Adds a new bitexact_log2tan() function which is much simpler, and
more accurate.
The new approximation has an RMS error of 0.0038 bits from the
correctly rounded result over the range of inputs we use, compared
to an RMS error of 0.013 for the old log2_frac() method.
The actual computation of delta is also changed to use FRAC_MUL16,
since this allows us to keep the full accuracy of the new method
while avoiding 16-bit overflow.
The old delta computation actually could overflow 16 bits: it needed
8 for the log2_frac() result, 1 for the sign of the difference, and
8 more for N.
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().
B contains the number of blocks _after_ splitting.
We were using it to decide a) when to use a uniform PDF instead of a
triangular one for theta and b) whether to bias the bit allocation
towards the lower bins.
Using B0 (the number of blocks before the split) instead for a)
gives a PEAQ gain of 0.003 ODG (as high as 0.1 ODG on s02a samples
006, 083, and 097) for 240-sample frames at 96kbps mono.
Using B0 instead for b) gives a gain of only 0.00002.
The mid = (lo+hi)>>1 line in the binary search would allow hi to drop
down to the same value as lo, meaning the rounding after the search
would be choosing between the same two values.
This patch changes it to (lo+hi+1)>>1.
This will allow lo to increase up to the value hi, but only in the
case that we can't possibly allocate enough pulses to meet the
target number of bits (in which case the rounding doesn't matter).
To pay for the extra add, this moves the +1 in the comparison to bits
to the other side, which can then be taken outside the loop.
The compiler can't normally do this because it might cause overflow
which would change the results.
This rarely mattered, but gives a 0.01 PEAQ improvement on 12-byte
120 sample frames.
It also makes the search process describable with a simple
algorithm, rather than relying on this particular optimized
implementation.
I.e., the binary search loop can now be replaced with
for(lo=0;lo+1<cache[0]&&cache[lo+1]<bits;lo++);
hi=lo+1;
and it will give equivalent results.
This was not true before.
This renames ec_dec_cdf() to ec_dec_icdf(), and changes the
functionality to use an "inverse" CDF table, where
icdf[i]=ft-cdf[i+1].
The first entry is omitted entirely.
It also adds a corresonding ec_enc_icdf() to the encoder, which uses
the same table.
One could use ec_encode_bin() by converting the values in the tables
back to normal CDF values, but the icdf[] table already has them in
the form ec_encode_bin() wants to use them, so there's no reason to
translate them and then translate them back.
This is done primarily to allow SILK to use the range coder with
8-bit probability tables containing cumulative frequencies that
span the full range 0...256.
With an 8-bit table, the final 256 of a normal CDF becomes 0 in the
"inverse" CDF.
It's the 0 at the start of a normal CDF which would become 256, but
this is the value we omit, as it already has to be special-cased in
the encoder, and is not used at all in the decoder.
The band where intensity stereo begins was being coded as an
absolute value, rather than relative to start, even though the
range of values in the bitstream was limited as if it was being
coded relative to start (meaning there would be desync if
intensity was sufficiently large).
The valid bands range from [start,end) everywhere, with start<end.
Therefore end should never be 0, and should be allowed to extend
all the way to mode->nbEBands.
This patch does _not_ enforce that start<end, and it does _not_
handle clearing oldBandE[] when the valid range changes, which
are separate issues.
cf874373 raised the limit from 7 to 8 for N>1 bands in
interp_bits2pulses(), but did not raise the corresponding limits
for N=1 bands, or for [un]quant_energy_finalise().
This commit raises all of the limits to the same value, 8.
This way if a band doesn't get the fine bits we want because it
wasn't allocated enough bits to start with, then we will still
give it priority for any spare bits after PVQ.
ec_byte_read() ec_byte_read_from_end() had different return types.
ec_dec_bits() was storing its return value as int instead of
ec_uint32, which will break if int is only 16 bits.
For our current usage, this doesn't matter, but is more consistent
with the rest of the API.
We may want to reduce this to an unsigned char[], but I'd rather
coordinate that optimization with SILK's planned reduction to
8-bit CDFs, as we may be able to use the same code.
Introduced by 30df6cf3.
This should have only affected the output in the case where the last
few extra bits caused us to bust, and wouldn't have prevented us
from detecting the error.
This simplifies a good bit of the error handling, and should make it
impossible to overrun the buffer in the encoder or decoder, while
still allowing tell() to operate correctly after a bust.
The encoder now tries to keep the range coder data intact after a
bust instead of corrupting it with extra bits data, though this is
not a guarantee (too many extra bits may have already been flushed).
It also now correctly reports errors when the bust occurs merging the
last byte of range coder and extra bits.
A number of abstraction barrier violations were cleaned up, as well.
This patch also includes a number of minor performance improvements:
ec_{enc|dec}_bits() in particular should be much faster.
Finally, tf_select was changed to be coded with the range coder
rather than extra bits, so that it is at the front of the packet
(for unequal error protection robustness).
This means we're "time-ordered" in all cases except when increasing
the time resolution on frames that already use short blocks.
There's no reordering when increasing the frequency resolution
on short blocks.
Dynalloc becomes 2x more likely every time we use it, until it
reaches a probability of 1/4. Allocation increments now have
a floor of 1/8 bit/sample and a ceiling of 1 bit/sample.
