[vorbis] glossary?

[Ralph Giles]

On Thu, 30 Nov 2000 10:12:52 -0800 Peter Jaques <klez23 at yahoo.com> wrote:

> hi; i'm a lurker & trying to make sense of the discussion... is there a
> glossary of terms available somewhere? mdct, wavelets, 4th dimensions &
> preechoes remind me of my time in the twilight zone;)

Not really. A book on digital audio compression/processing will explain
most of it. There are also some reasonable explainations on the web
(data-compression.com is a good place to start) but I've never found
anything as concise as a glossary.

It would be a good thing to have. Maybe you could put something together
as you learn and share it with us?

To get you started:

mdct - modified discrete cosine transform. Converts data from
amplitude-vs-time to intensity-vs-frequency over a limited window. Closely
related to the fourier transform.

wavelets - another kind of transform, more general. Converts vs-time data
to amplitude per frequency and "scale" with respect to some basis function
(the "wavelet"). A big field, just search on the name to drown in
descriptions.

4th dimensions - I don't really know, but I assume this refers to the
number of coefficients used in the vector quantization scheme, and thus
the dimension of the corresponding encoding space.

preecho - named for what it looks like. A general feature of
fourier-family transforms is that sharp changes in frequency data (as can
arise during the quantization process in lossy compression) result in
extra wiggly bits in the orginal time/space domain. In jpeg image
compression this artifact is called 'ringing' and looks like ripples
around sharp edges. In audio, it looks like an echo of stong peaks,
bluring sharp attacks. Echos after the attack just sound like extra
fuzziness, but those before really muddy the sound in an unnatural way.

Hope that helps,
 -ralph


--
giles at ashlu.bc.ca

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