#2686: Native AAC encoder collapses at high bitrates on some samples -------------------------------------+------------------------------------- Reporter: Kamedo2 | Owner: Type: defect | Status: open Priority: normal | Component: avcodec Version: git-master | Resolution: Keywords: aac | Blocked By: regression | Reproduced by developer: 1 Blocking: | Analyzed by developer: 0 | -------------------------------------+-------------------------------------
Comment (by klaussfreire): Sorry about the delay. I wanted to give you a 6b patch, since I was making good progress, but I got stalled yesterday. I managed to remove (or, rather, ameliorate) those bitrate outliers, without constraining VBR. I noticed they were related to silence parts. It seems in the absence of any significant signal, it will try to encode the noise, and being noise, it's quite hard to encode. What I did is I modeled the absolute hearing threshold in aacpsy, and now that's performing better. But there's still a tendency to waste bits on noisy transients. I couldn't quite yet confirm it's a waste, all my attempts at saving bits in those, run afoul quality-wise. As if those bits were really needed. But I suspect there's still some work to be done in that regard. So in essence, I achieved some extra efficiency by modeling absolute hearing thresholds. Since you never know what SPL will the sample be playing at, I matched the masking curve's lowest point to 16-bit quantization noise. That should correctly match most playback situations, but I'd like people to comment if there was an explicit reason why absolute thresholds haven't been accounted for. As for your testing plans, you tell me. I can give you the current state of the encoder (I don't expect to do any more progress quickly, I tried lots of things and failed to improve it, so unless I get some kind of inspiration the encoder will remain as is for a while), or you can test the current one. Bitrate-wise, they're similar. The newest one performs a little better since it's unconstrained, but it still has a disadvantage against ABR regarding tonal, quiet passages. -- Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2686#comment:193> FFmpeg <http://ffmpeg.org> FFmpeg issue tracker _______________________________________________ FFmpeg-trac mailing list FFmpeg-trac@avcodec.org http://avcodec.org/mailman/listinfo/ffmpeg-trac