[vorbis-dev] FLOAT_LOOKUP version of lsp_to_curve

[TIMMERMANS ANDRE]

> -----Original Message-----
> From:	TIMMERMANS ANDRE 
> Sent:	Wednesday, June 26, 2002 8:47 AM
> To:	'vorbis-dev at xiph.org'
> Subject:	RE: [vorbis-dev] FLOAT_LOOKUP version of lsp_to_curve
> 
> > -----Original Message-----
> > From:	TIMMERMANS ANDRE 
> > Sent:	Wednesday, June 26, 2002 8:40 AM
> > To:	vorbis-dev at xiph.org
> > Subject:	[vorbis-dev] FLOAT_LOOKUP version of lsp_to_curve
> > 
> > It seems to me that in this version of the function using
> > 
> >     float p=.7071067812f;
> >     float q=.7071067812f;
> > 
> > will just leads after applying all the products and squaring
> > to p and q having a common factor 0.5 which can be extracted
> > from the formulaes.
> > 
> > So I propose to replace:
> > 
> >     float p=.7071067812f;
> >     float q=.7071067812f;
> >     float w=vorbis_coslook(wdel*k);
> >     float *ftmp=lsp;
> >     int c=m>>1;
> > 
> > by:
> > 
> >     float w=vorbis_coslook(wdel*k);
> >     float p=lsp[0]-w;
> >     float q=lsp[1]-w;
> >     float *ftmp=lsp + 2;
> >     int c=(m>>1) - 1;
> > 
> > and:
> > 
> >     vorbis_invsq2explook(qexp+m)
> > 
> > by:
> > 
> >     vorbis_invsq2explook(qexp+m-1)
> > 
> I forget to add that strictly speaking the looping conditions should changed on
> 
>     do{
>       q*=ftmp[0]-w;
>       p*=ftmp[1]-w;
>       ftmp+=2;
>     }while(--c);
> 
> but if I understand it correctly m is always greater or equal than 8
> which means the loop will always be executed at least once.
> 
I have mad a confusion with windowing/mdct, from floor one I see
the 0 < m < 256. But is really m = 1 valid because in that case
not only my changes are problematic but the integer variant
of lsp_to_curve is incorrect.

André

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