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<?xml version='1.0'?> <!DOCTYPE rfc SYSTEM 'rfc2629.dtd'> <?rfc toc="yes" ?> <rfc ipr="full3978" docName="Constrained-Energy Lapped Transform (CELT) Codec"> <front> <title>draft-valin-celt-codec-00</title> <author initials="J-M" surname="Valin" fullname="Jean-Marc Valin"> <organization>Octasic Semiconductor</organization> <address> <postal> <street>4101, Molson Street, suite 300</street> <city>Montreal</city> <region>Quebec</region> <code>H1Y 3L1</code> <country>Canada</country> </postal> <email>jean-marc.valin@octasic.com</email> </address> </author> <!-- <author initials="et" surname="al." fullname="et al."> <organization></organization> </author> --> <date day="18" month="December" year="2008" /> <area>General</area> <workgroup>AVT Working Group</workgroup> <keyword>I-D</keyword> <keyword>Internet-Draft</keyword> <keyword>CELT</keyword> <abstract> <t> CELT <xref target="celt-website"/>is an open-source voice codec suitable for use in very low delay Voice over IP (VoIP) type applications. This document describes the encoding and decoding process. </t> </abstract> </front> <middle> <section anchor="Conventions used in this document" title="Conventions used in this document"> <t> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 <xref target="rfc2119"></xref>. </t> </section> <section anchor="Overview of the CELT Codec" title="Overview of the CELT Codec"> <t> CELT stands for "Constrained Energy Lapped Transform". It applies some of the CELP principles, but does everything in the frequency domain, which removes some of the limitations of CELP. CELT is suitable for both speech and music and currently features: </t> <t> <list style="symbols"> <t>Ultra-low latency (typically from 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</t> </list> </t> <t>CELT is designed for transmission over RTP <xref target="rfc3550"/></t> </section> <section anchor="CELT Encoder" title="CELT Encoder"> <t>Insert encoder overview</t> <t>The input audio first goes through a pre-emphasis filter, which attenuates the "spectral tilt". The filter is has the transfer function A(z)=1-alpha_p*z^-1, with alpha_p=0.8. The inverse of the pre-emphasis is applied at the decoder.</t> <section anchor="Range Coder" title="Range Coder"> </section> <section anchor="Forward MDCT" title="Forward MDCT"> <t>CELT is a transform codec, based on the Modified Discrete Cosine Transform <xref target="mdct"></xref>, which is based on a DCT-IV, with overlap and time-domain aliasing calcellation. The MDCT implementation has no special characteristic. The input is a windowed signal (after pre-emphasis) of 2*N samples and the output is N frequency-domain samples. A "low-overlap" window is used to reduce the algorithmc delay. It is composed of a smaller window with symmetric zero padding on both sides. The window 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 </t> </section> <section anchor="Energy Envelope Quantization" title="Energy Envelope Quantization"> <t>Coarse quantization with 6 dB resolution, prediction, Laplace distribution</t> <t>Fine quantization using resolution determined by the bit allocation</t> </section> <section anchor="Bit 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 manner in both the encoder and decoder to ensure that the result is always exactly the same. Any mismatch would cause an error in the decoded output.</t> </section> <section anchor="Pitch Prediction" title="Pitch Prediction"> </section> <section anchor="Spherical Vector Quantization" 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 haven't been predicted by the pitch predictor.</t> <section anchor="Index Encoding" title="Index Encoding"> </section> </section> <section anchor="Short windows" title="Short windows"> </section> </section> <section anchor="CELT Decoder" title="CELT Decoder"> <t> Some more text </t> <section anchor="Range Decoder" title="Range Decoder"> </section> <section anchor="Spherical VQ Decoder" title="Spherical VQ Decoder"> <t>CELT uses a Pyramid Vector Quantization (PVQ) [] codebook for quantising the details of the spectrum in each band that haven't been predicted by the pitch predictor.</t> </section> <section anchor="Index Decoding" title="Index Decoding"> </section> <section anchor="Backward MDCT" title="Backward MDCT"> </section> <section anchor="Packet Loss Concealment" title="Packet Loss Concealment (PLC)"> </section> </section> <section anchor="Security Considerations" title="Security Considerations"> <t> A potential denial-of-service threat exists for data encodings using compression techniques that have non-uniform receiver-end computational load. The attacker can inject pathological datagrams into the stream which are complex to decode and cause the receiver to be overloaded. However, this encoding does not exhibit any significant non-uniformity. </t> </section> <section anchor="Evaluation of CELT Implementations" title="Evaluation of CELT Implementations"> <t> Insert some text here. </t> </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"> <t> The authors would also like to thank the following members of the CELT and AVT communities for their input: </t> </section> </middle> <back> <references title="Normative References"> <reference anchor="rfc2119"> <front> <title>Key words for use in RFCs to Indicate Requirement Levels </title> <author initials="S." surname="Bradner" fullname="Scott Bradner"><organization/></author> </front> <seriesInfo name="RFC" value="2119" /> </reference> <reference anchor="rfc3550"> <front> <title>RTP: A Transport Protocol for real-time applications</title> <author initials="H." surname="Schulzrinne" fullname=""><organization/></author> <author initials="S." surname="Casner" fullname=""><organization/></author> <author initials="R." surname="Frederick" fullname=""><organization/></author> <author initials="V." surname="Jacobson" fullname=""><organization/></author> </front> <seriesInfo name="RFC" value="3550" /> </reference> </references> <references title="Informative References"> <reference anchor="celt-website"> <front> <title>The CELT ultra-low delay audio codec</title> <author><organization/></author> </front> <seriesInfo name="CELT website" value="http://www.celt-codec.org/" /> </reference> <reference anchor="mdct"> <front> <title>Modified Discrete Cosine Transform</title> <author><organization/></author> </front> <seriesInfo name="MDCT" value="http://en.wikipedia.org/wiki/Modified_discrete_cosine_transform" /> </reference> <reference anchor="PVQ"> <front> <title>A Pyramid Vector Quantizer</title> <author initials="T." surname="Fischer" fullname=""><organization/></author> <date month="July" year="1986" /> </front> <seriesInfo name="Pyramid Vector Quantizer" value="http://en.wikipedia.org/wiki/Modified_discrete_cosine_transform" /> </reference> </references> <section anchor="Reference Implementation" title="Reference Implementation"> <t>Insert a copy of the CELT source code here.</t> <!--<t><?rfc include="source/celt.c"?></t> <t><?rfc include="source/bands.c"?></t> --> </section> </back> </rfc>