[vorbis-dev] intro to compressed audio tutorial

[Beni Cherniavksy]

On 2002-02-06, Graham Mitchell wrote:

> > Hanging around #vorbis, I can tell you that a lot of people show up with
> > misconceptions about compressed (especially lossy) audio.  An introduction
> > to the area which uses Vorbis as the specific example would be a nice
> > thing. Most people learned about lossy audio from MP3 and some have
> > assigned attributes of MP3 to Vorbis incorrectly.  Such a document would
> > also have some tutorial aspects as well.  If this interests you, I (and
> > many others on the list) could suggest some topics for inclusion.
>
> This strikes my fancy.
>
> Rather than wait around for topics, I just wrote up a few pages since I had a
> free evening.  New topics are easily added, and if it's too long, it's always
> easier to cut than to fill.
>
> http://cs.leander.isd.tenet.edu/~mitchell/vorbis_intro.html
>
> Please provide corrections, feedback, suggestions for additions, flames, or
> whatever, either to the list or to me personally.
>
Very nice!  Thanks for the efforts.

At the end of the lossless passage, "Which this is ideal for some users,
the average user needs more." should probably say "While ...".

Where you choose to stick with vorbis, the openness of vorbis is
understaed, I think.  "is the only format among all the new ones that is
not heavily patented" might mean that it's only lightly patented ;) -
you should say that it's completely patent-free (while other are
heavily patented) and you never say that all the code is open-source.

CBR/ABR: True CBR means completely constant bitrate while ABR is constant
when averaged over some time window (a VBR, only required compensate).
True CBR is not used by any compressed format AFAIK!  The benefits of
ABR/CBR is for streaming (and simplified seeking) but it can always be
assumed that the player has some buffering.  This allows to use ABR,
improving the quality somewhat.  So everyone uses it.  MP3 in non-VBR mode
has a "bit reservoir" that allows to use one frame for bits belonging to
one onf several adjustant frames (or something like this).  So the term
CBR is almost always incorrectly used to describe ABR which is the right
term.  The vorbis position is to always say "ABR".  RC3 managed mode uses
a 2 second window IIRC.

It's also important to note that VBR means changing bitrate not inly
between parts of a file but between files.  So if a user learns that he
likes the quality -q X and it averages to bitrate Y on his test file, it's
better to encode other files with quality X than with bitrate Y (even if
there wouldn't be quality/size penalty for managed modes) because when
specifying quality, vorbis will automatically use different total bitrates
as needed to achieve same quality in all files (hopefully ;).


-- 
Beni Cherniavsky <cben at tx.technion.ac.il>
                 (also scben at t2 in Technion)

Best way to lose ~6GB music and other personal files: create a roaming
profile on another computer pointing to your My Documents instead of your
profile dir; logon - it won't find your settings; so logout - it will be
"saving your settings" very long time because it is busy deleting all
unrecognized things in your directory.  [Personally experienced on win2k].

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