Audio Video Transport WG Q. Xie
Internet-Draft D. Pearce
Expires: August 6, 2004 Motorola
February 06, 2004
RTP Payload Formats for European Telecommunications Standards
Institute (ETSI) European Standard ES 202 050, ES 202 211, and ES 202
212 Distributed Speech Recognition Encoding
draft-ietf-avt-rtp-dsr-codecs-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies RTP payload formats for encapsulating ETSI
Standard ES 202 050 DSR Advanced Front-end (AFE), ES 202 211 DSR
Extended Front-end (XFE), and ES 202 212 DSR Extended Advanced
Front-end (XAFE) 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 . . . . . . . . 4
2.2 ETSI ES 202 211 Extended DSR Front-end Codec . . . . . . . . 4
2.3 ETSI ES 202 212 Extended Advanced DSR Front-end Codec . . . 5
3. DSR RTP Payload Formats . . . . . . . . . . . . . . . . . . 5
3.1 Payload Format for ES 202 050 DSR . . . . . . . . . . . . . 5
3.1.1 Consideration on Number of FPs in Each RTP Packet . . . . . 6
3.1.2 Support for Discontinuous Transmission . . . . . . . . . . . 6
3.1.3 Frame Pair Formats . . . . . . . . . . . . . . . . . . . . . 6
3.1.4 RTP header usage . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Payload Format for ES 202 211 DSR . . . . . . . . . . . . . 8
3.2.1 Consideration on Number of FPs in Each RTP Packet . . . . . 9
3.2.2 Support for Discontinuous Transmission . . . . . . . . . . . 9
3.2.3 Frame Pair Formats . . . . . . . . . . . . . . . . . . . . . 9
3.2.4 RTP header usage . . . . . . . . . . . . . . . . . . . . . . 11
3.3 Payload Format ES 202 212 DSR . . . . . . . . . . . . . . . 12
3.3.1 Consideration on Number of FPs in Each RTP Packet . . . . . 12
3.3.2 Support for Discontinuous Transmission . . . . . . . . . . . 12
3.3.3 Frame Pair Formats . . . . . . . . . . . . . . . . . . . . . 12
3.3.4 RTP header usage . . . . . . . . . . . . . . . . . . . . . . 15
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15
4.1 Mapping MIME Parameters into SDP . . . . . . . . . . . . . . 16
5. Security Considerations . . . . . . . . . . . . . . . . . . 17
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 17
Normative References . . . . . . . . . . . . . . . . . . . . 17
Informative References . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . 20
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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
[5].
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 [10].
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 [11], and an RTP packetization for ES 201 108
frames is defined in [10] 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
[10]. The differences are in the DSR codec frame bit definition and
the payload type MIME registration.
The two further standards, ES 202 211 and ES 202 212, provided
extensions to each of the DSR front-end standards. The extensions
allow the speech waveform to be reconstructed for human audition and
can also be used to improve recognition performance for tonal
languages. This is done by sending additional pitch and voicing
information for each frame along with the recognition features.
The RTP packet format for these extended standards are also defined
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in this document.
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,
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 Energy.
Voice activity detection (VAD) for the classification 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.
2.2 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
ES 201 108 [11]. 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
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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
[2].
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 [2].
2.3 ETSI ES 202 212 Extended Advanced DSR Front-end Codec
ES 202 212 is the extension for the DSR Advanced Front-end ES 202 050
[1]. It provides the same capabilities as the extended mel-cepstrum
front-end described in section 2.2 but for the DSR Advanced
Front-end.
3. DSR RTP Payload Formats
3.1 Payload Format for ES 202 050 DSR
An ES 202 050 DSR RTP payload datagram uses exactly the same layout
as defined in Section 3 of [10], i.e., a standard RTP header followed
by a DSR payload containing a series of DSR FPs.
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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.
3.1.1 Consideration on Number of FPs in Each RTP Packet
Same considerations described in Section 3.1 of [10] apply to ES 202
050 RTP payload.
3.1.2 Support for Discontinuous Transmission
Same considerations described in Section 3.2 of [10] apply to ES 202
050 RTP payload.
3.1.3 Frame Pair Formats
3.1.3.1 Format of Speech and Non-speech FPs
The following mel-cepstral frame MUST be used, as defined in [1]:
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. 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 202 050 FP:
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
+-----+-----+-----+-----+-----+-----+-----+-----+
CRC for Frame #1 and Frame #2 and padding in FP:
================================================
(MSB) (LSB)
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| 0 | 0 | 0 | 0 | CRC | Octet 12
+-----+-----+-----+-----+-----+-----+-----+-----+
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 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 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 [10] when determining the proper number of FPs in an RTP
payload.
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3.1.3.2 Format of Null FP
Null FPs are sent to mark the end of a transmission segment. Details
on transmission segment and the use of Null FPs can be found in [10].
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.
3.1.4 RTP header usage
The format of the RTP header is specified in [8]. 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 [8].
The RTP header marker bit (M) should be set following the general
rules for audio codecs as defined in Section 4.1 in [9].
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.
3.2 Payload Format for ES 202 211 DSR
An ES 202 211 DSR RTP payload datagram is very similar to that
defined in Section 3 of [10], i.e., a standard RTP header followed by
a DSR payload containing a series of DSR FPs.
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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.
3.2.1 Consideration on Number of FPs in Each RTP Packet
Same considerations described in Section 3.1 of [10] apply to ES 202
211 RTP payload.
3.2.2 Support for Discontinuous Transmission
Same considerations described in Section 3.2 of [10] apply to ES 202
211 RTP payload.
3.2.3 Frame Pair Formats
3.2.3.1 Format of Speech and Non-speech FPs
The following mel-cepstral frame MUST be used, as defined in Section
6.2.4 in [2]:
As defined in Section 6.2.4 in [2], 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:
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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
+-----+-----+-----+-----+
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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 [2].
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 [10] when determining the proper number of FPs in an RTP
payload.
3.2.3.2 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. Null FPs are sent to mark the
end of a transmission segment. Details on transmission segment and
the use of Null FPs can be found in [10].
3.2.4 RTP header usage
The format of the RTP header is specified in [8]. 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
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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 202 211 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 [8].
The RTP header marker bit (M) should be set following the general
rules for audio codecs as defined in Section 4.1 in [9].
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.
3.3 Payload Format ES 202 212 DSR
Similar to other ETSI DSR front-end encoding schemes, the encoded DSR
feature stream of ES 202 212 is transmitted in a sequence of frame
pairs (FPs), where each FP represents two consecutive original voice
frames.
An ES 202 212 DSR RTP payload datagram is very similar to that
defined in Section 3 of [10], i.e., a standard RTP header followed by
a DSR payload containing a series of DSR FPs.
The size of each ES 202 212 FP is 112 bits or 14 octets (see Sections
3 below). This ensures that an ES 202 212 DSR RTP payload will always
end on an octet boundary.
3.3.1 Consideration on Number of FPs in Each RTP Packet
Same considerations described in Section 3.1 of [10] apply to ES 202
212 RTP payload.
3.3.2 Support for Discontinuous Transmission
Same considerations described in Section 3.2 of [10] apply to ES 202
212 RTP payload.
3.3.3 Frame Pair Formats
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3.3.3.1 Format of Speech and Non-speech FPs
The following mel-cepstral frame MUST be used, as defined in Section
7.2.4 in [3]:
As defined in Section 7.2.4 in [3], 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 next follow (Cidx1 and
Cidx2). Finally, a 2-bit CRC (PC-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. 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 padding brings the total size of a FP to 112 bits, or
14 octets. Note, this padding is separate from padding indicated by
the P bit in the RTP header.
The following diagram shows a complete ES 202 212 FP:
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
+-----+-----+-----+-----+-----+-----+-----+-----+
CRC for Frame #1 and Frame #2 in FP:
====================================
(MSB) (LSB)
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+
| CRC | Octet 12/1
+-----+-----+-----+-----+
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 codebook indices, VAD flag, pitch index, and class index are
specified in Section 6 of [3]. 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 7.2.4 in [3].
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
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fragmentation and SHOULD follow the recommendations given in Section
3.1 in [10] when determining the proper number of FPs in an RTP
payload.
3.3.3.2 Format of Null FP
A Null FP for the ES 202 212 front-end codec is defined by setting
all the 112 bits of the FP with 0's. Null FPs are sent to mark the
end of a transmission segment. Details on transmission segment and
the use of Null FPs can be found in [10].
3.3.4 RTP header usage
The format of the RTP header is specified in [8]. 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 202 212 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 202 212 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 [8].
The RTP header marker bit (M) should be set following the general
rules for audio codecs as defined in Section 4.1 in [9].
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.
4. IANA Considerations
For each of the three ETSI DSR front-end codecs covered in this
document, a new MIME subtype registration is required for the
corresponding payload type, as described below.
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Media Type name: audio
Media subtype name: dsr-es202050 for ES 202 050, dsr-es202211 for ES
202 211, and dsr-es202212 for ES 202 212 front-end, respectively.
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: see RFC3267 [7]. 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 [6].
Encoding considerations: This type is defined for transfer via RTP
[8] 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
4.1 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)
[6], which is commonly used to describe RTP sessions. When SDP is
used to specify sessions employing ES 202 050, ES 202 211, or ES 202
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212 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", "dsr-es202211", or
"dsr-es202212") goes in SDP "a=rtpmap" as the encoding name.
o The optional parameter "rate" also goes in "a=rtpmap" as clock
rate. If no rate is given, then the default value (i.e., 8000) is
used in SDP.
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
Example of usage of ES 202 211 DSR:
m=audio 49120 RTP/AVP 101
a=rtpmap:101 dsr-es202211/8000
a=maxptime:40
Example of usage of ES 202 212 DSR:
m=audio 49120 RTP/AVP 101
a=rtpmap:101 dsr-es202212/8000
a=maxptime:40
5. Security Considerations
Implementations using the payload defined in this specification are
subject to the security considerations discussed in the RTP
specification [8] and the RTP profile [9]. This payload does not
specify any different security services.
6. Acknowledgments
The design presented here is based on that of [10].
Normative References
[1] European Telecommunications Standards Institute (ETSI) Standard
ES 202 050, "Speech Processing, Transmission and Quality
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Aspects (STQ); Distributed Speech Recognition; Front-end
Feature Extraction Algorithm; Compression Algorithms", (http://
pda.etsi.org/pda/) , October 2002.
[2] European Telecommunications Standards Institute (ETSI) Standard
ES 202 211, "Speech Processing, Transmission and Quality
Aspects (STQ); Distributed Speech Recognition; Extended
front-end feature extraction algorithm; Compression algorithms;
Back-end speech reconstruction algorithm", (http://
pda.etsi.org/pda/) , November 2003.
[3] European Telecommunications Standards Institute (ETSI) Standard
ES 202 212, "Speech Processing, Transmission and Quality
aspects (STQ); Distributed speech recognition; Extended
advanced front-end feature extraction algorithm; Compression
algorithms; Back-end speech reconstruction algorithm", (http://
pda.etsi.org/pda/) , November 2003.
[4] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[7] Sjoberg, J., Westerlund, M., Lakaniemi, A. and Q. Xie,
"Real-Time Transport Protocol (RTP) Payload Format and File
Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive
Multi-Rate Wideband (AMR-WB) Audio Codecs", RFC 3267, June
2002.
[8] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
[9] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551, July 2003.
[10] 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
[11] European Telecommunications Standards Institute (ETSI) Standard
ES 201 108, "Speech Processing, Transmission and Quality
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Aspects (STQ); Distributed Speech Recognition; Front-end
Feature Extraction Algorithm; Compression Algorithms", (http://
webapp.etsi.org/pda/) , 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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Xie & Pearce Expires August 6, 2004 [Page 21]
