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

[Michael Niedermayer]

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 


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you fake or manipulate statistics in a paper in physics you will never
get a job again.
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a job for life at the pharma industry.
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