[vorbis-dev] regarding vorbis

[Ralph Giles]

On Sat, Nov 29, 2003 at 06:47:19AM +0000, satish rao wrote:

> iam satish and working on ogg vorbis audio decoder for blackfin processor as a part of my masters project. i would like to clarify certain doubts regarding the algorithmn, like floor, residue, LPC and LSP. iam studying through the documents posted in your website. But, still i need some help and understanding regarding the above topics. What exactly do you mean by floor, residues etc.., 

Good luck with your project!

Well, to answer briefly, the 'floor' is a rough model of the spectal shape of an audio block after 
it has been transformed to frequency space. In vorbis, this is just a piecewise-linear function. 
The residue is what's left over after subtracting the floor from the actual audio spectrum. In 
other words, the part of the signal the floor doesn't model. As with most compression schemes the 
idea is to re-arrange the data extracting features with respect to a model in a way that is easier
to entropy-code the information.

Beta versions of vorbis used a smooth function for the floor instead of a piecewise-linear 
function. This function's representation is known as LSP, for Line Spectal Pair(s). It also has  
a different representation known as LPC for Linear Predictive Coding, which is mentioned because 
it is more widely familiar. While a more sophisticated approach than the piecewise-linear floors 
used in vorbis 1.0, it actually doesn't work as well, so support for this is only needed for 
compatibility beta encoders. It is required for spec compliance, however.

These are all standard terms in the field of audio compression. I'd recommend finding a text on 
the subject if you are unfamiliar with them. We can of course answer specific questions, but 
a text is a better resource for general background and concepts.

Hope that helps,
 -r
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