On Wed, May 1, 2013 at 7:53 AM, Claudio Freire <klaussfre...@gmail.com> wrote: > On Wed, May 1, 2013 at 4:26 AM, Claudio Freire <klaussfre...@gmail.com> wrote: >>> But still, not perfect... I gave it 256k, and it produced 140k. Lots >>> of room for improvement still, but I thought I'd share this bit. I'll >>> keep looking. >> >> >> Aaand mostly finally in aaccoder.c:713, search_for_quantizers_twoloop >> ignores lambda, so there's no effective rate control. >> >> >> Replacing >> >> int destbits = avctx->bit_rate * 1024.0 / avctx->sample_rate / >> avctx->channels; >> >> with >> >> int destbits = avctx->bit_rate * 1024.0 / avctx->sample_rate / >> avctx->channels * lambda; >> >> Results in general bliss. Will check the other search methods. > > > Ok... I had to forfeit my right to sleep, but here it is. Managed to > make both the fast and 2loop methods work reasonably well (albeit I > don't know why anyone would use fast, because it's not faster. I > assume it's an experiment?). > > I don't have access to AAC standards, though, since they're paywalled > (at least the ones I found), so most of the changes were done blind, > assuming from what I know of audio codecs, and by trial and error. > > In particular, fast has some magic numbers there that I got from trial > and error, and nothing more... profound. > > In any case, attached it is, git diff.
Oh, forgot to explain a bit. I believe the reason why it works for channels=2 and not so much for >2, is that psy already does some rate control by adapting the model to the bit rate and bit reservoire. But that doesn't work for channels > 2, since psy doesn't see all channels at once in those cases, only pairs of channels, and it ends up introducing some significant bias towards an over-estimation. There's the lambda parameter compensating that... except AAC quantization parameter search methods mostly ignored it, save faac (which I didn't try). Fast had some rather nasty bugs too, resulting in entirely zeroed out zones in complex passages. Fixed that somewhat by tweaking numbers, adding bias against zeroing. _______________________________________________ Libav-user mailing list Libav-user@ffmpeg.org http://ffmpeg.org/mailman/listinfo/libav-user
