[FFmpeg-devel] [PATCH] AAC decoder

[Michael Niedermayer]

On Wed, May 28, 2008 at 12:37:04PM +0100, Robert Swain wrote:
> 2008/5/26 Michael Niedermayer <michaelni at gmx.at>:
> > On Mon, May 26, 2008 at 12:42:37PM +0100, Robert Swain wrote:
> >> The tables are still different as intensity uses pow(0.5, (i-100)/4.)
> >> and the other cases use pow(2.0, (i-100)/4.).
> >
> > pow(0.5, (i-100)/4.) == pow(2.0, (100-i)/4.)
> >
> > and
> >
> > pow(2.0, (100-i)/4.) / 1024 == pow(2.0, (100-i)/4.-10) ==pow(2.0, (100-i-40)/4.)
> >
> > possibly these allow the 2 tables to be merged, i mean
> >
> > pow2sf_tab[i] and intensity_tab[i]
> > to
> > pow2sf_tab[i+C] and pow2sf_tab[-i]
> 
> OK, I've thought about this a bit more. I think either sf_scale should
> be 'applied' just before downmixing/float_to_int16 conversion as in
> ac3dec.c or sf_scale can effectively be merged into this table almost
> entirely thanks to being representable as a power of 2 in either the C
> or SIMD float_to_int16 case.
> 
> >From what I see there are 3 cases.
> 
> - intensity table:
>   pow(0.5, (i-100)/4) = pow(2, (100-i)/4)
>   which would have indices [100-255, 100-0] = [-155, 100]
> 
> - sf table when sf_scale is -1/1024:
>   pow(2, (i-100)/4) * -pow(2, -10) = -pow(2, (i-140)/4)
>   ignoring the sign issue, it would have indices [0-140, 255-140] = [-140, 115]
> 
> - sf table when sf_scale is -1/(1024*32768):
>   pow(2, (i-100)/4) * -pow(2, -25) = -pow(2, (i-200)/4)
>   [0-200, 255-200] = [-200, 55]
> 
> So, the range of indices into the table should be [-200, 115],
> sf_scale can be replaced by a constant integer offset into the table
> and we handle the signs with a little branching or something. Does
> that sound like a good idea? Any suggestions for alterations before I
> implement it?

yes that was my idea (note ive not checked if the ranges are correct)

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No snowflake in an avalanche ever feels responsible. -- Voltaire
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