AVT F. de Bont Internet-Draft Philips Electronics Updates: 3640 (if approved) S. Doehla Intended status: Standards Track Fraunhofer IIS Expires: April 23, 2009 M. Schmidt Dolby Laboratories R. Sperschneider Fraunhofer IIS October 20, 2008 RTP Payload Format for Elementary Streams with MPEG Surround multi- channel audio draft-ietf-avt-rtp-mps-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 April 23, 2009. Abstract This memo describes extensions for the RTP payload format defined in RFC3640 for the transport of MPEG Surround multi-channel audio. Additional Media Type parameters are defined to signal backwards compatible transmission inside an MPEG-4 audio elementary stream. In addition a layered transmission scheme without using the MPEG-4 systems framework is presented to transport an MPEG Surround de Bont, et al. Expires April 23, 2009 [Page 1]
Internet-Draft Transport of MPEG Surround October 2008 elementary stream via RTP in parallel with an RTP stream containing the downmixed audio data. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Definitions and Abbreviations . . . . . . . . . . . . . . . . 3 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4 4. Transport of MPEG Surround . . . . . . . . . . . . . . . . . . 4 4.1. Embedded spatial audio data in AAC payloads . . . . . . . 5 4.2. MPEG Surround Elementary Stream . . . . . . . . . . . . . 6 4.2.1. Low Bit-rate MPEG Surround . . . . . . . . . . . . . . 7 4.2.2. High Bit-rate MPEG Surround . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5.1. Media Type registration . . . . . . . . . . . . . . . . . 9 5.2. Registration of Mode Definitions with IANA . . . . . . . . 9 5.3. Usage of SDP . . . . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Normative References . . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 12 de Bont, et al. Expires April 23, 2009 [Page 2]
Internet-Draft Transport of MPEG Surround October 2008 1. Introduction MPEG Surround (Spatial Audio Coding, SAC) [23003-1] is an International Standard that was finalized by MPEG in January 2007. It is capable of re-creating N channels based on M<N transmitted channels and additional control data. In the preferred modes of operating the spatial audio coding system, the M channels can either be a single mono channel or a stereo channel pair. The control data represents a significant lower data rate than the data rate required for transmitting all N channels, making the coding very efficient while at the same time ensuring compatibility with M channel devices. The MPEG Surround standard incorporates a number of tools enabling features that allow for broad application of the standard. A key feature is the ability to scale the spatial image quality gradually from very low spatial overhead towards transparency. Another key feature is that the decoder input can be made compatible to existing matrixed surround technologies. As an example, for 5.1 multi-channel audio, the MPEG Surround encoder creates a stereo (or mono) downmix signal and spatial information describing the full 5.1 material in a highly efficient parameterised format. The spatial information is transmitted alongside the downmix. By using MPEG Surround, existing services can easily be upgraded to provide surround sound in a backward compatible fashion. While a stereo decoder in an existing legacy consumer device ignores the MPEG Surround data and plays back the stereo signal without any quality degradation, an MPEG Surround enabled decoder will deliver high quality multi-channel audio. The MPEG Surround decoder can operate in modes that render the multi- channel signal to multi-channel output, stereo output or operate in a two-channel headphone mode to produce a virtual surround output signal. 2. Conventions 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 [RFC2119]. 3. Definitions and Abbreviations de Bont, et al. Expires April 23, 2009 [Page 3]
Internet-Draft Transport of MPEG Surround October 2008 3.1. Definitions This memo makes use of the definitions specified in [14496-1], [14496-3], [23003-1] and [RFC3640]. Frequently used terms are summed up for convenience: Access Unit: An MPEG Access Unit is the smallest data entity to which timing information is attributed. In the case of audio, an Access Unit is the smallest individually accessible portion of coded audio data within an elementary stream. AudioSpecificConfig(): Extends the class DecoderSpecificInfo(), as defined in [14496-1] when the objectType indication refers to a stream complying with [14496-3]. AudioSpecificConfig() is used as the configuration structure for MPEG-4 Audio as specified in [14496-3]. It contains the field audioObjectType that distincts between the different audio codecs defined in [14496-3], general audio information (e.g. the sampling frequency and number of channels) and further codec-dependent information structures. SpatialSpecificConfig(): Configuration structure for MPEG Surround audio coding as specified in [23003-1]. An AudioSpecificConfig() with an audioObjectType of value 30 contains a SpatialSpecificConfig() structure. 3.2. Abbreviations AOT: Audio Object Type ASC: AudioSpecificConfig() structure AU: Access Unit PLI: Profile and Level Indication SSC: SpatialSpecificConfig() structure 4. Transport of MPEG Surround From a top-level perspective MPEG Surround data can be subdivided into configuration data contained in the SpatialSpecificConfig() (SSC) and the SpatialFrame() that contains the MPEG Surround payload. The configuration data can be signaled in-band or out-of-band. In the case of in-band signaling the SSC is conveyed in an SacDataFrame() jointly with a SpatialFrame(). In the case of out-of- band signaling the SSC is transmitted to the decoder separately, e.g. by SDP means. SpatialFrame()s may be transmitted either embedded into the downmix stream (Section 4.1) or as an individual elementary stream besides the downmix audio stream (Section 4.2). de Bont, et al. Expires April 23, 2009 [Page 4]
Internet-Draft Transport of MPEG Surround October 2008 The buffer definition for AAC decoders limits the size of an AU as specified in [14496-3]. For high-bitrate applications that exceed this limit all MPEG Surround data MUST be put in a separate stream as defined in Section 4.2. 4.1. Embedded spatial audio data in AAC payloads [14496-3] define the extension_payload() as a mechanism for transport of extension data inside AAC payloads. Typical extension data include SBR data and MPEG Surround data, i.e. a SacDataFrame() in extension_payload()s of type EXT_SAC_DATA. extension_payload()s reside inside the downmix AAC elementary stream. The resulting single elementary stream is transported as specified in [RFC3640]. As AAC decoders are required to skip unknown extension data, MPEG Surround data can be embedded backwards compatible and be transported with the mechanism already described in [RFC3640]. The SacDataFrame() includes a SpatialFrame() and an optional header that contains an SSC. Any SSC in a SacDataFrame() MUST be identical to the SSC conveyed via SDP for that stream. No new mode is introduced for SpatialFrame()s being embedded into AAC payloads. Either the modes AAC-lbr or AAC-hbr SHOULD be used. The additional Media Type parameters as defined in Section 5.1 SHOULD be present when SpatialFrame()s are embedded into AAC payloads. For example: m=audio 5000 RTP/AVP 96 a=rtpmap:96 mpeg4-generic/48000/2 a=fmtp:96 streamType=5; profile-level-id=44; mode=AAC-hbr; config=131 056E598; sizeLength=13; indexLength=3; indexDeltaLength=3; constant Duration=2048; MPS-profile-level-id=55; MPS-config=F1B4CF920442029B 501185B6DA00; In this example the stream specifies the HE-AAC Profile at Level 2 [Profile and Level Indication (PLI) 44] and the config string contains the hexadecimal representation of the HE-AAC ASC [audioObjectType=2 (AAC LC); extensionAudioObjectType=5 (SBR); samplingFrequencyIndex=0x6 (24kHz); extensionSamplingFrequencyIndex=0x3 (48kHz); channelConfiguration=2 (2.0 channels)] of the downmix AAC elementary stream using explicit backward compatible signaling. Furthermore, the stream specifies the MPEG Surround Baseline Profile at Level 3 (PLI55) and the MPS-config string contains the hexadecimal representation of the MPEG Surround ASC [audioObjectType=30 (MPEG Surround); samplingFrequencyIndex=0x3 (48kHz); channelConfiguration=6 de Bont, et al. Expires April 23, 2009 [Page 5]
Internet-Draft Transport of MPEG Surround October 2008 (5.1 channels); sacPayloadEmbedding=1; SSC=(48 kHz; 32 slots; 525 tree; ResCoding=1; ResBands=[0,13,13,13])]. Note that the a=fmtp lines of the example above have been wrapped to fit the page; they comprise each a single line in the SDP file. 4.2. MPEG Surround Elementary Stream MPEG Surround SpatialFrame()s can be present in an individual elementary stream. This stream complements the stream containing the downmix audio data, which may be coded by an arbitrary coding scheme. MPEG Surround elementary streams are packetized as specified in [RFC3640]. The mode signaled and used for an MPEG Surround elementary stream MUST be either MPS-hbr or MPS-lbr. The MPS-hbr mode SHALL be used when the frame size may exceed 63 bytes, e.g. when high-bitrate residual coding is in use. The dependency relationships between the MPEG Surround elementary stream and the downmix stream are signaled as specified in [I-D.ietf-mmusic-decoding-dependency]. The media clocks of the MPEG Surround elementary stream and the downmix stream SHALL operate in the same clock domain, i.e. the clocks are derived from a common clock and MUST NOT drift. RTCP sender reports MUST indicate that the stream timestamps are not drifting, i.e. that a single sender report for each stream is sufficient to establish unambiguous timing. The sampling rate of the MPEG Surround signal and the decoded downmix signal MUST be identical. If HE-AAC is used as the coding scheme for the downmix, the RTP clock-rate of the downmix MAY be the sampling rate of the AAC core, i.e. the clock-rate of the MPEG Surround elementary stream is an integer multiple of the clock-rate of the downmix stream. Note that separate RTP streams have different random RTP timestamp offsets and therefore RTCP MUST be used to synchronize the coded downmix audio data and the MPEG surround elementary stream. For example: de Bont, et al. Expires April 23, 2009 [Page 6]
Internet-Draft Transport of MPEG Surround October 2008 a=group:DDP L1 L2 m=audio 5000 RTP/AVP 96 a=rtpmap:96 mpeg4-generic/48000/2 a=fmtp:96 streamType=5; profile-level-id=44; mode=AAC-hbr; config=2B1 18800; sizeLength=13; indexLength=3; indexDeltaLength=3; constantDu ration=2048 a=mid:L1 m=audio 5002 RTP/AVP 97 a=rtpmap:97 mpeg4-generic/48000/6 a=fmtp:97 streamType=5; profile-level-id=55; mode=MPS-hbr; config=F1B 0CF920460029B601189E79E70; sizeLength=13; indexLength=3; indexDelt aLength=3; constantDuration=2048 a=mid:L2 a=depend:97 lay L1:96 In this example the first stream specifies the High Efficiency AAC Profile at Level 2 (PLI44) and the config string contains the hexadecimal representation of the HE-AAC ASC [audioObjectType=2 (AAC LC); extensionAudioObjectType=5 (SBR); samplingFrequencyIndex=0x6 (24kHz); extensionSamplingFrequencyIndex=0x3 (48kHz); channelConfiguration=2 (2.0 channels)]. The second stream specifies Baseline MPEG Surround Profile at Level 3 (PLI55) and the config string contains the hexadecimal represenation of the ASC [AOT=30(MPEG Surround); 48 kHz; 5.1-ch; sacPayloadEmbedding=0; SSC=(48 kHz; 32 slots; 525 tree; ResCoding=1; ResBands=[7,7,7,7])] Note that the a=fmtp lines of the example above have been wrapped to fit the page; they comprise each a single line in the SDP file. 4.2.1. Low Bit-rate MPEG Surround This mode is signaled by mode=MPS-lbr. This mode supports the transport of one or more complete Access Units, each consisting of a single MPEG Surround SpatialFrame(). The AUs can be variably sized and interleaved. The maximum size of a SpatialFrame() is 63 bytes. Fragmentation MUST NOT be used in this mode. Receivers MUST support de-interleaving. The payload configuration is the same as in the AAC-lbr mode. It consists of the AU Header Section, followed by concatenated AUs. Note that Access Units are byte-aligned. The Auxiliary Section MUST be empty in the MPS-lbr mode. The one-octet AU-header MUST provide: de Bont, et al. Expires April 23, 2009 [Page 7]
Internet-Draft Transport of MPEG Surround October 2008 1. the size of each AAC frame encoded as 6 bits 2. 2 bits index information for computing the sequence (and hence timing) of each SpatialFrame(). The concatenated AU-header Section MUST be preceded by the 16-bit AU- header-length field. In addition to the required Media format parameters, the following parameters MUST be present with fixed values: sizeLength (fixed value 6), indexLength (fixed value 2) and indexDeltaLength (fixed value 2). The parameter maxDisplacement MUST be present when interleaving. SpatialFrame()s always have a fixed duration per AU; the fixed duration MUST be signaled by the Media format parameter constantDuration. The value of the "config" parameter is the hexadecimal representation of the ASC, as defined in [14496-3] with an AOT of 30 and the sacPayloadEmbedding flag set to 0. The "profile-level-id" parameter SHALL contain a valid PLI for MPEG Surround as specified in [14496-3]. 4.2.2. High Bit-rate MPEG Surround This mode is signaled by mode=MPS-hbr. This mode supports the transportation of either one fragment of an Access Unit or one complete AU or several complete AUs. Each AU consists of a single MPEG Surround SpatialFrame(). The AUs can be variably sized and interleaved. The maximum size of a SpatialFrame() is 8191 bytes. Receivers MUST support de-interleaving. The payload configuration is the same as in the AAC-hbr mode. It consists of the AU Header Section, followed by either one SpatialFrame(), a fragment of a SpatialFrame() or several concatenated SpatialFrame()s. Note that Access Units are byte- aligned. The Auxiliary Section MUST be empty in the MPS-hbr mode. The two-octet AU-header MUST provide: 1. the size of each AAC frame encoded as 13 bits 2. 3 bits index information for computing the sequence (and hence timing) of each SpatialFrame(), i.e. the AU-Index or AU-Index- delta field. Each AU-Index field MUST be coded with the value 0. The concatenated AU-header Section MUST be preceded by the 16-bit AU-header-length field. de Bont, et al. Expires April 23, 2009 [Page 8]
Internet-Draft Transport of MPEG Surround October 2008 In addition to the required Media format parameters, the following parameters MUST be present with fixed values: sizeLength (fixed value 13), indexLength (fixed value 3) and indexDeltaLength (fixed value 3). The parameter maxDisplacement MUST be present when interleaving. SpatialFrame()s always have a fixed duration per AU; the fixed duration MUST be signaled by the Media format parameter constantDuration. The value of the "config" parameter is the hexadecimal representation of the ASC, as defined in [14496-3] with an AOT of 30 and the sacPayloadEmbedding flag set to 0. The "profile-level-id" parameter SHALL contain a valid PLI for MPEG Surround as specified in [14496-3]. 5. IANA Considerations This memo defines additional optional format parameters to the Media subtype mpeg4-generic. These parameters SHALL only be used in combination with the AAC-lbr or AAC-hbr modes (cf. [RFC3640] section 3.3). 5.1. Media Type registration This memo defines the following additional optional parameters which SHALL be used if MPEG Surround data is present inside the payload of an AAC elementary stream. MPS-profile-level-id: A decimal representation of the MPEG Surround Profile Level indication as defined in [14496-3]. This parameter MUST be used in the capability exchange or session set-up procedure to indicate the MPEG Surround Profile and Level that the decoder must be capable in order to decode the stream. MPS-config: A hexadecimal representation of an octet string that expresses the AudioSpecificConfig (ASC) as defined in [14496-3] for MPEG Surround. The ASC is mapped onto the hexadecimal octet string in an MSB-first basis. The AOT in this ASC SHALL have the value 30. The SSC inside the ASC MUST have the sacPayloadEmbedding flag set to 1. 5.2. Registration of Mode Definitions with IANA This memo defines the modes MPS-hbr and MPS-lbr. de Bont, et al. Expires April 23, 2009 [Page 9]
Internet-Draft Transport of MPEG Surround October 2008 5.3. Usage of SDP It is assumed that the Media format parameters are conveyed via an SDP message as specified in [RFC3640], section 4.4. 6. Security Considerations RTP packets using the payload format defined in this memo are subject to the security considerations of the RTP specification [RFC3550] and [RFC3640] which is extended with this memo. This implies that confideniality of the media streams is achieved by encryption. Because the data compression used with this payload format is applied end-to-end, encryption may be performed on the compressed data so there is no conflict between the two operations. 7. Normative References [14496-1] MPEG, "ISO/IEC International Standard 14496-1 - Coding of audio-visual objects, Part 1 Systems", 2004. [14496-3] MPEG, "ISO/IEC International Standard 14496-3 - Coding of audio-visual objects, Part 3 Audio", 2005. [23003-1] MPEG, "ISO/IEC International Standard 23003-1 - MPEG Surround (MPEG D)", 2007. [I-D.ietf-mmusic-decoding-dependency] Schierl, T. and S. Wenger, "Signaling media decoding dependency in Session Description Protocol (SDP)", I-D ietf-mmusic-decoding-dependency, February 2008. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3640] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, "RTP Payload Format for Transport of MPEG-4 Elementary Streams", RFC 3640, November 2003. de Bont, et al. Expires April 23, 2009 [Page 10]
Internet-Draft Transport of MPEG Surround October 2008 Authors' Addresses Frans de Bont Philips Electronics High Tech Campus 5 5656 AE Eindhoven, NL Phone: ++31 40 2740234 Email: frans.de.bont@philips.com Stefan Doehla Fraunhofer IIS Am Wolfmantel 33 91058 Erlangen, DE Phone: +49 9131 776 6042 Email: stefan.doehla@iis.fraunhofer.de Malte Schmidt Dolby Laboratories Deutschherrnstr. 15-19 90537 Nuernberg, DE Phone: +49 911 928 91 42 Email: malte.schmidt@dolby.com Ralph Sperschneider Fraunhofer IIS Am Wolfmantel 33 91058 Erlangen, DE Phone: +49 9131 776 6167 Email: ralph.sperschneider@iis.fraunhofer.de de Bont, et al. Expires April 23, 2009 [Page 11]
Internet-Draft Transport of MPEG Surround October 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. de Bont, et al. Expires April 23, 2009 [Page 12]
