[FFmpeg-devel] [PATCH] add E-AC-3 support to AC-3 decoder

[Michael Niedermayer]

On Sat, Jul 12, 2008 at 01:12:41PM -0400, Justin Ruggles wrote:
> Hi,
> 
> Michael Niedermayer wrote:
> > On Sat, Jun 07, 2008 at 10:30:31AM -0400, Justin Ruggles wrote:
> >> +        for (i = 1; i < 6; i++) {
> >> +            tmp += ((int64_t)idct_cos_tab[blk][i-1] * (int64_t)s->pre_mantissa[i][ch][bin]) >> 23;
> >> +        }
> >> +        s->fixed_coeffs[ch][bin] = tmp >> s->dexps[ch][bin];
> >> +    }
> >> +}
> > 
> > there are symmetries in the idct, this brute force solution is a little
> > umm ...
> 
> The only solution I could come up with which takes advantage of some
> symmetry is still brute-force for the first 3 blocks, but essentially
> just copies the data with a sign flip for odd index values in the second
> 3 blocks.  This increases the speed of the function by 40% and overall
> E-AC3 decoding by 7% when AHT is used.  Is that adequate?

The transformation matrix is:

1.000000  1.366025  1.224745  1.000000  0.707107  0.366025 
1.000000  1.000000  0.000000 -1.000000 -1.414214 -1.000000 
1.000000  0.366025 -1.224745 -1.000000  0.707107  1.366025 
1.000000 -0.366025 -1.224745  1.000000  0.707107 -1.366025 
1.000000 -1.000000 -0.000000  1.000000 -1.414214  1.000000 
1.000000 -1.366025  1.224745 -1.000000  0.707107 -0.366025 

after one pass of butterflies thats just
1.000000 0.000000  1.224745  0.000000  0.707107  0.000000 
1.000000 0.000000 -0.000000  0.000000 -1.414214  0.000000 
1.000000 0.000000 -1.224745  0.000000  0.707107  0.000000 
0.000000 0.366025  0.000000 -1.000000  0.000000  1.366025 
0.000000 1.000000  0.000000 -1.000000  0.000000 -1.000000 
0.000000 1.366025  0.000000  1.000000  0.000000  0.366025 

Even a lame brute force implementation of that should be more than 40%
faster, that is due to the simple and repeated coefficients

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

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle
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