From: jeh@cmkrnl.com
Newsgroups: rec.audio
Subject: Re:  Dolby C vs. dbx
Date: 4 Oct 92 02:15:11 PDT
Organization: Kernel Mode Consulting, San Diego, CA

In article <1992Oct1.185829.10974@pilot.dmg.ml.com>, steveh@pilot.dmg.ml.com (Steve Hannoford) writes:
> I can tell you how Dolby works, but I can't really say how dbx
> works, [...]

Several replies have already noted the flaws in this person's statements about
dbx.  As for Dolby B and C...

> Dolby labs researched tape noise and isolated the frequencies
> where the noise typically occurrs.  

Yes, but...

> When you make a recording
> with Dolby, the signal is boosted in that range, so that the 
> signal to noise ratio is increased. (The signal is boosted so
> that it 'covers up' the noise.)  This results in a top heavy
> (increased treble range) recording.  During playback, the 
> signal is reduced by the same amount, restoring the proper
> treble levels, but the noise isn't there (well not as much
> of it).  

This isn't quite correct.  It isn't just a matter of boosting the treble on
record and cutting it on playback -- that would be merely a different EQ curve,
and all tape decks already apply EQ on recording and apply the reverse EQ on
playback.  Simply boosting the treble even more would cause tape saturation. 

Rather, Dolby B and C apply *compression* to the high frequencies on record,
and expansion on playback.  Parts of the program that already have a lot of
high-freq. content won't be much affected by Dolby.  But in parts in which the
highs are of low level, highs are boosted on record and cut on playback. 
During the "cut on playback", the tape hiss, which is also mostly high freqs,
gets cut too.  Voila.  (When there is a lot of high-freq content in the 
program material, you don't notice the hiss nearly so much, as you're paying
attention to the program.  This is a psychoacoustic affect known as "masking".)

To get an idea of how compression and expansion works, imagine that you're
recording a series of numbers in the range 0 through 1.  But your recording
scheme somehow has a problem with the lower numbers -- anything much below 0.9
will be corrupted by noise on playback.  So you first divide the numbers by 10
and then add 0.9.  Now you are recording numbers in the range 0.9 to 1.0.  On
playback you reverse the procedure and recover the original numbers. 

(This by the way would be a ten-to-one compression ratio.  Most companders used
in audio work are far less aggressive than that, 2:1 or so.  The compander that
is part of VHS HiFi is, I believe, about 2.5:1.  THe main difference between
Dolby B and C, btw, is in the compression ratio used -- I don't know the
exact numbers but C has about twice the compression ratio of B.)

Now, anyone who works with floating point numbers in programming will spot a
flaw: In digital computers you only have so many bits of precision per number. 
Since you can't record with infinite precision you will end up with some
rounding errors on playback.  Suppose, for instance, you can only record two
decimal places after the decimal point.  In the "companding" scheme, inputs
0.91 and 0.92 will both be recorded as 0.99.  So on playback you don't know
quite what the original number was, and you have to settle for something close,
but you regard this as an ok tradeoff in exchange for avoiding the problems you
used to have with recording small-valued numbers. 

The same thing happens in analog recording -- we aren't getting something for
nothing here; we are merely shifting the errors to a domain where they're less
objectionable.  In companding schemes (Dolby B, C, S, and A, and dbx, are all
companding schemes) we are getting rid of tape-hiss "errors" but we might not
be getting all of the pieces of the original program played back at exactly the
right levels with respect to one another.  This is called "tracking error" and
it is usually an acceptable tradeoff, as the hiss is far more bothersome. Most

Dolby C implementations seem to be an exception:  Dolby C has proven to be
extremely difficult to get right as far as level-tracking is concerned. Even
slight head misalignments between the recording and playback decks are enough
to cause bad tracking errors.  (In the "digital" example above, the effect of
head misalignment would be to shift the entire range of recorded numbers
downward -- say from 0.9 to 1.0 down to 0.7 through 0.8.  Since the playback
decoder is being presented with bad data it can't possibly recover the original
numbers.) In fact, the cassette mechanism itself is sloppy enough so that
record and playback on the same deck sometimes causes problems.  

This is why Dolby is trying to push Dolby S -- it is just as effective as Dolby
C, maybe more so, but is far less subject to tracking errors due to head
misalignment. It also sounds quite acceptable when played through a Dolby B
deck.  If the Digital Compact Cassette (DCC) turns out to be a dud, you may see
the prerecorded cassette industry switch to Dolby S.) 

In some companding schemes, notably that used by VHS HiFi to get its wide
dynamic range, other errors are common.  These frequently show up as noise
"tails" at the beginning of quiet sections following crescendos, and in an
audible modulation of the background noise level (known as "breathing" and
"pumping").  

	--- 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