From: rvkl60@aus18a25.sps.mot.com (Tom Mathes)
Subject: Re:  Dolby C vs. dbx
Organization: Motorola Inc, Austin, Texas
Distribution: na
Date: Mon, 5 Oct 1992 15:01:16 GMT

As the original poster of this query, it's really amazing how things have gotten
of track.  I was asking the subjective opinions out there.  So far, I've found
that most folks that object to dbx hate the noise pumping artifacts.  If noise
pumping doesn't bother the listener, he/she prefers dbx over Dolby C.

Noise pumping artifacts bother me, so I'll skip on dbx.  Thanks to all who
responded to my query by e-mail.  All of you were very helpful.

In article <1992Oct4.021511.709@cmkrnl.com> jeh@cmkrnl.com writes:
>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".)

You've got your theory right, but terminology wrong.  Dolby amplifies, or
expands the high frequencies during recording, then compresses them during
playback.  That's one reason to critically check the recording levels during
recording with all Dolby NR systems.  Too high a level will saturate the tape 
and distort like crazy.  That's where Dolby HX helps; it can help reduce the
tape saturation effects of high frequencies by attenuating the recording bias
during high-level high-frequency passages.

>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

You're right: the biggest problem with Dolby C is tape sensitivity mismatches.  
If it's easy to set this on your deck, along with bias level, Dolby C works great.
Problem is that most decks don't allow easy adjustments for tape sensitivity.
My deck has these external pots, so it's easy to tweek the deck.

>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

I've read in "Audio" that Dolby Laboratories is being very picky as to what
tape decks will be allowed to use the Dolby S circuit.  The manufacturer has
to make the prospective deck with very tight tolerances of head alignment, and
it also has to have wide frequency repsonse.  Also, the deck has to have some
type of provision for tape sensitivity adjustment, either automatic or manual.
Dolby Labs learned from the problems of Dolby C to do this; I'm glad they are
doing it.  Unfortunately, only high-end decks ($900+) contain Dolby S right now.

Subjective tests in "Audio" and "Stereo Review" of Dolby S decks rank them almost
as good as DAT.  On an oscilloscope it was easy to tell the analog deck, but to
the ear, the reviewers said it sounded practically as good as DAT.

Tom
Newsgroups: rec.audio