[FFmpeg-devel] [PATCH] ac3dec_fixed: always use the USE_FIXED=1 variant of the AC3DecodeContext

[Andreas Cadhalpun]

Hi,

On 13.03.2015 22:46, Christophe Gisquet wrote:
> 2015-03-13 22:28 GMT+01:00 Andreas Cadhalpun <andreas.cadhalpun at googlemail.com>:
>> -int ff_eac3_parse_header(AC3DecodeContext *s);
>> +static int ff_eac3_parse_header(AC3DecodeContext *s);
> 
> It's somewhat cosmetics, but if these functions become static, they
> would better drop the ff_ prefix.

I don't mind the names, but I tried to keep the changes minimal, to
ease backporting the fix.
The names can be changed in a follow-up patch.

>> -            float accum = 0.0f;
>> +            INTFLOAT accum = 0.0f;
> [...]
>>              for (i = 0; i < bandsize; i++) {
>> -                float coeff = s->transform_coeffs[ch][bin++];
>> +                INTFLOAT coeff = s->transform_coeffs[ch][bin++];
>>                  accum += coeff * coeff;
> [...]
>> -            float nscale = s->spx_noise_blend[ch][bnd] * rms_energy[bnd] * (1.0f / INT32_MIN);
>> -            float sscale = s->spx_signal_blend[ch][bnd];
>> +            INTFLOAT nscale = s->spx_noise_blend[ch][bnd] * rms_energy[bnd] * (1.0f / INT32_MIN);
>> +            INTFLOAT sscale = s->spx_signal_blend[ch][bnd];
>>              for (i = 0; i < s->spx_band_sizes[bnd]; i++) {
>> -                float noise  = nscale * (int32_t)av_lfg_get(&s->dith_state);
>> +                INTFLOAT noise  = nscale * (int32_t)av_lfg_get(&s->dith_state);
> 
> Does that work at all? I mean, if it's fixedpoint, I would have
> expected some renormalization, various things to avoid overflows,
> potential warnings because of casts, etc.
> 
> And is the output on eac3 samples, eg here:
> http://samples.mplayerhq.hu/A-codecs/AC3/eac3/
> (particularly the spx ones)
> anything listenable?
> 
> Maybe the extensions should simply be declared as unsupported by the fp version?

It works well enough, I think.
Without this patch:
$ ffmpeg -c:a ac3_fixed -i ./rio_bravo_mono_64_spx.ac3 broken.ac3
[eac3 @ 0x2401560] Estimating duration from bitrate, this may be inaccurate
Input #0, eac3, from '/tmp/rio_bravo_mono_64_spx.ac3':
  Duration: 00:04:22.18, start: 0.000000, bitrate: 64 kb/s
    Stream #0:0: Audio: ac3, 48000 Hz, mono, s16p, 64 kb/s
Output #0, ac3, to '/tmp/broken.ac3':
  Metadata:
    encoder         : Lavf56.25.101
    Stream #0:0: Audio: ac3, 48000 Hz, mono, fltp, 96 kb/s
    Metadata:
      encoder         : Lavc56.26.100 ac3
Stream mapping:
  Stream #0:0 -> #0:0 (ac3 (ac3_fixed) -> ac3 (native))
Press [q] to stop, [?] for help
[ac3_fixed @ 0x24024c0] exponent out-of-range
[ac3_fixed @ 0x24024c0] error decoding the audio block
...
many more such errors
...
[ac3_fixed @ 0x24024c0] exponent out-of-range
[ac3_fixed @ 0x24024c0] error decoding the audio block
Segmentation fault (core dumped)


Make sure the volume is at a low level, when playing the
broken.ac3, or your ears will hurt.

With this patch:
$ ffmpeg -c:a ac3_fixed -i ./rio_bravo_mono_64_spx.ac3 out.ac3
[eac3 @ 0xb5b560] Estimating duration from bitrate, this may be inaccurate
Input #0, eac3, from '/tmp/rio_bravo_mono_64_spx.ac3':
  Duration: 00:04:22.18, start: 0.000000, bitrate: 64 kb/s
    Stream #0:0: Audio: ac3, 48000 Hz, mono, s16p, 64 kb/s
Output #0, ac3, to '/tmp/out.ac3':
  Metadata:
    encoder         : Lavf56.25.101
    Stream #0:0: Audio: ac3, 48000 Hz, mono, fltp, 96 kb/s
    Metadata:
      encoder         : Lavc56.26.100 ac3
Stream mapping:
  Stream #0:0 -> #0:0 (ac3 (ac3_fixed) -> ac3 (native))
Press [q] to stop, [?] for help
size=    3072kB time=00:04:22.17 bitrate=  96.0kbits/s    
video:0kB audio:3072kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%

The out.ac3 sounds much like the input.

Best regards,
Andreas