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energy decoding

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Jean-Marc Valin 2011-02-23 17:29:42 -05:00
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commit 19caaddba8
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doc/draft-ietf-codec-opus.xml
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@ -506,25 +506,26 @@ Insert decoder figure.
<ttcol align='center'>Symbol(s)</ttcol>
<ttcol align='center'>PDF</ttcol>
<ttcol align='center'>Condition</ttcol>
<c>silence</c> <c>logp=15</c> <c></c>
<c>post-filter</c> <c>logp=1</c> <c></c>
<c>silence</c> <c>[32767, 1]/32768</c> <c></c>
<c>post-filter</c> <c>[1, 1]/2</c> <c></c>
<c>octave</c> <c>uniform (6)</c><c>post-filter</c>
<c>period</c> <c>raw bits (4+octave)</c><c>post-filter</c>
<c>gain</c> <c>raw bits (3)</c><c>post-filter</c>
<c>tapset</c> <c>[2, 1, 1]/4</c><c>post-filter</c>
<c>transient</c> <c>logp=3</c><c></c>
<c>transient</c> <c>[7, 1]/8</c><c></c>
<c>intra</c> <c>[7, 1]/8</c><c></c>
<c>coarse energy</c><c><xref target="energy-decoding"/></c><c></c>
<c>tf_change</c> <c><xref target="transient-decoding"/></c><c></c>
<c>tf_select</c> <c>logp=1</c><c><xref target="transient-decoding"/></c>
<c>tf_select</c> <c>[1, 1]/2</c><c><xref target="transient-decoding"/></c>
<c>spread</c> <c>[7, 2, 21, 2]/32</c><c></c>
<c>dyn. alloc.</c> <c><xref target="allocation"/></c><c></c>
<c>alloc. trim</c> <c>[2, 2, 5, 10, 22, 46, 22, 10, 5, 2, 2]/128</c><c></c>
<c>skip (*)</c> <c>logp=1</c><c><xref target="allocation"/></c>
<c>skip (*)</c> <c>[1, 1]/2</c><c><xref target="allocation"/></c>
<c>intensity (*)</c><c>uniform</c><c><xref target="allocation"/></c>
<c>dual (*)</c> <c>logp=1</c><c></c>
<c>dual (*)</c> <c>[1, 1]/2</c><c></c>
<c>fine energy</c> <c><xref target="energy-decoding"/></c><c></c>
<c>residual</c> <c><xref target="PVQ-decoder"/></c><c></c>
<c>anti-collapse</c><c>logp=1</c><c>stereo && transient</c>
<c>anti-collapse</c><c>[1, 1]/2</c><c>stereo && transient</c>
<c>finalize</c> <c><xref target="energy-decoding"/></c><c></c>
<postamble>Order of the symbols in the CELT section of the bit-stream</postamble>
</texttable>
@ -555,23 +556,71 @@ tf_change flags.
</section>
<section anchor="energy-decoding" title="Energy Envelope Decoding">
<t>
The energy of each band is extracted from the bit-stream in two steps according
to the same coarse-fine strategy used in the encoder. First, the coarse energy is
decoded in unquant_coarse_energy() (quant_bands.c)
based on the probability of the Laplace model used by the encoder.
</t>
<t>
After the coarse energy is decoded, the same allocation function as used in the
encoder is called. This determines the number of
bits to decode for the fine energy quantization. The decoding of the fine energy bits
is performed by unquant_fine_energy() (quant_bands.c).
Finally, like the encoder, the remaining bits in the stream (that would otherwise go unused)
are decoded using unquant_energy_finalise() (quant_bands.c).
It is important to quantize the energy with sufficient resolution because
any energy quantization error cannot be compensated for at a later
stage. Regardless of the resolution used for encoding the shape of a band,
it is perceptually important to preserve the energy in each band. CELT uses a
three-step coarse-fine-fine strategy for encoding the energy in the base-2 log
domain, as implemented in quant_bands.c</t>
<section anchor="coarse-energy-decoding" title="Coarse energy decoding">
<t>
Coarse quantization of the energy uses a fixed resolution of 6 dB
(integer part of base-2 log). To minimize the bitrate, prediction is applied
both in time (using the previous frame) and in frequency (using the previous
bands). The part of the prediction that is based on the
previous frame can be disabled, creating an "intra" frame where the energy
is coded without reference to prior frames. The decoder first reads the intra flag
to determine what prediction is used.
The 2-D z-transform of
the prediction filter is: A(z_l, z_b)=(1-a*z_l^-1)*(1-z_b^-1)/(1-b*z_b^-1)
where b is the band index and l is the frame index. The prediction coefficients
applied depend on the frame size in use when not using intra energy and a=0 b=4915/32768
when using intra energy.
The time-domain prediction is based on the final fine quantization of the previous
frame, while the frequency domain (within the current frame) prediction is based
on coarse quantization only (because the fine quantization has not been computed
yet). The prediction is clamped internally so that fixed point implementations with
limited dynamic range to not suffer desynchronization.
We approximate the ideal
probability distribution of the prediction error using a Laplace distribution
with seperate parameters for each frame size in intra and inter-frame modes. The
coarse energy quantization is performed by unquant_coarse_energy() and
unquant_coarse_energy_impl() (quant_bands.c). The encoding of the Laplace-distributed values is
implemented in ec_laplace_decode() (laplace.c).
</t>
</section>
<section anchor="fine-energy-decoding" title="Fine energy quantization">
<t>
The number of bits assigned to fine energy quantization in each band is determined
by the bit allocation computation described in <xref target="allocation"></xref>.
Let B_i be the number of fine energy bits
for band i; the refinement is an integer f in the range [0,2^B_i-1]. The mapping between f
and the correction applied to the coarse energy is equal to (f+1/2)/2^B_i - 1/2. Fine
energy quantization is implemented in quant_fine_energy() (quant_bands.c).
</t>
<t>
When some bits are left "unused" after all other flags have been decoded, these bits
are assigned to a "final" step of fine allocation. In effect, these bits are used
to add one extra fine energy bit per band per channel. The allocation process
determines two <spanx style="emph">priorities</spanx> for the final fine bits.
Any remaining bits are first assigned only to bands of priority 0, starting
from band 0 and going up. If all bands of priority 0 have received one bit per
channel, then bands of priority 1 are assigned an extra bit per channel,
starting from band 0. If any bit is left after this, they are left unused.
This is implemented in unquant_energy_finalise() (quant_bands.c).
</t>
</section> <!-- fine energy -->
</section> <!-- Energy decode -->
<section anchor="allocation" title="Bit allocation">
<t>Bit allocation is performed based only on information available to both
the encoder and decoder. The same calculations are performed in a bit-exact