Commit 154486bb authored by Jean-Marc Valin's avatar Jean-Marc Valin
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CELT decoder doc

parent 2b5dc862
......@@ -574,10 +574,119 @@ Copy from SILK draft.
<section title="CELT Decoder">
Copy from CELT draft.
Insert decoder figure.
The decoder extracts information from the range-coded bit-stream in the same order
as it was encoded by the encoder. In some circumstances, it is
possible for a decoded value to be out of range due to a very small amount of redundancy
in the encoding of large integers by the range coder.
In that case, the decoder should assume there has been an error in the coding,
decoding, or transmission and SHOULD take measures to conceal the error and/or report
to the application that a problem has occurred.
<section anchor="energy-decoding" title="Energy Envelope Decoding">
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.
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).
<section anchor="pitch-decoding" title="Pitch prediction decoding">
If the pitch bit is set, then the pitch period is extracted from the bit-stream. The pitch
gain bits are extracted within the PVQ decoding as encoded by the encoder. When the folding
bit is set, the folding prediction is computed in exactly the same way as the encoder,
with the same gain, by the function intra_fold() (vq.c).
<section anchor="PVQ-decoder" title="Spherical VQ Decoder">
In order to correctly decode the PVQ codewords, the decoder must perform exactly the same
bits to pulses conversion as the encoder.
<section anchor="cwrs-decoder" title="Index Decoding">
The decoding of the codeword from the index is performed as specified in
<xref target="PVQ"></xref>, as implemented in function
decode_pulses() (cwrs.c).
<section anchor="normalised-decoding" title="Normalised Vector Decoding">
The spherical codebook is decoded by alg_unquant() (vq.c).
The index of the PVQ entry is obtained from the range coder and converted to
a pulse vector by decode_pulses() (cwrs.c).
<t>The decoded normalized vector for each band is equal to</t>
<t>X' = y/||y||,</t>
This operation is implemented in mix_pitch_and_residual() (vq.c),
which is the same function as used in the encoder.
<section anchor="denormalization" title="Denormalization">
Just like each band was normalized in the encoder, the last step of the decoder before
the inverse MDCT is to denormalize the bands. Each decoded normalized band is
multiplied by the square root of the decoded energy. This is done by denormalise_bands()
<section anchor="inverse-mdct" title="Inverse MDCT">
<t>The inverse MDCT implementation has no special characteristics. The
input is N frequency-domain samples and the output is 2*N time-domain
samples, while scaling by 1/2. The output is windowed using the same window
as the encoder. The IMDCT and windowing are performed by mdct_backward
(mdct.c). If a time-domain pre-emphasis
window was applied in the encoder, the (inverse) time-domain de-emphasis window
is applied on the IMDCT result. After the overlap-add process,
the signal is de-emphasized using the inverse of the pre-emphasis filter
used in the encoder: 1/A(z)=1/(1-alpha_p*z^-1).
<section anchor="Packet Loss Concealment" title="Packet Loss Concealment (PLC)">
Packet loss concealment (PLC) is an optional decoder-side feature which
SHOULD be included when transmitting over an unreliable channel. Because
PLC is not part of the bit-stream, there are several possible ways to
implement PLC with different complexity/quality trade-offs. The PLC in
the reference implementation finds a periodicity in the decoded
signal and repeats the windowed waveform using the pitch offset. The windowed
waveform is overlapped in such a way as to preserve the time-domain aliasing
cancellation with the previous frame and the next frame. This is implemented
in celt_decode_lost() (mdct.c).
<section anchor="security" title="Security Considerations">
......@@ -705,6 +814,14 @@ Christopher Montgomery, Karsten Vandborg Soerensen, and Timothy Terriberry.
<seriesInfo name="Ph.D. thesis" value="Dept. of Electrical Engineering, Stanford University" />
<reference anchor="PVQ">
<title>A Pyramid Vector Quantizer</title>
<author initials="T." surname="Fischer" fullname=""><organization/></author>
<date month="July" year="1986" />
<seriesInfo name="IEEE Trans. on Information Theory, Vol. 32" value="pp. 568-583" />
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