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From: fc@lexicon.com (Frank Cunningham)
Newsgroups: comp.music
Subject: dbx vs. Dolby C Noise Reduction
Message-ID: <590@lexicon.com>
Date: 17 Jul 90 20:58:13 GMT
Organization: Lexicon, Inc., Waltham, MA
Lines: 44

>  dbx
If I recall the term correctly, both dbx and dolby are homomorphic
filters, in that their control is derived from the signal itself.
They are both invertable - the expansion circuits do the inverse of
the compression circuits. They both are reasonably resistant to
corruption of the encoded signal by the channel, although Dolby is
more sensitive to high-frequency losses since it only works on them.

dbx uses a log amp to control the gain of a VCA so the conversion
is linear in dB. dbx also bandpasses to control signal side chain
so that it is more resistant to frequency responce variations than
a full-band control signal would be.

>    - 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?

No separate signal. It simple expands linearly in dB the signal it
previously compressed. There are some time constants in RMS convertors
that are matched.

> 1. How does Dolby C work?
By some double compression of the high end only, above some frequency,
below some threshold (where dbx is level independent for reasonable
setups). I've forgotten the details. It's basically Dolby B done twice.

> 2. What are the pros and cons of Dolby C vs. dbx, particularly with
>    respect to the application of multitrack recording?

I've never actually compared them. For a reasonable machine and most
signals either will do. Dolby C must be calibrated probably to better
than 1/2 dB. see dbx#3 below

> 3. dbx having a "tendency to cut off leading edges of transient
>    What causes this problem and how serious is it?
I think this occurs when you are pushing the channel. The RMS / log
control circuit allows the edge to pass uncompressed to the tape,
which saturates. The expansion circuit then cannot restore the 
original edge.
-- 
-Frank Cunningham   smart: fc@lexicon.com	phone: (617) 891-6790
		    dumb: {husc6,linus,harvard,bbn}!spdcc!lexicon!fc
		    snail: Lexicon Inc.  100 Beaver St.  Waltham MA 02174
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