[vorbis] what is floor? HERE IT COMES :-)

[Alan MacDonald]

Vincent wrote:

>There is still something that i don't understand :
>The floor is an approximation of the spectral image of a chanel, and the
>residue is the real spectrum of the chanel minus the floor.
>Then only the residue is encoded ?
>  
>
No both the floor and residue are encoded.

>If so, when decoding, how the decoder can know the shape of the floor that
>has been used to obtain the residue?
>  
>
See above.

>If not, both the floor and the residue are encoded, then it means it takes
>the same space than encoding the real spectrum, isn't it?
>
>  
>
Not at all.  The psycho-acoustic processing removes some spectral 
components.  Addtionally, the floor is actually encoded as just a small 
number of "post" values (around 30 I believe).   Each  post is like a 
point along the curve.  The whole floor curve is then drawn by using 
those post values.   This is for floor type 1, there is also floor type 
0 which was only used in some pre-release encoders, but is still 
supported by the Vorbis decoder spec.   I may have over simplified the 
explanation a bit, you can find more details here:

http://xiph.org/ogg/vorbis/doc/vorbis-spec-floor1.html
as it states:   "Vorbis floor type one uses a piecewise straight-line 
representation to encode a spectral envelope curve. The representation 
plots this curve mechanically on a linear frequency axis and a 
logarithmic (dB) amplitude axis. The integer plotting algorithm used is 
similar to Bresenham's algorithm."

Hope that helps,
Alan

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