Some details on the MDCT, fixed a bunch of warnings
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Jean-Marc Valin
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4c9a007251
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1 changed files with 47 additions and 71 deletions
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@ -12,7 +12,6 @@
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<author initials="J-M" surname="Valin" fullname="Jean-Marc Valin">
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<organization>Octasic Semiconductor</organization>
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<address>
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<email>jean-marc.valin@octasic.com</email>
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<postal>
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<street>4101, Molson Street, suite 300</street>
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<city>Montreal</city>
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@ -20,12 +19,14 @@
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<code>H1Y 3L1</code>
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<country>Canada</country>
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</postal>
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<email>jean-marc.valin@octasic.com</email>
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</address>
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</author>
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<author initials="et" surname="al." fullname="et al.">
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<!-- <author initials="et" surname="al." fullname="et al.">
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<organization></organization>
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</author>
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-->
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<date day="18" month="December" year="2008" />
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@ -37,7 +38,7 @@
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<keyword>CELT</keyword>
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<abstract>
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<t>
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CELT is an open-source voice codec suitable for use in very low delay
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CELT <xref target="celt-website"/>is an open-source voice codec suitable for use in very low delay
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Voice over IP (VoIP) type applications. This document describes the encoding
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and decoding process.
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</t>
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@ -72,18 +73,32 @@ CELT stands for "Constrained Energy Lapped Transform". It applies some of the CE
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</list>
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</t>
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<t>CELT is designed for transmission over RTP <xref target="rfc3550"/></t>
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</section>
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<section anchor="CELT Encoder" title="CELT Encoder">
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<t>Insert encoder overview</t>
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<t>Pre-emphasis</t>
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<t>The input audio first goes through a pre-emphasis filter, which attenuates the
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"spectral tilt". The filter is has the transfer function A(z)=1-alpha_p*z^-1, with
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alpha_p=0.8. The inverse of the pre-emphasis is applied at the decoder.</t>
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<section anchor="Range Coder" title="Range Coder">
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</section>
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<section anchor="Forward MDCT" title="Forward MDCT">
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<t>CELT is a transform codec, based on the Modified Discrete Cosine Transform
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<xref target="mdct"></xref>, which is based on a DCT-IV, with overlap and time-domain
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aliasing calcellation. The MDCT implementation has no special characteristic. The
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input is a windowed signal (after pre-emphasis) of 2*N samples and the output is N
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frequency-domain samples. A "low-overlap" window is used to reduce the algorithmc delay.
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It is composed of a smaller window with symmetric zero padding on both sides. The window
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is the same as the one used in the Vorbis codec and defined as: W(n)=[sin(pi/2*sin(pi/2*(n+.5)/L))]^2
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</t>
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</section>
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<section anchor="Energy Envelope Quantization" title="Energy Envelope Quantization">
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@ -101,8 +116,8 @@ that the result is always exactly the same. Any mismatch would cause an error in
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</section>
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<section anchor="Spherical Vector Quantization" title="Spherical Vector Quantization">
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CELT uses a Pyramid Vector Quantization (PVQ) [] codebook for quantising the details
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of the spectrum in each band that haven't been predicted by the pitch predictor.
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<t>CELT uses a Pyramid Vector Quantization (PVQ) <xref target="PVQ"></xref> codebook for quantising the details
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of the spectrum in each band that haven't been predicted by the pitch predictor.</t>
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<section anchor="Index Encoding" title="Index Encoding">
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</section>
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@ -125,8 +140,8 @@ Some more text
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</section>
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<section anchor="Spherical VQ Decoder" title="Spherical VQ Decoder">
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CELT uses a Pyramid Vector Quantization (PVQ) [] codebook for quantising the details
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of the spectrum in each band that haven't been predicted by the pitch predictor.
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<t>CELT uses a Pyramid Vector Quantization (PVQ) [] codebook for quantising the details
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of the spectrum in each band that haven't been predicted by the pitch predictor.</t>
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</section>
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<section anchor="Index Decoding" title="Index Decoding">
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@ -139,8 +154,6 @@ of the spectrum in each band that haven't been predicted by the pitch predictor.
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<section anchor="Packet Loss Concealment" title="Packet Loss Concealment (PLC)">
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</section>
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<t>De-emphasis</t>
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</section>
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@ -197,7 +210,7 @@ CELT and AVT communities for their input:
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<reference anchor="rfc2119">
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<front>
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<title>Key words for use in RFCs to Indicate Requirement Levels </title>
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<author initials="S." surname="Bradner" fullname="Scott Bradner"></author>
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<author initials="S." surname="Bradner" fullname="Scott Bradner"><organization/></author>
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</front>
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<seriesInfo name="RFC" value="2119" />
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</reference>
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@ -205,69 +218,14 @@ CELT and AVT communities for their input:
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<reference anchor="rfc3550">
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<front>
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<title>RTP: A Transport Protocol for real-time applications</title>
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<author initials="H." surname="Schulzrinne" fullname=""></author>
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<author initials="S." surname="Casner" fullname=""></author>
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<author initials="R." surname="Frederick" fullname=""></author>
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<author initials="V." surname="Jacobson" fullname=""></author>
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<author initials="H." surname="Schulzrinne" fullname=""><organization/></author>
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<author initials="S." surname="Casner" fullname=""><organization/></author>
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<author initials="R." surname="Frederick" fullname=""><organization/></author>
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<author initials="V." surname="Jacobson" fullname=""><organization/></author>
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</front>
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<seriesInfo name="RFC" value="3550" />
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</reference>
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<reference anchor="rfc2045">
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<front>
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<title>Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
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<author initials="" surname="" fullname=""></author>
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</front>
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<date month="November" year="1998" />
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<seriesInfo name="RFC" value="2045" />
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</reference>
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<reference anchor="rfc2327">
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<front>
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<title>SDP: Session Description Protocol</title>
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<author initials="V." surname="Jacobson" fullname=""></author>
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<author initials="M." surname="Handley" fullname=""></author>
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</front>
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<date month="April" year="1998" />
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<seriesInfo name="RFC" value="2327" />
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</reference>
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<reference anchor="H323">
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<front>
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<title>Packet-based Multimedia Communications Systems</title>
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<author initials="" surname="" fullname=""></author>
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</front>
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<date month="" year="1998" />
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<seriesInfo name="ITU-T Recommendation" value="H.323" />
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</reference>
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<reference anchor="H245">
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<front>
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<title>Control of communications between Visual Telephone Systems and Terminal Equipment</title>
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<author initials="" surname="" fullname=""></author>
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</front>
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<date month="" year="1998" />
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<seriesInfo name="ITU-T Recommendation" value="H.245" />
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</reference>
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<reference anchor="rfc3551">
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<front>
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<title>RTP Profile for Audio and Video Conferences with Minimal Control.</title>
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<author initials="H." surname="Schulzrinne" fullname=""></author>
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<author initials="S." surname="Casner" fullname=""></author>
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</front>
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<date month="July" year="2003" />
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<seriesInfo name="RFC" value="3551" />
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</reference>
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<reference anchor="rfc3534">
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<front>
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<title>The application/ogg Media Type</title>
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<author initials="L." surname="Walleij" fullname=""></author>
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</front>
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<date month="May" year="2003" />
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<seriesInfo name="RFC" value="3534" />
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</reference>
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</references>
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@ -276,11 +234,29 @@ CELT and AVT communities for their input:
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<reference anchor="celt-website">
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<front>
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<title>The CELT ultra-low delay audio codec</title>
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<author><organization/></author>
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</front>
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<seriesInfo name="CELT website" value="http://www.celt-codec.org/" />
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</reference>
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</references>
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<reference anchor="mdct">
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<front>
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<title>Modified Discrete Cosine Transform</title>
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<author><organization/></author>
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</front>
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<seriesInfo name="MDCT" value="http://en.wikipedia.org/wiki/Modified_discrete_cosine_transform" />
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</reference>
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<reference anchor="PVQ">
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<front>
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<title>A Pyramid Vector Quantizer</title>
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<author initials="T." surname="Fischer" fullname=""><organization/></author>
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<date month="July" year="1986" />
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</front>
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<seriesInfo name="Pyramid Vector Quantizer" value="http://en.wikipedia.org/wiki/Modified_discrete_cosine_transform" />
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</reference>
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</references>
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<section anchor="Reference Implementation" title="Reference Implementation">
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