Audio Video Transport WG Q. Xie Internet-Draft D. Pearce Expires: April 18, 2004 Motorola October 19, 2003 RTP Payload Format for European Telecommunications Standards Institute (ETSI) European Standard ES 202 211 Extended Distributed Speech Recognition Encoding (XFE) draft-xie-avt-xdsr-es202211-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 18, 2004. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document specifies an RTP payload format for encapsulating ETSI Standard ES 202 211 extended advanced front-end signal processing feature streams for distributed speech recognition (DSR) systems. Xie & Pearce Expires April 18, 2004 [Page 1] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 Table of Contents 1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 ETSI ES 202 211 Extended DSR Front-end Codec . . . . . . . . . 3 3. ES 202 211 DSR RTP Payload Format . . . . . . . . . . . . . . 4 3.1 Consideration on Number of FPs in Each RTP Packet . . . . . . 5 3.2 Support for Discontinuous Transmission . . . . . . . . . . . . 5 4. Frame Pair Formats . . . . . . . . . . . . . . . . . . . . . . 5 4.1 Format of Speech and Non-speech FPs . . . . . . . . . . . . . 5 4.2 Format of Null FP . . . . . . . . . . . . . . . . . . . . . . 7 4.3 RTP header usage . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5.1 Mapping MIME Parameters into SDP . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 Normative References . . . . . . . . . . . . . . . . . . . . . 10 Informative References . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . 12 Xie & Pearce Expires April 18, 2004 [Page 2] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 1. Conventions The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [3]. The following acronyms are used in this document: DSR - Distributed Speech Recognition ETSI - the European Telecommunications Standards Institute FP - Frame Pair DTX - Discontinuous Transmission 2. Introduction Distributed speech recognition (DSR) technology is intended for a remote device acting as a thin client, also known as the front-end, to communicate with a speech recognition server, also called a speech engine, over a network connection to obtain speech recognition services. More details on DSR over Internet can be found in [7]. To achieve interoperability with different client devices and speech engines, the first ETSI standard DSR front-end ES 201 108 was published in early 2000 [8], and an RTP packetization for ES 201 108 frames is defined in [7] in IETF. In ES 202 211 [1], ETSI issues another standard for an Extended DSR front-end that provides additional fundamental frequency and voicing class information along with the front-end features of ES 201 108. The purpose of this information is to enable the reconstruction of speech waveform at the back-end and may also be useful in enhancing the recognition accuracy of tonal languages, e.g., Mandarin, Cantonese, and Thai. The RTP packetization for ES 202 211 front-end defined in this document uses the same RTP packet format layout as that defined in [7] but with the addition of the compressed information for the extension in the frame bit definition for the payload. It also has a different payload type MIME registration. 2.1 ETSI ES 202 211 Extended DSR Front-end Codec Some relevant characteristics of ES 202 211 Extended DSR front-end codec are summarized below. ES 202 211 is an extension of the mel-cepstrum DSR Front-end standard Xie & Pearce Expires April 18, 2004 [Page 3] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 ES 201 108 [8]. The mel-cepstrum front-end provides the features for speech recognition but these are not available for human listening. The purpose of the extension is allow the reconstruction of the speech waveform from these features so that they can be replayed. The front-end feature extraction part of the processing is exactly the same as for ES 201 108. To allow speech reconstruction additional fundamental frequency (perceived as pitch) and voicing class (e.g. non-speech, voiced, unvoiced and mixed) information is needed. This is the extra information that is provided by the extended front-end processing algorithms at the device side that is compressed and transmitted along with the front-end features to the server. This extra information may also be useful for improved speech recognition performance with tonal languages such as Mandarin, Cantonese and Thai. Full information about the client side signal processing algorithms used in the standard are described in the specification ES 202 211 [1]. The additional fundamental frequency and voicing class information is compressed for each frame pair. The pitch for the first frame of the FP is quantised to 7 bits and the second frame is differentially quantized with 5 bits. The voicing class is indicated with one bit for each frame. The total for the extension information for a frame pair therefore consists of 14 bits plus and additional 2 bits of CRC error protection computed over these extension bits only. The total information for the frame pair is made up of 92 bits for the two compressed front-end feature frames (including 4 bits for their CRC) plus 16 bits for the extension (including 2 bits for their CRC) and 4 bits of null padding to give a total of 14 octets per frame pair. As for ES 201 208 the extended frame pair also corresponds to 20ms of speech. The extended front-end supports three raw sampling rates: 8 kHz, 11 kHz, and 16 kHz. The quantized vectors from two consecutive frames are put into an FP, as described in more detail in Section 4.1 below. The parameters received at the remote server from the RTP extended DSR payload specified here can be used to synthesize an intelligible speech waveform for replay. The algorithms to do this are described in the specification ES 202 211 [1]. 3. ES 202 211 DSR RTP Payload Format An ES 202 211 DSR RTP payload datagram is very similar to that defined in Section 3 of [7], i.e., a standard RTP header followed by a DSR payload containing a series of DSR FPs. Xie & Pearce Expires April 18, 2004 [Page 4] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 The size of each ES 202 211 FP is 112 bits or 14 octets (see Sections 4 below). This ensures that a DSR RTP payload will always end on an octet boundary. 3.1 Consideration on Number of FPs in Each RTP Packet Same considerations described in Section 3.1 of [7] apply to ES 202 211 RTP payload. 3.2 Support for Discontinuous Transmission Same considerations described in Section 3.2 of [7] apply to ES 202 211 RTP payload. 4. Frame Pair Formats 4.1 Format of Speech and Non-speech FPs The following mel-cepstral frame MUST be used, as defined in Section 6.2.4 in [1]: As defined in Section 6.2.4 in [1], after two frames (Frame #1 and Frame #2) worth of codebook indices, or 88 bits, a 4-bit CRC calculated on these 88 bits immediately follows it. The pitch indices of the first frame (Pidx1: 7 bits) and the second frame (Pidx2: 5 bits) of the frame pair then follow. The class indices of the two frames in the frame pair worth 1 bit each (Cidx1 and Cidx2) next follow. Finally, a 2-bit CRC calculated on the pitch and class bits (total: 14 bits) of the frame pair using the binary polynomial g(X) = 1 + X + X2 is included (PC-CRC). The total number of bits in frame pair packet is therefore 44 + 44 + 4 + 7 + 5 + 1 + 1 + 2 = 108. At the end, each FP MUST be padded with 4 zeros to the MSB 4 bits of the last octet in order to make the FP aligned to the 32-bit word boundary. The following diagram shows a complete ES 202 211 FP: Xie & Pearce Expires April 18, 2004 [Page 5] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 Frame #1 in FP: =============== (MSB) (LSB) 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ : idx(2,3) | idx(0,1) | Octet 1 +-----+-----+-----+-----+-----+-----+-----+-----+ : idx(4,5) | idx(2,3) (cont) : Octet 2 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(6,7) |idx(4,5)(cont) Octet 3 +-----+-----+-----+-----+-----+-----+-----+-----+ idx(10,11) | idx(8,9) | Octet 4 +-----+-----+-----+-----+-----+-----+-----+-----+ : idx(12,13) | idx(10,11) (cont) : Octet 5 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(12,13) (cont) : Octet 6/1 +-----+-----+-----+-----+ Frame #2 in FP: =============== (MSB) (LSB) 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+ : idx(0,1) | Octet 6/2 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(2,3) |idx(0,1)(cont) Octet 7 +-----+-----+-----+-----+-----+-----+-----+-----+ : idx(6,7) | idx(4,5) | Octet 8 +-----+-----+-----+-----+-----+-----+-----+-----+ : idx(8,9) | idx(6,7) (cont) : Octet 9 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(10,11) |idx(8,9)(cont) Octet 10 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(12,13) | Octet 11 +-----+-----+-----+-----+-----+-----+-----+-----+ CRC for Frame #1 and Frame #2 in FP: ==================================== (MSB) (LSB) 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+ | CRC | Octet 12/1 +-----+-----+-----+-----+ Xie & Pearce Expires April 18, 2004 [Page 6] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 Extension information and padding in FP: ======================================== (MSB) (LSB) 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+ : Pidx1 | Octet 12/2 +-----+-----+-----+-----+-----+-----+-----+-----+ | Pidx2 | Pidx1 (cont) : Octet 13 +-----+-----+-----+-----+-----+-----+-----+-----+ | 0 | 0 | 0 | 0 | PC-CRC |Cidx2|Cidx1| Octet 14 +-----+-----+-----+-----+-----+-----+-----+-----+ The 4-bit CRC and the 2-bit PC-CRC in the FP MUST be calculated using the formula (including the bit-order rules) defined in 6.2.4 in [1]. Therefore, each FP represents 20ms of original speech. Note, as shown above, each FP MUST be padded with 4 zeros to the MSB 4 bits of the last octet in order to make the FP aligned to the 32-bit word boundary. This makes the total size of an FP 112 bits, or 14 octets. Note, this padding is separate from padding indicated by the P bit in the RTP header. Any number of FPs MAY be aggregate together in an RTP payload and they MUST be consecutive in time. However, one SHOULD always keep the RTP payload size smaller than the MTU in order to avoid IP fragmentation and SHOULD follow the recommendations given in Section 3.1 in [7] when determining the proper number of FPs in an RTP payload. 4.2 Format of Null FP A Null FP for the ES 202 211 front-end codec is defined by setting all the 112 bits of the FP with 0's. 4.3 RTP header usage The format of the RTP header is specified in [5]. This payload format uses the fields of the header in a manner consistent with that specification. The RTP timestamp corresponds to the sampling instant of the first sample encoded for the first FP in the packet. The timestamp clock frequency is the same as the sampling frequency, so the timestamp unit is in samples. As defined by ES 202 211 front-end codec, the duration of one FP is 20 ms, corresponding to 160, 220, or 320 encoded samples with sampling rate of 8, 11, or 16 kHz being used at the front-end, Xie & Pearce Expires April 18, 2004 [Page 7] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 respectively. Thus, the timestamp is increased by 160, 220, or 320 for each consecutive FP, respectively. The DSR payload for ES 202 211 front-end codes is always an integral number of octets. If additional padding is required for some other purpose, then the P bit in the RTP in the header may be set and padding appended as specified in [5]. The RTP header marker bit (M) should be set following the general rules for audio codecs as defined in Section 4.1 in [6]. The assignment of an RTP payload type for this new packet format is outside the scope of this document, and will not be specified here. It is expected that the RTP profile under which this payload format is being used will assign a payload type for this encoding or specify that the payload type is to be bound dynamically. 5. IANA Considerations One new MIME subtype registration is required for this payload type, as described below. Media Type name: audio Media subtype name: dsr-es202211 Required parameters: none Optional parameters: rate: Indicates the sample rate of the speech. Valid values include: 8000, 11000, and 16000. If this parameter is not present, 8000 sample rate is assumed. maxptime: The maximum amount of media which can be encapsulated in each packet, expressed as time in milliseconds. The time shall be calculated as the sum of the time the media present in the packet represents. The time SHOULD be a multiple of the frame pair size (i.e., one FP => 20ms). If this parameter is not present, maxptime is assumed to be 80ms. Note, since the performance of most speech recognizers are extremely sensitive to consecutive FP losses, if the user of the payload format expects a high packet loss ratio for the session, it MAY consider to explicitly choose a maxptime value for the session that is shorter than the default value. Xie & Pearce Expires April 18, 2004 [Page 8] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 ptime: see RFC2327 [4]. Encoding considerations: This type is defined for transfer via RTP [5] as described in Sections 3 and 4 of RFC XXXX. Security considerations: See Section 6 of RFC XXXX. Person & email address to contact for further information: Qiaobing.Xie@motorola.com Intended usage: COMMON. It is expected that many VoIP applications (as well as mobile applications) will use this type. Author/Change controller: * Qiaobing.Xie@motorola.com * IETF Audio/Video transport working group 5.1 Mapping MIME Parameters into SDP The information carried in the MIME media type specification has a specific mapping to fields in the Session Description Protocol (SDP) [4], which is commonly used to describe RTP sessions. When SDP is used to specify sessions employing ES 202 050 DSR codec, the mapping is as follows: o The MIME type ("audio") goes in SDP "m=" as the media name. o The MIME subtype ("dsr-es202211") goes in SDP "a=rtpmap" as the encoding name. o The optional parameter "rate" also goes in "a=rtpmap" as clock rate. o The optional parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively. Example of usage of ES 202 211 DSR: m=audio 49120 RTP/AVP 101 a=rtpmap:101 dsr-es202211/8000 a=maxptime:40 Xie & Pearce Expires April 18, 2004 [Page 9] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 6. Security Considerations Implementations using the payload defined in this specification are subject to the security considerations discussed in the RTP specification [5] and the RTP profile [6]. This payload does not specify any different security services. 7. Acknowledgments The design presented here is based on that of [7]. Normative References [1] European Telecommunications Standards Institute (ETSI) Standard ES 202 211 v0.1.1, "Speech Processing, Transmission and Quality Aspects (STQ); Distributed Speech Recognition; Extended Front-end Feature Extraction Algorithm; Compression Algorithms", (http://pda.etsi.org/pda/home.asp?wki_id=??) , April 2003. [2] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [5] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. [6] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003. [7] Xie, Q., "RTP Payload Format for European Telecommunications Standards Institute (ETSI) European Standard ES 201 108 Distributed Speech Recognition Encoding", RFC 3557, July 2003. Informative References [8] European Telecommunications Standards Institute (ETSI) Standard ES 201 108, "Speech Processing, Transmission and Quality Aspects (STQ); Distributed Speech Recognition; Front-end Feature Extraction Algorithm; Compression Algorithms", http:// webapp.etsi.org/pda/home.asp?wki_id=9948 , April 2000. Xie & Pearce Expires April 18, 2004 [Page 10] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 Authors' Addresses Qiaobing Xie Motorola, Inc. 1501 W. Shure Drive, 2-F9 Arlington Heights, IL 60004 US Phone: +1-847-632-3028 EMail: qxie1@email.mot.com David Pearce Motorola Labs UK Research Laboratory Jays Close Viables Industrial Estate Basingstoke, HANTS RG22 4PD UK Phone: +44 (0)1256 484 436 EMail: bdp003@motorola.com Xie & Pearce Expires April 18, 2004 [Page 11] Internet-Draft RTP Payload Format for ES202211 DSR October 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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