• Claudio Freire's avatar
    AAC encoder: Extensive improvements · 01ecb717
    Claudio Freire authored
    This finalizes merging of the work in the patches in ticket #2686.
    
    Improvements to twoloop and RC logic are extensive.
    
    The non-exhaustive list of twoloop improvments includes:
     - Tweaks to distortion limits on the RD optimization phase of twoloop
     - Deeper search in twoloop
     - PNS information marking to let twoloop decide when to use it
       (turned out having the decision made separately wasn't working)
     - Tonal band detection and priorization
     - Better band energy conservation rules
     - Strict hole avoidance
    
    For rate control:
     - Use psymodel's bit allocation to allow proper use of the bit
       reservoir. Don't work against the bit reservoir by moving lambda
       in the opposite direction when psymodel decides to allocate more/less
       bits to a frame.
     - Retry the encode if the effective rate lies outside a reasonable
       margin of psymodel's allocation or the selected ABR.
     - Log average lambda at the end. Useful info for everyone, but especially
       for tuning of the various encoder constants that relate to lambda
       feedback.
    
    Psy:
     - Do not apply lowpass with a FIR filter, instead just let the coder
       zero bands above the cutoff. The FIR filter induces group delay,
       and while zeroing bands causes ripple, it's lost in the quantization
       noise.
     - Experimental VBR bit allocation code
     - Tweak automatic lowpass filter threshold to maximize audio bandwidth
       at all bitrates while still providing acceptable, stable quality.
    
    I/S:
     - Phase decision fixes. Unrelated to #2686, but the bugs only surfaced
       when the merge was finalized. Measure I/S band energy accounting for
       phase, and prevent I/S and M/S from being applied both.
    
    PNS:
     - Avoid marking short bands with PNS when they're part of a window
       group in which there's a large variation of energy from one window
       to the next. PNS can't preserve those and the effect is extremely
       noticeable.
    
    M/S:
     - Implement BMLD protection similar to the specified in
       ISO-IEC/13818:7-2003, Appendix C Section 6.1. Since M/S decision
       doesn't conform to section 6.1, a different method had to be
       implemented, but should provide equivalent protection.
     - Move the decision logic closer to the method specified in
       ISO-IEC/13818:7-2003, Appendix C Section 6.1. Specifically,
       make sure M/S needs less bits than dual stereo.
     - Don't apply M/S in bands that are using I/S
    
    Now, this of course needed adjustments in the compare targets and
    fuzz factors of the AAC encoder's fate tests, but if wondering why
    the targets go up (more distortion), consider the previous coder
    was using too many bits on LF content (far more than required by
    psy), and thus those signals will now be more distorted, not less.
    
    The extra distortion isn't audible though, I carried extensive
    ABX testing to make sure.
    
    A very similar patch was also extensively tested by Kamendo2 in
    the context of #2686.
    01ecb717
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