Commit c10565bd authored by Jean-Marc Valin's avatar Jean-Marc Valin
Browse files

ietf doc: PVQ search

parent 59f67687
......@@ -84,19 +84,7 @@ audio with very low delay. It is suitable for encoding both
speech and music and rates starting at 32 kbit/s. It is primarly designed for transmission
over packet networks and protocols such as RTP <xref target="rfc3550"/>, but also includes
a certain amount of robustness to bit errors, where this could be done at no significant
cost. The codec features are:
</t>
<t>
<list style="symbols">
<t>Ultra-low algorithmic delay (typically 3 to 9 ms)</t>
<t>Full audio bandwidth (44.1 kHz and 48 kHz)</t>
<t>Support for both voice and music</t>
<t>Stereo support</t>
<t>Packet loss concealment</t>
<t>Constant bit-rates from 32 kbps to 128 kbps and above</t>
<t>Free software/open-source/royalty-free</t>
</list>
cost.
</t>
<t>The novel aspect of CELT compared to most other codecs is its very low delay,
......@@ -134,10 +122,19 @@ the codec (version 0.3.2 and 0.5.1, respectively), the principles remain the sam
</t>
<t>CELT is a transform codec, based on the Modified Discrete Cosine Transform
<xref target="mdct"/>, which is based on a DCT-IV, with overlap and time-domain
aliasing calcellation.</t>
<xref target="mdct"/>, derived from the DCT-IV, with overlap and time-domain
aliasing calcellation. The main characteristics of CELT are as follows:
<list style="symbols">
<t>Ultra-low algorithmic delay (typically 3 to 9 ms)</t>
<t>Full audio bandwidth (44.1 kHz and 48 kHz)</t>
<t>Support for both speech and music</t>
<t>Stereo support</t>
<t>Robustness to packet loss</t>
<t>Constant bit-rate from 32 kbps to 128 kbps and above</t>
<t>Open source, with no known intellectual property issue</t>
</list>
</t>
</section>
......@@ -265,7 +262,7 @@ The CELT codec has several optional features that be switched on of off, some of
<ttcol align='center'>P</ttcol>
<ttcol align='center'>S</ttcol>
<ttcol align='center'>F</ttcol>
<ttcol align='center'>Encoding</ttcol>
<ttcol align='right'>Encoding</ttcol>
<c>0</c><c>0</c><c>0</c><c>1</c><c>00</c>
<c>0</c><c>1</c><c>0</c><c>1</c><c>01</c>
<c>1</c><c>0</c><c>0</c><c>1</c><c>110</c>
......@@ -435,20 +432,45 @@ In bands where no pitch and no folding is used, the PVQ is used directly to enco
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:
prediction or a folding vector P is used, the quantized unit vector X' becomes:
</t>
<t>X = P + g_f * y,</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>This is described in mix_pitch_and_residual() (<xref target="vq.c">vq.c</xref>).</t>
<t>The combination of the pitch with the pvq codeword is described in
mix_pitch_and_residual() (<xref target="vq.c">vq.c</xref>) and is used in
both the encoder and the decoder.
</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.
implementation computes an initial codeword y1 by projecting the residual signal
R = X - P onto the codebook pyramid of K-1 pulses:
</t>
<t>
y0 = round_towards_zero( (K-1) * R / sum(abs(R)))
</t>
<t>
Depending on N, K and the input data, the initial codeword y0 may contain from
0 to K-1 non-zero values. All the remaining pulses, with the exception of the last one,
are found iteratively with a greedy search that minimizes the normalised correlation
between y and R:
</t>
<t>
J = -R^T*y / ||y||
</t>
<t>
The last pulse is the only one considering the pitch and minimizes the cost function <xref target="celt-tasl"></xref>:
</t>
<t>
J = -g_f * R^T*y + (g_f)^2 * ||y||^2
</t>
<section anchor="Index Encoding" title="Index Encoding">
......@@ -570,6 +592,8 @@ significant non-uniformity.
</section>
<!--
<section anchor="Evaluation of CELT Implementations" title="Evaluation of CELT Implementations">
<t>
......@@ -578,18 +602,7 @@ Insert some text here.
</section>
<section anchor="Issues that need to be addressed" title="Issues that need to be addressed">
<t>
<list>
<t>Dynamic bit allocation</t>
<t>Stereo coupling</t>
</list>
</t>
</section>
-->
<section anchor="Acknowledgments" title="Acknowledgments">
......
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