[FFmpeg-devel] [PATCH] G.729A (now fixed-point) decoder

[Vladimir Voroshilov]

On Wed, Mar 19, 2008 at 10:46 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, Mar 19, 2008 at 10:13:05PM +0600, Vladimir Voroshilov wrote:
>  > On Wed, Mar 19, 2008 at 8:53 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>  > > On Wed, Mar 19, 2008 at 08:34:56PM +0600, Vladimir Voroshilov wrote:
>  > >  > On Wed, Mar 19, 2008 at 8:23 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>  > >  > > On Wed, Mar 19, 2008 at 07:27:51PM +0600, Vladimir Voroshilov wrote:
>  > >  > >  [...]
>  > >  > >
>  > >  > > > >  And yes there are issues except spelling, for example why is it not
>  > >  > >  > >  binary identical? Is it either * bugs or * it would be slow/messy/complex?
>  > >  > >  >
>  > >  > >  > Small quick example:
>  > >  > >  > reference code uses L_shr() routine everywhere instead of ">>"
>  > >  > >  > L_shr(-1)==0, L_shr(-3)==2
>  > >  > >
>  > >  > >  L_shr takes 2 parameters
>  > >  > >
>  > >  >
>  > >  > Yes, my fault :)
>  > >  > I meant L_shr(-1,1) and L_shr(-3,1) there
>  > >
>  > >  L_shr(a,b) == a / (1<<b)
>  > >
>  > >  gcc will simplify it IIRC :)
>  > >
>  >
>  > -3/(1<<1) == -1 for me, not -2 :P
>  >
>  > Reference code uses ~((~a)>>b) for negative numbers.
>
>  argh, right, L_shr(a,b) == a>>b
>  iam silly but you are as well because L_shr(-1,1)!=0

L_shr(-1,1) IS equal to zero and
L_shr(-3,1) IS equal to -2
in reference code.

And i'm sure there are other similar things in reference code.
Thus bitexact result is not so easy (at least for me).

>  > Moreover bitexact results can be archived, imho, only if (along with
>  > other things) math operations will be done with the same precision as
>  > in reference code at every stage.
>
>  No, we will have to check them all one by one and see if sane code
>  works as well, only as last resort should we use the braindamaged nonsense
>  the reference is useing ...

What is the criteria of "works as well" ?


-- 
Regards,
Vladimir Voroshilov mailto:voroshil at gmail.com
JID: voroshil at gmail.com, voroshil at jabber.ru
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