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
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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.
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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
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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,
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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]:
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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
+-----+-----+-----+-----+
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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.
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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.
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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
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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
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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
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