[FFmpeg-devel] [PATCH] Move Kaiser-Bessel Derived window to mdct.c

[Robert Swain]

Hello,

On 12/01/2008, Uoti Urpala <uoti.urpala at pp1.inet.fi> wrote:
> On Sat, 2008-01-12 at 15:12 +0000, Robert Swain wrote:
> > On 11/01/2008, Michael Niedermayer <michaelni at gmx.at> wrote:
> > > On Fri, Jan 11, 2008 at 11:02:05PM +0000, Robert Swain wrote:
> > > > I suppose I should test how many iterations are required to achieve
> > > > the maximum precision of float, right?
> > >
> > > thats an option
> >
> > I've had a bit of a think about this and I see three options:
> >
> > - Calculate an estimate of the error and decide the number of
> > iterations dynamically based on that and a specified error threshold.
> >
> > Testing with the various alpha, n for aac.c and ac3dec.c showed the
> > limit of float precision of the final window coefficients was reached
> > in no more than 25 iterations.
> > - Hard code to 50 iterations to be super safe.
> > - Use a #define using 50 iterations to be super safe but allow easy
> > alteration if we find this number at all problematic.
>
> The number of iterations needed depends mostly on alpha.
> alpha * pi/2 + 7.6 * sqrt(alpha) iterations should be enough with normal
> doubles, so 50 iterations should be more than enough until at least
> alpha=14.

Thanks for the analysis, this is useful to know.

Rob