From: Bill Press <press@lip.wustl.edu>
Subject: DBX - how it works (and Dolby too)
Reply-To: press@v1.wustl.edu
Organization: The Internet
Date: Sun, 24 Jan 1993 15:24:30 GMT

Though I don't know the exact details of DBX, I can give a heuristic
description as to how it works. Basically, the noise your tape-head
picks up falls within a particular range of frequencies. Thus, any
music you record on the tape at those same frequencies will be played
back with the noise superimposed (tape hiss). Now, DBX takes the incoming
music signal and compresses it. What do I mean by this? Say your music
ranges from 20-20k Hz... compressing it can mean doing some sort of
transformation so that your music now ranges from 20-10k Hz. These aren't
the ranges DBX uses, but it should give you a feel for what I'm talking
about. Now this compressed signal ideally has very little information
loss... that is, we can later decompress it to recover the original
signal (for the most part). So we have this compressed signal, which
ranges from 20-10k Hz, and we're going to write this to the tape. And
we've got our old friend noise, too. But he resides at, say 12k-15k Hz
(again, these aren't the actual numbers...). So when we record everything
on the tape, we've got a 20-10k Hz signal and a 12k-15k Hz noise. Notice
that *they do not overlap*. Thus when we play back with DBX, the tape
deck only looks at the 20-10k Hz portion of the signal (the music) and
ignores everything else (ie the noise). It then decompresses it, giving
us sound that is *much* better than no-noise-reduction or dolby (as those
of you who use DBX know -- in fact, I think it's almost comparable to 
digital, though not quite). Clearly, though, DBX music sounds *terrible*
if you don't decode it, so *never* send a DBX tape to somebody if they
don't have DBX capabilities.
        We had a Dolby thread a while ago. I won't go over whether you should
use it or not (and *please* don't start that thread again), but I will
briefly describe what it does. Dolby, instead of shifting the signal 
before it's recorded, boosts the frequencies of the signal that are     
coincident with noise. So your 10-20k Hz signal will be louder in the
12-15k Hz range than would normally be. Yet this boosting occurs before
the musics written to tape (obviously), so the noise gets recorded at its 
normal level. When you play back the music with Dolby, the boosted range
gets dropped back down to its original level. Now the noise, in the same
range being reduced, was recorded as it normally was, so when you play
back with Dolby, it drops way down in audibility. The difference between
Dolby B and Dolby C is primarily the range of frequencies that are affected
by this scheme (Dolby C better matches the range the noise falls in). I
think Dolby C may also be a greater change in amplitude of the affected
signals, but I'm not sure of this.
        I hope this was somewhat useful to people. Basically, if you've got
DBX, you're psyched. It really does work wonders, and was only phased out
because digital was coming to town. A shame really (although I *love*
digital, and prefer it to any analog recordings anyway).

	Hail Eris! Hail hail! Hail YES!
	Bill