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From: a577@mindlink.UUCP (Curt Sampson)
Newsgroups: comp.music
Subject: Re: dbx vs. Dolby C Noise Reduction
Message-ID: <2700@mindlink.UUCP>
Date: 2 Aug 90 17:39:00 GMT
Organization: MIND LINK! - British Columbia, Canada
Lines: 38

> muller@Alliant.COM writes:
> 
> >    dbx
> >    - If it uses a dynamic gain device, then:
> >    - How does the playback expander know when and by how much to expand?
> >    - Is there a separate control signal recorded that is used to
> >      dynamically adjust playback level?
> >Don't know.
> 
> This has been answered before, but...  In general, expansion on playback uses
> the signal itself compared an aboslute reference level.  There is
> no control signal.  I am not sure exactly what dbx per se does.

dbx compress on record and expands on playback with a fixed 2:1 ratio. I
believe the reference level is -5 dBV for type II (cassette) dbx.

>  >    Dolby C
> >My understanding is this...  Dolby B is much like an enhancer in that
> >it boosts the highs before putting them on tape.  Then on playback, B
> >cuts them again.  My understanding is that Dolby C is much like Dolby
> >B applied twice to the same signal.
> 
> No.  It *can't* work that way.  If that was all there was to it, you
> could just establish an appropriate EQ curve and use it.  In fact, you
> should already be trying to saturate the tape, so just boosting the highs on
> recording and reducing them on playback will saturate things more.
> There must be some dynamic effect, but there is some question of what
> frequencies you are using to measure your level and whether you compress all
> the frequencies.  This is the basic difference between Dolby B and C,
> and possibly dbx too.  Maybe someone should post that again?  I know it
> just went up a few weeks ago, but...  AAAAAaaaaarrgh!

Dolby B raises the level of all signals above a certain frequency (1500 Hz, I
belive) when the level falls below a certain point.  Otherwise it does not
touch the signal.  Dolby C works in a similar manner except that it uses two
seperate "zones," from 1.5 KHz to 6 KHz, and above 6 KHz, I believe.  The
original Dolby A used five "zones" spread across the frequency spectrum.
        -cjs    ( Curt_Sampson@mindlink.UUCP )