celt源码，实现编码 解码 过程

AVT Working Group J-M. Valin Internet-Draft Octasic Semiconductor Expires: November 12, 2009 G. Maxwell Juniper Networks May 11, 2009 draft-valin-celt-rtp-profile-01 RTP Payload Format for the CELT Codec Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 12, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Valin & Maxwell Expires November 12, 2009 [Page 1]  Internet-Draft draft-valin-celt-rtp-profile-01 May 2009 Abstract CELT is an open-source voice codec suitable for use in very low delay audio communication applications, including Voice over IP (VoIP). This document describes the payload format for CELT generated bit streams within an RTP packet. Also included here are the necessary details for the use of CELT with the Session Description Protocol (SDP). At the time of this writing, the CELT bit-stream has NOT been finalized yet, and compatibility is usually broken with every new release of the codec. Table of Contents 1. Conventions used in this document . . . . . . . . . . . . . . 3 2. Overview of the CELT Codec . . . . . . . . . . . . . . . . . . 4 3. RTP payload format for CELT . . . . . . . . . . . . . . . . . 5 3.1. RTP Header . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. CELT payload . . . . . . . . . . . . . . . . . . . . . . . 6 3.3. Multiple CELT frames in a RTP packet . . . . . . . . . . . 7 3.4. Multiple channels . . . . . . . . . . . . . . . . . . . . 8 4. MIME registration of CELT . . . . . . . . . . . . . . . . . . 10 5. SDP usage of CELT . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Multichannel Mapping . . . . . . . . . . . . . . . . . . . 13 5.2. Low-Overhead Mode . . . . . . . . . . . . . . . . . . . . 15 6. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 16 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 9.1. Normative References . . . . . . . . . . . . . . . . . . . 19 9.2. Informative References . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 Valin & Maxwell Expires November 12, 2009 [Page 2]  Internet-Draft draft-valin-celt-rtp-profile-01 May 2009 1. Conventions used in this document 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 [rfc2119]. Valin & Maxwell Expires November 12, 2009 [Page 3]  Internet-Draft draft-valin-celt-rtp-profile-01 May 2009 2. Overview of the CELT Codec 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: o Ultra-low algorithmic delay (as low as 2 ms) o Full audio bandwidth (up to 20 kHz audio bandwidth) o Support for both voice and music o Stereo support o Packet loss concealment o Constant bitrates from under 32 kbps to 128 kbps and above o Free software/open-source Valin & Maxwell Expires November 12, 2009 [Page 4]  Internet-Draft draft-valin-celt-rtp-profile-01 May 2009 3. RTP payload format for CELT For RTP based transportation of CELT encoded audio the standard RTP header [rfc3550] is followed by one or more payload data blocks. An optional padding terminator may also be used. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | one or more frames of CELT .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3.1. RTP Header 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The RTP header is defined in the RTP specification [rfc3550]. This section defines how fields in the RTP header are used. Padding (P): 1 bit If the padding bit is set, the packet contains one or more additional padding octets at the end which are not part of the payload. The last octet of the padding contains a count of how many padding octets should be ignored, including itself. Padding may be needed by some encryption algorithms with fixed block sizes or for carrying several RTP packets in a lower-layer protocol data unit. Extension (X): 1 bit If the extension, X, bit is set, the fixed header MUST be followed by Valin & Maxwell Expires November 12, 2009 [Page 5]  Internet-Draft draft-valin-celt-rtp-profile-01 May 2009 exactly one header extension, with a format defined in Section 5.3.1. of [rfc3550]. Marker (M): 1 bit The M bit MUST be set to zero in all packets. The