Commit 9ac1673c authored by Jean-Marc Valin's avatar Jean-Marc Valin
Browse files

PVQ doc

parent 52cb5fb3
......@@ -271,12 +271,12 @@ It is derived from a basic (with full overlap) window that is the same as the on
</t>
</section>
<section anchor="Bands and Normalization" title="Bands and Normalization">
<section anchor="normalization" title="Bands and Normalization">
<t>
The MDCT output is divided into bands that are designed to match the ear's critical bands,
with the exception that they have to be at least 3 bins wide. For each band, the encoder
computes the energy, that will later be encoded. Each band is then normalized by the
square root of the <spanx style="strong">unquantized</spanx> energy, such that each band now forms a unit vector.
square root of the <spanx style="strong">unquantized</spanx> energy, such that each band now forms a unit vector X.
The energy and the normalization are computed by compute_band_energies()
and normalise_bands() (<xref target="bands.c">bands.c</xref>), respectively.
</t>
......@@ -360,12 +360,28 @@ compute_pitch_gain() (<xref target="bands.c">bands.c</xref>).
<section anchor="pvq" title="Spherical Vector Quantization">
<t>CELT uses a Pyramid Vector Quantization (PVQ) <xref target="PVQ"></xref>
codebook for quantising the details of the spectrum in each band that have not
been predicted by the pitch predictor. The PVQ codebook consists of all combinations
of K pulses signed in a vector of N samples.
been predicted by the pitch predictor. The PVQ codebook consists of all sums
of K signed pulses in a vector of N samples, where two pulses at the same position
are required to have the same sign. We can thus say that the codebook includes
all codevectors y of N dimensions that satisfy sum(abs(y(j))) = K.
</t>
<t>
The search is performed by alg_quant() (<xref target="vq.c">vq.c</xref>).
In bands where no pitch and no folding is used, the PVQ is used directly to encode
the unit vector that results from the normalisation in
<xref target="normalization"></xref>. Given a PVQ codevector y, the unit vector X is
obtained as X = y/||y||. Where ||.|| denotes the L2 norm. In the case where a pitch
prediction or a folding vector P is used, the unit vector X becomes:
</t>
<t>X = P + g_f * y,</t>
<t>where g_f = ( sqrt( (y^T*P)^2 + ||y||^2*(1-||P||^2) ) - y^T*P ) / ||y||^2. </t>
<t>
The search for the best codevector y is performed by alg_quant()
(<xref target="vq.c">vq.c</xref>). There are several possible approaches to the
search with a tradeoff between quality and complexity. The method used in the reference
implementation consists of first projecting the residual signal R = X - P onto the codebook
pyramid.
</t>
<section anchor="Index Encoding" title="Index Encoding">
......
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