AAC encoder: improve SF range utilization

Multimedia / FFmpeg - Claudio Freire [gmail.com] - 2 December 2015 04:47 UTC

This patch does 4 things, all of which interact and thus it woudln't be possible to commit them separately without causing either quality regressions or assertion failures.

Fate comparison targets don't all reflect improvements in quality, yet listening tests show substantially improved quality and stability.

1. Increase SF range utilization.

The spec requires SF delta values to be constrained within the range -60..60. The previous code was applying that range to the whole SF array and not only the deltas of consecutive values, because doing so requires smarter code: zeroing or otherwise skipping a band may invalidate lots of SF choices.

This patch implements that logic to allow the coders to utilize the full dynamic range of scalefactors, increasing quality quite considerably, and fixing delta-SF-related assertion failures, since now the limitation is enforced rather than asserted.

2. PNS tweaks

The previous modification makes big improvements in twoloop's efficiency, and every time that happens PNS logic needs to be tweaked accordingly to avoid it from stepping all over twoloop's decisions. This patch includes modifications of the sort.

3. Account for lowpass cutoff during PSY analysis

The closer PSY's allocation is to final allocation the better the quality is, and given these modifications, twoloop is now
very efficient at avoiding holes. Thus, to compute accurate thresholds, PSY needs to account for the lowpass applied implicitly during twoloop (by zeroing high bands).

This patch makes twoloop set the cutoff in psymodel's context the first time it runs, and makes PSY account for it during threshold computation, making PE and threshold computations closer to the final allocation and thus achieving better subjective quality.

4. Tweaks to RC lambda tracking loop in relation to PNS

Without this tweak some corner cases cause quality regressions. Basically, lambda needs to react faster to overall bitrate efficiency changes since now PNS can be quite successful in enforcing maximum bitrates, when PSY allocates too many bits to the lower bands, suppressing the signals RC logic uses to lower lambda in those cases and causing aggressive PNS.

This tweak makes PNS much less aggressive, though it can still use some further tweaks.

Also update MIPS specializations and adjust fuzz

Also in lavc/mips/aacpsy_mips.h: remove trailing whitespace

ca203e9 AAC encoder: improve SF range utilization
libavcodec/aaccoder.c | 60 +++++++++-----
libavcodec/aaccoder_twoloop.h | 136 +++++++++++++++++++++----------
libavcodec/aacenc.c | 2 +-
libavcodec/aacenc_is.c | 11 ++-
libavcodec/aacenc_utils.h | 63 ++++++++++++++
libavcodec/aacpsy.c | 20 +++--
libavcodec/mips/aaccoder_mips.c | 172 ++++++++++++++++++++++++++-------------
libavcodec/mips/aacpsy_mips.h | 78 +++++++++---------
libavcodec/psymodel.c | 1 +
libavcodec/psymodel.h | 1 +
tests/fate/aac.mak | 26 +++---
11 files changed, 393 insertions(+), 177 deletions(-)

  • Share