[FFmpeg-devel] AAC decoder round 9

[Michael Niedermayer]

On Wed, Aug 20, 2008 at 07:44:24PM +0100, Robert Swain wrote:
> 2008/8/20 Robert Swain <robert.swain at gmail.com>:
> > 2008/8/20 Vitor Sessak <vitor1001 at gmail.com>:
> >> Robert Swain wrote:
[...]
> >>> It seems the various conditions on in[] and f0 cause it to exit with
> >>> an error with the coef[] vectors I've tried and as they're potentially
> >>> valid, that's disconcerting for using this code.
> >>>
> >>> compute_lpc_coefs() accepts a two dimensional lpc array as an
> >>> argument. I'm guessing this so that the coefficients are available for
> >>> various orders ready for testing to choose which is best or something
> >>> like that.
> >>>
> >>> Do you have any advice for how to proceed? I'm going to keep prodding
> >>> eval_lpc_coeffs() in the mean time.
> >>
> >> Can the following patch be modified to work with AAC too?
> >
> > I don't like the patch as it is, but you've helped me quite a bit to
> > figure this out. :)
> >
> > lpc[0] = 1 isn't used so I'd like to remove it and start the useful
> > coefficients from index 0. With this, the above-quoted loop can be
> > rewritten to use part of compute_lpc_coefs():
> >
> > // tns_decode_coef
> > for (m = 0; m < order; m++) {
> >    float tmp;
> >    lpc[m] = tns->coef[w][filt][m];
> >    for (i = 0; i < m/2; i++) {
> >        tmp = lpc[i];
> >        lpc[i]     += lpc[m] * lpc[m-1-i];
> >        lpc[m-1-i] += lpc[m] * tmp;
> >    }
> >    if(m & 1)
> >        lpc[i]     += lpc[m] * lpc[i];
> > }
> >
> > Doing it this way avoids the need for the b[] temporary array and
> > produces identical output I think. You should be able to see clearly
> > that this is the same as the main loops within compute_lpc_coefs().
> > So, now the question is how to refactor this code to satisfy everyone.
> >
> > Michael - would you like a single generic macro like the one Vitor
> > proposes? Or maybe two macros, one for this inner set of loops and one
> > for the extra autocorrelation function return value normalisation to
> > convert to autocorrelation coefficients and checks that are needed for
> > the other implementations? Or something else?
> 
> Actually, I think an inline function will be better. I'll have a look
> a go at implementing this after I've eaten.

thats exactly what i wanted to suggest, all these macros are ugly ...
(the reason behind vitors macros was IIRC a compression loss with float
 vs.double in relation to flac but i didnt had the time to investigate
 what excatly was causing it or if there is a better solution ...)

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

No great genius has ever existed without some touch of madness. -- Aristotle
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