[FFmpeg-devel] [Issue 664] [PATCH] Fix AAC PNS Scaling

[Michael Niedermayer]

On Mon, Oct 06, 2008 at 09:52:51PM -0400, Alex Converse wrote:
> On Mon, Oct 6, 2008 at 9:39 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Mon, Oct 06, 2008 at 08:52:06PM -0400, Alex Converse wrote:
> >> On Mon, Oct 6, 2008 at 8:22 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> > On Mon, Oct 06, 2008 at 03:46:55PM -0400, Alex Converse wrote:
> >> >> On Tue, Sep 30, 2008 at 11:25 PM, Alex Converse <alex.converse at gmail.com> wrote:
> >> >> > Hi,
> >> >> >
> >> >> > The attached patch should fix AAC PNS scaling [Issue 664]. It will not
> >> >> > fix PNS conformance.
> >> >>
> >> >> Here's a sightly updated patch (sqrtf instead of sqrt). The current
> >> >> method of PNS will never conform because sample energy simpl doesn't
> >> >> converge to it's mean fast enough. The spec explicitly states that PNS
> >> >> should be normalized per band. Not doing it that way causes PNS-1
> >> >> conformance to fail for 45 bands.
> >> >
> >> > elaborate, what part of the spec says what?
> >>
> >> 14496-3:2005/4.6.13.3 p184 (636 of the PDF)
> >>
> >> > what is PNS-1 conformance ?
> >>
> >> 14496-4:2004/6.6.1.2.2.4 p94 (102 PDF)
> >> 14496-5/conf_pns folder
> >
> > do you happen to have URLs for these?
> >
> >
> >>
> >> > the part that feels a little odd is normalizing random data on arbitrary
> >> > and artificial bands, this simply makes things non random.
> >> > This would be most extreemly vissibly with really short bands of 1 or 2
> >> > coeffs ...
> >> > another way to see the issue is to take 100 coeffs and split them into
> >> > 10 bands, if you now normalize litterally these 10 bands then the 100
> >> > coeffs will no longer be random at all, they will be significantly
> >> > correlated. This may be inaudible, it may or may not sound better and
> >> > may or may not be what the spec wants but still it feels somewhat wrong
> >> > to me ...
> >> >
> >>
> >> Ralph Sperschneider from FhG/MPEG spelled it all out:
> >> http://lists.mpegif.org/pipermail/mp4-tech/2003-June/002358.html
> >>
> >> I'm not saying it's a smart way to design a CODEC but it's what MPEG picked.
> >
> > yes, so i guess the most sensible solution would be to precalculate
> > a second of noise normalized to the band sizes and randomly pick from
> > these.
> >
> 
> That sounds messy and overly complex. What's wrong with doing it the
> way MPEG tells us to? 

that is what mpeg tells us to do, they do not mandate any specific way
to calculate random values. And i do not like doing sqrt() per band ...


> Or just sticking with what we have it sounds
> fine and is fast.

well if its conformant thats fine, but it seemed to me that it is not
though iam still waiting for a quote from the spec to confirm this.


> 
> >
> >>
> >> >
> >> >>
> >> >> However with this patch there appears to be no audible difference
> >> >> between the approaches.
> >> >
> >> >> I don't know the ideal mean energy so I'm
> >> >> using the sample mean energy for 1024 iterations of the LCG.
> >> >
> >> > i assume cpu cycles got more expensive if people can only spare a few
> >> > thousand
> >> >
> >>
> >> How many do you propose then? I tried running it over the whole period
> >> and the result seemed low, I think it's a classic case of adding too
> >> many equal size floating point values.
> >
> > real mathematicans tend not to use floats that are bound to rounding errors
> >
> > try:
> > for(i=min; i<=max; i++){
> >    uint64_t a= i*i;
> >    var += a;
> >    if(var < a){
> >            var2++;
> >    }
> > }
> >
> 
> That will only hold 5 or 6 big values 2^64/((2^31)^2) = 4.

read the code again please, especially var2
also, just to make sure you have the types correct

int min= -(1<<31)+1;
int max=  (1<<31)-1;
int64_t i;
uint64_t var=0;
uint64_t var2=0;

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

The misfortune of the wise is better than the prosperity of the fool.
-- Epicurus
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