Neither the encoder nor decoder were incrementing the side-channel
index for a mid-only frame.
Unfortunately, they used that index to index the VAD flags and LBRR
flags.
This made the VAD flags for the side channel particularly useless,
as you couldn't tell which frame a flag belonged to without
decoding most of the packet.
It also desynched the LBRR information, as look at the wrong LBRR
flags to decide when it had to code a mid-only flag.
If some frames were skipped in the side channel, then the last few
VAD flags and LBRR flags would be garbage, but still get encoded.
This patch fixes this by continuing to increment nFramesDecoded or
nFramesEncoded, even when skipping a frame in the side channel.
This makes the side-channel VAD and LBRR flags correspond to the
correct time periods for frames greater than 20 ms.
It also fixes a bug where if DTX was not used on the packet where
the side channel got turned off, it would never get used again
until the encoder attempted to encode something in the side
channel again.
b24e5746 introduced changes to LastGainIndex which broke
conditional coding for side frames after a mid-only frame (i.e.,
in a 60 ms frame where the side is coded, not coded, then coded
again).
These rules were a mess in general, however, because the side
channel state kept a different nFramesDecoded count from the mid
channel state, and had no way to tell if the prior side frame was
coded.
This patch attempts to rationalize them by moving the conditional
coding decision up to the top level, where all this information is
available.
The first coded side frame after an uncoded side frame now always
uses independent coding.
If such a frame is also not the first side frame in an Opus frame,
then it doesn't include an LTP scaling parameter (because the LTP
state is well-defined).
A number of fixes and additions, including
* Ensure all usage of the word "mode" refer only to the choice of
SILK/hybrid/CELT, not audio bandwidth, frame size, channel count,
or anything else.
There's still a bunch of usage of "mode" in CELT to refer to
things in the CELTMode struct (e.g., band layout, etc.).
* Document the LSF reordering for silk_NLSF2A_find_poly().
* Document the DC response check during LSF stabilization.
* Fix the excitation scaling to give decoded SILK output in the
range -1.0...1.0.
* Rewrite the mode-switching section.
Ironically the title of the section still implies "mode" means
than just SILK/hybrid/CELT, but I couldn't come up with a
better one.
* Minor clean-ups to the acknowledgements.
- There was a bug where the decoder resampler was not properly initialized
when fs_kHz == API_fs_kHz. In that case the resampler would continue to
upsample, and the output was corrupt.
- The delay value in the decoder was taken from the state before it was
potentially updated. This caused the decoder to apply the new dalay value one
frame late
- The encoder and decoder states are now updated more consistently, when
the sampling rate changes (pesq liked these changes)
- Properly resetting the side channel encoder and decoder for the first
frame with side coding active again
- Faster updating the "ratio" value in the LR_to_MS() code for large
prediction values means that for certain extreme/artificial input
signals the output looks better
- compile warning in opus_decoder.c
- decoder state reduced by ~3 kB (by merging buffers, as Tim suggested)
- some minor decoder optimizations (only the PLC is non-bit exact, so should be ok)
- increases the max pitch lag by 1 (the thing Tim pointed out). this brings the decoder in sync with the old one
- avoids that the first stereo frame is collapsed to mono
No longer encoding the redundancy flag for SILK since we can infer
redundancy from the length of the frame. Also, we skip encoding the
flag for hybrid mode when we know the decoder will not read it.
1) averaging the output of the left and right resampling states when
switching to mono
2) averaging the the delay buffers from left and right when switching
Adds SILK delay compensation that depends on encode and decode sampling
rate, as well as SILK internal coding rate. This ensures that the SILK
part of Opus is always in sync with the CELT part no matter what the
sampling rates are. It also increases the resampling delay to 1.15 ms
(was previously 0.48 ms).
This patch
* expands sections on LPC and LTP synthesis into something that can
actually be implemented
* fixes an error in the excitation reconstruction
* reverts an erroneous simplification of the subframe gain
decoding, and
* updates the LPC gain limiting description to reflect the new,
more accurate approach for computing the reflection
coefficients.
It also includes a number of general clean-ups, such as
* correcting the description of the sample rates various pieces
run at (e.g., we can decode directly to rates other than 48 kHz)
* the usage of "sampling rate" vs. "sample rate"
* capitalization consistency in TOC titles, and
* better selection of which sections appear in the TOC.
