draft-ietf-payload-rtp-aptx-01.txt   draft-ietf-payload-rtp-aptx-02.txt 
Internet-Draft J. Lindsay Internet-Draft J. Lindsay
A/V Transport Payloads Working Group H.Foerster A/V Transport Payloads Working Group H.Foerster
Intended status: Standards Track APT Ltd Intended status: Standards Track APT Ltd
Expires: March 5, 2014 September 5, 2013 Expires: April 14, 2014 October 14, 2013
RTP Payload Format for Standard apt-X and Enhanced apt-X Codecs RTP Payload Format for Standard apt-X and Enhanced apt-X Codecs
draft-ietf-payload-rtp-aptx-01 draft-ietf-payload-rtp-aptx-02
Abstract Abstract
This document specifies a scheme for packetizing Standard apt-X, or This document specifies a scheme for packetizing Standard apt-X, or
Enhanced apt-X, encoded audio data into Real-time Transport Protocol Enhanced apt-X, encoded audio data into Real-time Transport Protocol
(RTP) packets. The document describes a payload format that permits (RTP) packets. The document describes a payload format that permits
transmission of multiple related audio channels in a single RTP transmission of multiple related audio channels in a single RTP
payload, and a means of establishing Standard apt-X and Enhanced payload, and a means of establishing Standard apt-X and Enhanced
apt-X connections through the Session Description Protocol (SDP). apt-X connections through the Session Description Protocol (SDP).
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on February 22, 2014. This Internet-Draft will expire on April 14, 2014.
Submission Compliance for Internet-Drafts. Submission Compliance for Internet-Drafts.
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Copyright and License Notice Copyright and License Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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bit in each field is the most significant. The numbering starts from bit in each field is the most significant. The numbering starts from
0 and ascends, where bit 0 will be the most significant. 0 and ascends, where bit 0 will be the most significant.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. Standard apt-X and Enhanced apt-X Codecs 3. Standard apt-X and Enhanced apt-X Codecs
Standard apt-X and Enhanced apt-X are proprietary audio coding Standard apt-X and Enhanced apt-X are proprietary audio coding
algorithms, licensed by CSR plc and widely deployed in a variety of algorithms, which can be licensed from CSR plc and are widely
audio processing equipment. For commercial reasons, the detailed deployed in a variety of audio processing equipment.
internal operations of these algorithms are not described in For commercial reasons, the detailed internal operations of these
standards or reference documents. However, the data interfaces to algorithms are not described in standards or reference documents.
implementations of these algorithms are very simple, and allow easy However, the data interfaces to implementations of these algorithms
RTP packetization of data coded with the algorithms, without a are very simple, and allow easy RTP packetization of data coded
detailed knowledge of the actual coded audio stream syntax. with the algorithms, without a detailed knowledge of the actual
coded audio stream syntax.
Both the Standard apt-X and Enhanced apt-X coding algorithms are Both the Standard apt-X and Enhanced apt-X coding algorithms are
based on Adaptive Differential Pulse Code Modulation principles. based on Adaptive Differential Pulse Code Modulation principles.
They produce a constant coded bit rate that is scaled according to They produce a constant coded bit rate that is scaled according to
the sample frequency of the uncoded audio. This constant rate is 1/4 the sample frequency of the uncoded audio. This constant rate is 1/4
of the bit rate of the uncoded audio, irrespective of the resolution of the bit rate of the uncoded audio, irrespective of the resolution
(number of bits) used to represent an uncoded audio sample. For (number of bits) used to represent an uncoded audio sample. For
example, a 1.536 Mbit/s stereo audio stream, composed of 2 channels example, a 1.536 Mbit/s stereo audio stream, composed of 2 channels
of 16-bit Pulse Code Modulated (PCM) audio that is sampled at a of 16-bit Pulse Code Modulated (PCM) audio that is sampled at a
frequency of 48 kHz, is encoded at 384 kbit/s. frequency of 48 kHz, is encoded at 384 kbit/s.
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structure, and the coded data forms a continuous coded sample stream structure, and the coded data forms a continuous coded sample stream
with each coded sample capable of regenerating 4 PCM samples when with each coded sample capable of regenerating 4 PCM samples when
decoded. The Standard apt-X algorithm encodes 4 successive 16-bit decoded. The Standard apt-X algorithm encodes 4 successive 16-bit
PCM samples from each audio channel into a single 16-bit coded sample PCM samples from each audio channel into a single 16-bit coded sample
per audio channel. The Enhanced apt-X algorithm encodes 4 successive per audio channel. The Enhanced apt-X algorithm encodes 4 successive
16-bit or 24-bit PCM samples from each audio channel and respectively 16-bit or 24-bit PCM samples from each audio channel and respectively
produces a single 16-bit or 24-bit coded sample per channel. The produces a single 16-bit or 24-bit coded sample per channel. The
same RTP packetisation rules apply for each of these algorithmic same RTP packetisation rules apply for each of these algorithmic
variations. variations.
. Standard apt-X and Enhanced apt-X coded data streams can optionally Standard apt-X and Enhanced apt-X coded data streams can optionally
carry synchronisation information and an auxiliary data channel carry synchronisation information and an auxiliary data channel
within the coded audio data without additional overhead. These within the coded audio data without additional overhead. These
mechanisms can, for instance, be used when the IP system is cascaded mechanisms can, for instance, be used when the IP system is cascaded
with another transportation system and the decoder is acting as a with another transportation system and the decoder is acting as a
simple bridge between the two systems. Since auxiliary data channel simple bridge between the two systems. Since auxiliary data channel
and synchronisation information are carried within the coded audio and synchronisation information are carried within the coded audio
data without additional overhead, RTP payload format rules do not data without additional overhead, RTP payload format rules do not
change if they are present. Out-of-band signalling is required change if they are present. Out-of-band signalling is required
however to notify the receiver end when autosync and auxiliary data however to notify the receiver end when autosync and auxiliary data
have been embedded in the apt-X stream. have been embedded in the apt-X stream.
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channel samples. By default apt-X channels pairing is not enabled. channel samples. By default apt-X channels pairing is not enabled.
Out-of-band signalling is required to notify the receiver when the Out-of-band signalling is required to notify the receiver when the
option is being used. option is being used.
Standard apt-X and Enhanced apt-X decoders that have not be set up Standard apt-X and Enhanced apt-X decoders that have not be set up
with the correct embedded autosync, auxiliary data and stereo pairing with the correct embedded autosync, auxiliary data and stereo pairing
information will playout uncoded PCM samples with a loss of decoding information will playout uncoded PCM samples with a loss of decoding
quality. In the case of standard apt-X the loss of quality can be quality. In the case of standard apt-X the loss of quality can be
significant. significant.
Further details on the algorithm operation can be obtained from
CSR plc.
Corporate HQ
Churchill House
Cambridge Business Park
Cowley Road
Cambridge
CB4 0WZ
UK
Tel: +44 1223 692000
Fax: +44 1223 692001
http://www.csr.com
4. Payload Format Capabilities 4. Payload Format Capabilities
This RTP payload format carries an integer number of Standard apt-X This RTP payload format carries an integer number of Standard apt-X
or Enhanced apt-X coded audio samples. When multiple related audio or Enhanced apt-X coded audio samples. When multiple related audio
channels are being conveyed within the payload, each channel channels are being conveyed within the payload, each channel
contributes the same integer number of apt-X coded audio samples to contributes the same integer number of apt-X coded audio samples to
the total carried by the payload. the total carried by the payload.
4.1. Use of Forward Error Correction (FEC) 4.1. Use of Forward Error Correction (FEC)
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fields are defined as follows. fields are defined as follows.
o V - As per [RFC3550] o V - As per [RFC3550]
o P - As per [RFC3550] o P - As per [RFC3550]
o X - As per [RFC3550] o X - As per [RFC3550]
o CC - As per [RFC3550] o CC - As per [RFC3550]
o M - This payload format defines no use for this bit. Senders o M - As per [RFC3551] Section 4.1
SHOULD set this bit to zero in each outgoing packet.
o PT - A dynamic payload type, i.e. one within the range [96..127], o PT - A dynamic payload type, MUST be used. [RFC3551]
MUST be used. [RFC3551]
o SN - As per [RFC3550] o SN - As per [RFC3550]
o Timestamp - As per [RFC3550]. The RTP timestamp reflects the o Timestamp - As per [RFC3550]. The RTP timestamp reflects the
instant at which the first audio sample in the packet was sampled, instant at which the first audio sample in the packet was sampled,
that is, the oldest information in the packet. that is, the oldest information in the packet.
Header field abbreviations are defined as follows. Header field abbreviations are defined as follows.
V - Version Number V - Version Number
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The type name ("audio") goes in SDP "m=" as the media name. The type name ("audio") goes in SDP "m=" as the media name.
The subtype name ("aptx") goes in SDP "a=rtpmap" as the encoding The subtype name ("aptx") goes in SDP "a=rtpmap" as the encoding
name. name.
The parameter "rate" also goes in "a=rtpmap" as clock rate. The parameter "rate" also goes in "a=rtpmap" as clock rate.
The parameter "channels" also goes in "a=rtpmap" as channel count. The parameter "channels" also goes in "a=rtpmap" as channel count.
The parameter "maxptime" when present, MUST be included in the
SDP "a=maxptime" attribute.
The required parameters "variant" and "bitresolution" MUST be The required parameters "variant" and "bitresolution" MUST be
included in the SDP "a=fmtp" attribute and MUST follow the included in the SDP "a=fmtp" attribute.
delivery-method that applies.
The optional parameters "stereo-channel-pairs", "embedded- The optional parameters "stereo-channel-pairs", "embedded-
autosync-channels", "embedded-aux-channels", and "maxptime" when autosync-channels", "embedded-aux-channels" when present,
present, MUST be included in the SDP "a=fmtp" attribute and MUST MUST be included in the SDP "a=fmtp" attribute.
follow the delivery-method that applies.
The parameter "ptime", when present, goes in a separate SDP The parameter "ptime", when present, goes in a separate SDP
attribute field and is signalled as "a=ptime:<value>", where attribute field and is signalled as "a=ptime:<value>", where
<value> is the number of millseconds of audio represented by one <value> is the number of millseconds of audio represented by one
RTP packet. See Section 6 of [RFC4566]. RTP packet. See Section 6 of [RFC4566].
6.2.1. SDP Usage Examples 6.2.1. SDP Usage Examples
Some example SDP session descriptions utilizing apt-X encodings Some example SDP session descriptions utilizing apt-X encodings
follow. In these examples, long a=fmtp lines are folded to meet the follow. In these examples, long a=fmtp lines are folded to meet the
column width constraints of this document. column width constraints of this document.
Example 1: A standard apt-X stream that encodes two independent Example 1: A standard apt-X stream that encodes two independent
44.1kHz 16-bit PCM channels into a 4ms RTP packet. 44.1kHz 16-bit PCM channels into a 4ms RTP packet.
.
m=audio 5004 RTP/AVP 98 m=audio 5004 RTP/AVP 98
a=rtpmap:98 aptx/44100/2 a=rtpmap:98 aptx/44100/2
a=fmtp:98 variant=standard; bitresolution=16; a=fmtp:98 variant=standard; bitresolution=16;
a=ptime:4 a=ptime:4
Example 2: An enhanced apt-X stream that encodes two 48kHz 24-bit Example 2: An enhanced apt-X stream that encodes two 48kHz 24-bit
stereo channels into a 4ms RTP packet and that carries both an stereo channels into a 4ms RTP packet and that carries both an
embedded autosync and auxiliary data channel. embedded autosync and auxiliary data channel.
m=audio 5004 RTP/AVP 98 m=audio 5004 RTP/AVP 98
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