[FFmpeg-devel] CNG (consistent noise generation) patch for AC-3 decoder

[Jonathan Campbell]

CRC computation isn't fast enough? What should I use then? A sum of byte 
values?

Jonathan Campbell
CASTUS

On 09/02/2016 04:50 AM, Michael Niedermayer wrote:
> On Thu, Sep 01, 2016 at 06:59:46PM -0700, Jonathan Campbell wrote:
>> I finished the consistent noise generation patch for AC-3 decoding.
>>
>> Set AVOption "cons_noisegen" to 1 (true) to enable it.
>>
>> Git repository:
>> https://github.com/joncampbell123/FFmpeg.git
>>
>> commit dbd086586f0ad1591ea2013293bbb6e4dbfd0455
>> Author: Jonathan Campbell <jonathan at castus.tv>
>> Date:   Thu Sep 1 18:46:16 2016 -0700
>>
>>      AC-3 consistent noise generation: avopt -cons_noisegen <number>
>>
>>      When -cons_noisegen 1, the linear feedback generator used for AC-3
>>      dithering is seeded with the contents of the AC-3 frame. Seeding from
>>      the AC-3 frame ensures the dithering noise comes out exactly the same
>>      when given the same AC-3 frame data, which can then be used by
>>      non-linear editing software to reliably decode discontinuous
>> segments of
>>      an AC-3 bitstream without gaps or discontinuities.
>>
>> Jonathan Campbell
>>
>> Patch follows, hope Thunderbird doesn't garble it:
>>
>> diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
>> index fac189b..28048d7 100644
>> --- a/libavcodec/ac3dec.c
>> +++ b/libavcodec/ac3dec.c
>> @@ -1419,6 +1419,19 @@ static int ac3_decode_frame(AVCodecContext *
>> avctx, void *data,
>>                               (const uint16_t *) buf, cnt);
>>       } else
>>           memcpy(s->input_buffer, buf, FFMIN(buf_size,
>> AC3_FRAME_BUFFER_SIZE));
>> +
>> +    /* if consistent noise generation is enabled, seed the linear
>> feedback generator
>> +     * with the contents of the AC-3 frame so that the noise is
>> identical across
>> +     * decodes given the same AC-3 frame data, for use with
>> non-linear edititing software. */
>> +    if (s->consistent_noise_generation) {
>> +        const AVCRC *avcrc = av_crc_get_table(AV_CRC_32_IEEE);
>> +
>> +        if (avcrc != NULL)
>> +            av_lfg_init(&s->dith_state, av_crc(avcrc, 0,
>> s->input_buffer, FFMIN(buf_size, AC3_FRAME_BUFFER_SIZE)));
> av_lfg_init involves md5 which is slow and not apropriate to be
> executed per frame, this would double the time to decode a frame
>
>   965641 decicycles in rndint,      64 runs,      0 skips
> 2375023 decicycles in FRAME,      64 runs,      0 skips
>
>
> write/use a faster way to init the LFG please
>
>
> [...]
>
>
>
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