Commit 76469c64 authored by Committed by Jean-Marc ValinBrowse files
Prevent busts at low bitrates.
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().