Dolby A.) The equipment is expensive and
- has elaborate setup and calibration requirements to eliminate tracking error.
-
- The very newest, most expensive consumer cassette decks have a new variant,
- Dolby S. I don't have the details on just how it differs "internally" from B
- or C. It's supposed to eliminate the tracking problems of Dolby C while
- providing similar noise reduction figures, but claims reasonable playback
- compatibility with Dolby B and even non-Dolby decks. (I'm not sure how they
- accomplish all of this at once!) But as it is much more expensive for the mfr
- to build in than B or C, so far it hasn't been seen on any but high-end decks.
- Dolby is hoping that the record companies will adopt Dolby S for use in
- mass-duplicated cassette tapes.
-
- Both the record companies and the consumer equipment manufacturers are pretty
- much holding back on Dolby S, because they're waiting to see how Digital
- Compact Cassettes fare in the marketplace. The record companies see no point
- in investing in Dolby S equipment if a changeover to DCC is imminent. As for
- the equipment manufacturers, if they are going to be pitching digital tape
- recording (in the form of DCC) to the masses, they don't want to be confusing
- the issue by also pitching expensive cassette decks which some test reports
- have claimed can reproduce music just as well as a DCC deck can.
-
- Personally I would rather see Dolby S succeed than DCC. DCC relies on the
- previously-mentioned "masking" effect and throws away not just noise but parts
- of the program material that you aren't supposed to be able to notice! I'd
- rather stick with noise-reduced analog or non-data-reduced digital. However
- the technical superiority of one format over another has never been the
- deciding factor; if it were, we'd all be using Beta VCRs. Everybody in the
- industry thinks that the magic word "digital" makes DCC a reasonable bet to
- replace cassettes, even in the face of higher costs for both blank and
- prerecorded tapes (early reports are that prerecorded DCC tapes will cost about
- the same as CDs). However the abject failure of DAT in the consumer
- marketplace has made everybody cautious -- no one is simply assuming that DCC
- will fly.
-
- (Yes, folks, the *real* reason that the record companies want DCC is that the
- format change will give them an excuse to raise prices -- just like they did
- in the LP->CD conversion -- even though CDs now cost far less to produce than
- LPs ever did. Don't you just love it?)
-
- > PS - I hope this doesn't start some kind of holy war thread....
-
- nope, that's for rec.audio!
-
- --- 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
-