[FFmpeg-devel] [PATCH] AAC decoder

[Robert Swain]

2008/5/21 Robert Swain <robert.swain at gmail.com>:
> 2008/4/2 Michael Niedermayer <michaelni at gmx.at>:
>> On Tue, Apr 01, 2008 at 04:56:48PM +0200, Andreas ?man wrote:
>>> Andreas ?man wrote:
>
> [...]
>
>>> +/**
>>> + * Decode scale_factor_data
>>> + * reference: Table 4.47
>>> + */
>>> +static int scale_factor_data(AACContext * ac, GetBitContext * gb, float mix_gain, unsigned int global_gain, ics_struct * ics, const int cb[][64], float sf[][64]) {
>>
>> Maybe rename the function to what the comment says
>>
>>
>>> +    int g, i;
>>> +    unsigned int intensity = 100; // normalization for intensity_tab lookup table
>>> +    int noise = global_gain - 90;
>>> +    int noise_flag = 1;
>>> +    ics->intensity_present = 0;
>>> +    for (g = 0; g < ics->num_window_groups; g++) {
>>> +        for (i = 0; i < ics->max_sfb; i++) {
>>> +            if (cb[g][i] == ZERO_HCB) {
>>> +                sf[g][i] = 0.;
>>
>>> +            } else if (cb[g][i] == INTENSITY_HCB || cb[g][i] == INTENSITY_HCB2) {
>>> +                ics->intensity_present = 1;
>>> +                intensity += get_vlc2(gb, ac->mainvlc.table, 7, 3) - 60;
>>> +                if(intensity > 255) {
>>> +                    av_log(ac->avccontext, AV_LOG_ERROR,
>>> +                           "Intensity (%d) out of range", intensity);
>>> +                    return -1;
>>> +                }
>>> +                sf[g][i] = ac->intensity_tab[intensity];
>>> +            } else if (cb[g][i] == NOISE_HCB) {
>>> +                if (noise_flag) {
>>> +                    noise_flag = 0;
>>> +                    noise += get_bits(gb, 9) - 256;
>>> +                } else {
>>> +                    noise += get_vlc2(gb, ac->mainvlc.table, 7, 3) - 60;
>>> +                }
>>> +                sf[g][i] = pow(2.0, 0.25 * noise) * ac->sf_scale;
>>> +            } else {
>>> +                global_gain += get_vlc2(gb, ac->mainvlc.table, 7, 3) - 60;
>>> +                if(global_gain > 255) {
>>> +                    av_log(ac->avccontext, AV_LOG_ERROR,
>>> +                           "Global gain (%d) out of range", global_gain);
>>> +                    return -1;
>>> +                }
>>> +                sf[g][i] = ac->pow2sf_tab[global_gain];
>>> +            }
>>
>> The identical code is implemented here 3 times, 2 times with seperate
>> LUTs and once by directly calling pow()
>> The differenes look like bugs. like forgetting to check validity or
>> performing the scaling needed for the float2int16 transform.
>>
>> if(cb[g][i] == NOISE_HCB && noise_flag-- > 0)
>>    gain[index] += get_bits(gb, 9) - 256;
>> else
>>    gain[index] += get_vlc2(gb, ac->mainvlc.table, 7, 3) - 60;
>> if(gain[index] > 255)
>>    ...
>> sf[g][i] = ac->pow2sf_tab[gain[index]];
>
> I tried this and audibly it sounds fine on a few files I tested. I
> want to compare the two tables and pow() * ac->sf_scale call before
> submitting a patch though.

The intensity case is different to the noise and global cases. See
attached patch.

Rob
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