Audio Video Transport WG Q. Xie Internet-Draft D. Pearce Expires: April 16, 2004 Motorola October 17, 2003 RTP Payload Format for European Telecommunications Standards Institute (ETSI) European Standard ES 202 050 Distributed Speech Recognition Encoding draft-ietf-avt-dsr-es202050-01.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 16, 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 202 050 advanced front-end signal processing feature streams for distributed speech recognition (DSR) systems. Xie & Pearce Expires April 16, 2004 [Page 1] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 Table of Contents 1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 ETSI ES 202 050 Advanced DSR Front-end Codec . . . . . . . . . 3 3. ES 202 050 DSR RTP Payload Format . . . . . . . . . . . . . . 4 3.1 Consideration on Number of FPs in Each RTP Packet . . . . . . 4 3.2 Support for Discontinuous Transmission . . . . . . . . . . . . 4 4. Frame Pair Formats . . . . . . . . . . . . . . . . . . . . . . 4 4.1 Format of Speech and Non-speech FPs . . . . . . . . . . . . . 4 4.2 Format of Null FP . . . . . . . . . . . . . . . . . . . . . . 6 4.3 RTP header usage . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5.1 Mapping MIME Parameters into SDP . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 Normative References . . . . . . . . . . . . . . . . . . . . . 9 Informative References . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . 11 Xie & Pearce Expires April 16, 2004 [Page 2] Internet-Draft RTP Payload Format for ES202050 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 VAD - Voice Activity Detection 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 202 050 [1], ETSI issues another standard for an Advanced DSR front-end that provides substantially improved recognition performance when background noise is present. The codecs in ES 202 050 uses a slightly different frame format from that of ES 201 108 and thus the two do not inter-operate with each other. The RTP packetization for ES 202 050 front-end defined in this document uses the same RTP packet format layout as that defined in [7]. The differences are in the DSR codec frame bit definition and the payload type MIME registration. 2.1 ETSI ES 202 050 Advanced DSR Front-end Codec Some relevant characteristics of ES 202 050 Advanced DSR front-end codec are summarized below. The front-end calculation is a frame-based scheme that produces an output vector every 10 ms. In the front-end feature extraction, noise reduction by two stages of Wiener filtering is performed first. Then, Xie & Pearce Expires April 16, 2004 [Page 3] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 waveform processing is applied to the de-noised signal and mel-cepstral features are calculated. At the end, blind equalization is applied to the cepstral features. The front-end algorithm produces at its output a mel-cepstral representation in the same format as ES 210 108, i.e., 12 cepstral coeffients [C1 - C12], C0 and log Engergy. Voice activity detection (VAD) for the clasification of each frame as speech or non-speech is also implemented in Feature Extraction. The VAD information is included in the payload format for each frame pair to be sent to the remote recognition engine as part of the payload. This information may optionally be used by the receiving recognition engine to drop non-speech frames. The front-end supports three raw sampling rates: 8 kHz, 11 kHz, and 16 kHz (It is worthwhile to note that unlike some other speech codecs, the feature frame size of DSR presented to RTP packetization is not dependent on the number of speech samples used in each 10 ms sample frame. This will become more evident in the following sections). After calculation of the mel-cepstral representation, the representation is first quantized via split-vector quantization to reduce the data rate of the encoded stream. Then, the quantized vectors from two consecutive frames are put into an FP, as described in more detail in Section 4.1 below. 3. ES 202 050 DSR RTP Payload Format An ES 202 050 DSR RTP payload datagram uses exactly the same layout as defined in Section 3 of [7], i.e., a standard RTP header followed by a DSR payload containing a series of DSR FPs. The size of each ES 202 050 FP is still 96 bits or 12 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 202 050 RTP payload. 3.2 Support for Discontinuous Transmission Same considerations described in Section 3.2 of [7] apply to ES 202 050 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 [1]: Xie & Pearce Expires April 16, 2004 [Page 4] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 As defined in [1], pairs of the quantized 10ms mel-cepstral frames MUST be grouped together and protected with a 4-bit CRC, forming a 92-bit long FP: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | Frame Pair (88 bits) = Frame #1 + Frame #2 | + +-+-+-+-+-+-+-+-+ | | CRC |0|0|0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Here Frame #1 and Frame #2 above MUST use the following mel-cepstral frame formats: 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)| VAD | idx(8,9) | Octet 4 +-----+-----+-----+-----+-----+-----+-----+-----+ : idx(12,13) | idx(10,11) (cont) : Octet 5 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(12,13) (cont) : Octet 6/1 +-----+-----+-----+-----+ Xie & Pearce Expires April 16, 2004 [Page 5] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 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) | VAD |idx(8,9)(cont) Octet 10 +-----+-----+-----+-----+-----+-----+-----+-----+ | idx(12,13) | Octet 11 +-----+-----+-----+-----+-----+-----+-----+-----+ The 4-bit CRC in the FP MUST be calculated using the formula (including the bit-order rules) defined in 7.2 in [1]. Therefore, each FP represents 20ms of original speech. Note, as shown above, each FP MUST be padded with 4 zeros to the LSB 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 96 bits, or 12 octets. Note, this padding is separate from padding indicated by the P bit in the RTP header. The definition of the indices and 'VAD' flag are described in [1] and their value is only set and examined by the codecs in the front-end client and the recognizer. 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 202 050 front-end codec is defined by setting the content of the first and second frame in the FP to null (i.e., filling the first 88 bits of the FP with 0's). The 4-bit CRC MUST be calculated the same way as described in 7.2.4 in [1], and 4 zeros MUST be padded to the end of the Null FP to made it 32-bit word aligned. Xie & Pearce Expires April 16, 2004 [Page 6] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 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 202 050 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, respectively. Thus, the timestamp is increased by 160, 220, or 320 for each consecutive FP, respectively. The DSR payload for ES 202 050 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-es202050 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. Xie & Pearce Expires April 16, 2004 [Page 7] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 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. 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-es202050") goes in SDP "a=rtpmap" as the encoding name. o The optional parameter "rate" also goes in "a=rtpmap" as clock Xie & Pearce Expires April 16, 2004 [Page 8] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 rate. o The optional parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively. Example of usage of ES 202 050 DSR: m=audio 49120 RTP/AVP 101 a=rtpmap:101 dsr-es202050/8000 a=maxptime:40 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 202 050, "Speech Processing, Transmission and Quality Aspects (STQ); Distributed Speech Recognition; Front-end Feature Extraction Algorithm; Compression Algorithms", (http:// pda.etsi.org/pda/home.asp?wki_id=6402) , October 2002. [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 Xie & Pearce Expires April 16, 2004 [Page 9] Internet-Draft RTP Payload Format for ES202050 DSR October 2003 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. 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 16, 2004 [Page 10] Internet-Draft RTP Payload Format for ES202050 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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