• Timothy B. Terriberry's avatar
    Prevent busts at low bitrates. · 76469c64
    Timothy B. Terriberry authored
    This patch makes all symbols conditional on whether or not there's
     enough space left in the buffer to code them, and eliminates much
     of the redundancy in the side information.
    A summary of the major changes:
    * The isTransient flag is moved up to before the the coarse energy.
      If there are not enough bits to code the coarse energy, the flag
       would get forced to 0, meaning what energy values were coded
       would get interpreted incorrectly.
      This might not be the end of the world, and I'd be willing to
       move it back given a compelling argument.
    * Coarse energy switches coding schemes when there are less than 15
       bits left in the packet:
      - With at least 2 bits remaining, the change in energy is forced
         to the range [-1...1] and coded with 1 bit (for 0) or 2 bits
         (for +/-1).
      - With only 1 bit remaining, the change in energy is forced to
         the range [-1...0] and coded with one bit.
      - If there is less than 1 bit remaining, the change in energy is
         forced to -1.
        This effectively low-passes bands whose energy is consistently
         starved; this might be undesirable, but letting the default be
         zero is unstable, which is worse.
    * The tf_select flag gets moved back after the per-band tf_res
       flags again, and is now skipped entirely when none of the
       tf_res flags are set, and the default value is the same for
       either alternative.
    * dynalloc boosting is now limited so that it stops once it's given
       a band all the remaining bits in the frame, or when it hits the
       "stupid cap" of (64<<LM)*(C<<BITRES) used during allocation.
    * If dynalloc boosing has allocated all the remaining bits in the
       frame, the alloc trim parameter does not get encoded (it would
       have no effect).
    * The intensity stereo offset is now limited to the range
       [start...codedBands], and thus doesn't get coded until after
       all of the skip decisions.
      Some space is reserved for it up front, and gradually given back
       as each band is skipped.
    * The dual stereo flag is coded only if intensity>start, since
       otherwise it has no effect.
      It is now coded after the intensity flag.
    * The space reserved for the final skip flag, the intensity stereo
       offset, and the dual stereo flag is now redistributed to all
       bands equally if it is unused.
      Before, the skip flag's bit was given to the band that stopped
       skipping without it (usually a dynalloc boosted band).
    In order to enable simple interaction between VBR and these
     packet-size enforced limits, many of which are encountered before
     VBR is run, the maximum packet size VBR will allow is computed at
     the beginning of the encoding function, and the buffer reduced to
     that size immediately.
    Later, when it is time to make the VBR decision, the minimum packet
     size is set high enough to ensure that no decision made thus far
     will have been affected by the packet size.
    As long as this is smaller than the up-front maximum, all of the
     encoder's decisions will remain in-sync with the decoder.
    If it is larger than the up-front maximum, the packet size is kept
     at that maximum, also ensuring sync.
    The minimum used now is slightly larger than it used to be, because
     it also includes the bits added for dynalloc boosting.
    Such boosting is shut off by the encoder at low rates, and so
     should not cause any serious issues at the rates where we would
     actually run out of room before compute_allocation().
quant_bands.h 3.17 KB