From: jeh@cmkrnl.com
Newsgroups: sci.electronics
Subject: Re: Dolby B/C? (A??)
Date: 26 Jul 92 18:29:32 GMT
Organization: Kernel Mode Consulting, San Diego, CA

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
Newsgroups: sci.electronics