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

[Justin Ruggles]

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?

>> +    // initialize ungrouping table for 1.67-bit GAQ gain codes
>> +    for(i=0; i<32; i++) {
>> +        gaq_ungroup_tab[i][0] = i / 9;
>> +        gaq_ungroup_tab[i][1] = (i % 9) / 3;
>> +        gaq_ungroup_tab[i][2] = i % 3;
>> +    }
> 
> i guess theres no way to factorize this and b1_mantissas related code without
> loosing speed?

Only way I can see is to hard-code the table in ac3dec_data.c and reuse
it to init the b1_mantissas table.

-Justin