From: btdy@vax5.cit.cornell.edu
Newsgroups: rec.music.phish
Subject: Re: What Dolby and DBX actually do
Date: 13 Apr 92 23:36:45 EDT
Distribution: rec
Organization: Cornell University

In article <1992Apr13.003533.15581@uvaarpa.Virginia.EDU>,
William Press <press@cns.caltech.edu> writes: 
> 
> The hiss of tape noise resides in the higher frequencies: when you
> record with Dolby, the music's high frequencies are boosted compared
> to the rest of the music -- the hiss caused by the tape head is
> unchanged.  Thus when you play back *with* Dolby, the high end is
> reduced to its normal volume, and the hiss becomes considerably less
> audible. This is why playing back a Dolby tape on a non-Dolby system
> will give that boosted treble sound. And by the way, the only
> difference between Dolby B and Dolby C is the amount by which they
> boost and reduce the higher frequencies: Dolby B uses about a 10 db
> difference, and Dolby C uses about 20 db.

Mostly true.  But Dolby C doesn't boost/cut by 20 db--that would be a bit
much.  But it does boost/cut at a different level tho.  Dolby
C covers a wider frequency range--it reduces noise at lower frequencies
than Dolby B.  Typical overall S/N ratios might be 60 dB for no NR,
70-80 dB for Dolby B, and 75-85 dB for Dolby C.  
> 
> While in theory this is all well and good, and should yield tasty
> sounding dubs, a couple of problems arise in practice. First of all,
> different tape deck manufacturers will use different implementations
> and hardware in trying to effect the same results. Thus, if you
> "encode" your tape with Dolby when you record a show for a friend,
> there is no guarentee that *his* or *her* tape deck will accurately
> "decode" the music. Your tape decks' Dolbys may have different
> roll-offs and frequency responses, resulting in a muffled, tinny, or
> simply *strange* sounding tape. Secondly, the decoding effected by a
> particular machine does not always accurately match its encoding
> scheme. My personal experience is that Dolby tapes often play back
> somewhat muffled, with some loss in the higher frequencies, even on
> the original tape deck. I do not know why this seems to be such a
> common problem, but these two caveats have been enough to scare me
> away from Dolby for fear of simply *ruining* my tapes.
> 
> DBX, on the other hand, is a different story altogether. This scheme
> takes all of the music, compresses the information, and completely
> stores it at bandwidths *different* than the hiss. Thus, when you play
> it back, the system simply cuts out all of the hiss frequencies (where
> *no* music is stored), and decodes the original signal. This system is
> amazingly effective, yielding a signal to noise of something like 98
> db. In fact, DBX rivaled digital signal-to-noise, and was for some
> reason phased out when the CD was released. There is, however, one
> serious problem. You cannot play back a DBX tape without a DBX tape
> deck. Period. If you do, the music will sound pretty fucked up...
> particularly tinny. Thus if you are making DBX tapes for someone, make
> sure they have a DBX tape deck, which is pretty uncommon these days.
> If you both have DBX, though, you'll probably be very happy with the
> results.

BTW, does anybody know where to get DBX dynamic range expanders (new or used)?
(not the NR unit--the ones used with *gasp* ANALOG discs)

Getting it clearly through alternate paths...  (I hope)

- Jeff