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 <jonat...@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. If you fake or manipulate statistics in a paper in medicin you will get a job for life at the pharma industry.
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