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From: sjc@angband.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: Re: one real CD interpolation filter
Message-ID: <78@angband.UUCP>
Date: Mon, 2-Sep-85 17:32:50 EDT
Article-I.D.: angband.78
Posted: Mon Sep  2 17:32:50 1985
Date-Received: Wed, 4-Sep-85 06:08:20 EDT
References: <519@amdimage.UUCP> <496@spar.UUCP>
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Organization: S-1 Project, LLNL
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> In article <519@amdimage.UUCP> steve@amdimage.UUCP (Steve eidson) writes:
> >Then, there's a trick that Philips uses. They 
> >apply a noise shaping filter (no details provided) to the
> >interpolator output...
> I puzzled over this for a while after I read it and decided that they
> must be doing dithering.

Philips may also use dithering, but their noise-shaping is something
else.  Because Philips rounds each 16-bit sample to 14 high-order bits
before presenting it to the D/A, circuitry prior to the D/A accumulates
the error from each sample (that is, "16-bit sample" minus "16-bit
sample rounded to 14 bits") and, when the error becomes big enough,
subtracts it from the next 14-bit number being sent to the D/A
converter. Thus, over a time scale of many samples, the integrated
output of the D/A is the same as if the D/A accepted 16 bits.

Dithering is a well-known technique which adds noise or random values
to a signal to reduce digitization errors at the cost of S/N ratio.
One cheap way to anti-alias an image for display on a raster monitor,
for example, is to add noise to the edges of things.

Anyone interested in the technology of the CD will want to read not only
the paper by Peek in the February 1985 "IEEE Communications Magazine", but
also Volume 40 No. 6 of the "Philips Technical Review" (1982), a collection
of several papers on the CD.  They are a giant step above what usually
passes for technical information among audio manufacturers.

-- 
                                                           --Steve Correll
sjc@s1-b.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc