[FFmpeg-devel] [PATCH] Common ACELP code & G.729 [2/7] - pitch lag decoding

[Michael Niedermayer]

On Fri, Jun 27, 2008 at 09:59:49PM +0700, Vladimir Voroshilov wrote:
> 2008/6/27 Michael Niedermayer <michaelni at gmx.at>:
> > On Fri, Jun 27, 2008 at 08:30:56PM +0700, Vladimir Voroshilov wrote:
> >> 2008/6/27 Michael Niedermayer <michaelni at gmx.at>:
> >> > On Fri, Jun 27, 2008 at 06:30:00AM +0700, Vladimir Voroshilov wrote:
> >> >> 2008/6/27 Michael Niedermayer <michaelni at gmx.at>:
> >> >> > On Fri, Jun 27, 2008 at 12:00:01AM +0700, Vladimir Voroshilov wrote:
> >> >>
> >> >> [...]
> >> >>
> >> >> >> +
> >> >> >> +    for(i=0; i<ma_pred_order; i++)
> >> >> >> +        energy += quant_energy[i] * ma_prediction_coeff[i];
> >> >> >> +
> >> >> >> +    innov_energy = sum_of_squares(fc_v, subframe_size, 0, 0);
> >> >> >> +#ifdef G729_BITEXACT
> >> >> >
> >> >> >> +    energy +=  MULL(ff_log2(innov_energy), -24660) << 10;
> >> >> >
> >> >> > s/24660/6165/
> >> >> > and i wonder if a normal * would do with the correctly simplified constant?
> >> >>
> >> >> Breaks bitexactness.
> >> >
> >> > you mean (x*2)>>1 != x then you have a overflow in your code
> >>
> >> No, i meant ((x*2)>>2)<<1 != (x>>2)<<2
> >> You didn't take in account MULL macro.
> >> The reason is rounding, not overflow.
> >
> > you should not use MULL!
> > if you do use it, you would have to use it correctly, its not ok to
> > multiply all constants by 4 because you forgot a #define at the top of
> > your code. But as said you should not use MULL at all, use
> > *6165 or *6165LL
> 
> I've rechecked code again.
> And you was right :)
> There is 32bit overflow too. 6165LL solves it, but
> I have question about rounding. What is better:
> (a >> 13)<<10
> or
> (a>>3) & ~0x3ff
> ?
> Such code is required for bitexactness.


i prefer the &~ variant

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

In a rich man's house there is no place to spit but his face.
-- Diogenes of Sinope
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