Change #275177

Comments

avcodec/aacenc: rework NMR stereo decisions, retune coded bandwidth
Stereo decisions are made per band before quantization, from the psy
model's spectra, and carry cross-frame memory (EMA-smoothed statistics,
per-grid mode banks, leave-hysteresis) so the image holds instead of
churning:

  * M/S adopts content-driven and rate-free (side under half the mid);
    for mid-dominant bands M/S is simply the better coding at every
    rate, and a wandering L/R fraction reads as image instability.
  * I/S competes with M/S above 6.1 kHz instead of only seeing M/S
    rejects (which are exactly the wide bands it cannot render), and
    engages under SUSTAINED strain only: the pressure ramp gated by the
    lambda floor, so pressure spikes at a comfortable operating point
    cannot flood it onto content where coding the side is affordable
    and strictly better. Unengaged candidates fall back to M/S.
  * Pairs whose joint-tool candidacy fraction stays low decouple:
    block switching goes per-channel and M/S stops, matching how
    independent coding wins on diffuse decorrelated content.
  * PNS in a pair is reserved for clearly-wide bands (it renders
    uncorrelated noise per channel).

Block switching for a pair is decided through the psy window_pair()
hook so common_window survives transients.

The NMR rate-to-bandwidth table is retuned upward at >= 48 kbps/ch to
track the bandwidth strong encoders deliver; both quality metrics
improve on a 16-clip battery and wider regresses. Lower rates are
unchanged.

Changed files

• libavcodec/aacenc.c
• tests/fate/aac.mak
• tests/ref/fate/id3v2-reenc-delete-metadata
• tests/ref/fate/id3v2-reenc-delete-metadata-keep
• tests/ref/fate/id3v2-reenc-delete-metadata-keep-format
• tests/ref/fate/id3v2-reenc-delete-metadata-keep-stream
• tests/ref/fate/id3v2-reenc-delete-metadata-map-metadata