Annex 2 to AVC-673R

ITU-T SG15 Experts Group	ISO/IEC JTC1/SC29/WG11	
TD No.7 rev (Grimstad)	MPEG94/262	
July 18, 1994	July 18 1994	

Source:	ITU-T SG15 Experts Group for Video Coding and Systems in ATM and Other Network Environments
Title:	Support for ITU-T Control and Indication Signals
Purpose:	Discussion/Proposal
Relevant sub-group: Systems

1.	Background

To accommodate the requirements of audiovisual communications systems in ATM environments (ITU-T Rec. H.32X), ITU-T Rec. H.222.0 | ISO/IEC 13818-1 and ITU-T Rec. H.222.1 should provide the following in addition to the generic functionalities already covered by ITU-T Rec. H.222.0 | ISO/IEC 13818-1:

•	provision of a control channel for audiovisual communication control,
•	use of audio and video coding algorithms other than ISO/IEC 13818 and ISO/IEC 11172,
•	inclusion of control and indication signals synchronized with the presentation of audiovisual frames,
•	change of audio coding and other communication modes during communication.

This document discusses and proposes necessary provisions in ITU-T H.222.0 | ISO/IEC 13818-1 to allow the above functions.

2.	Introduction

At the June 1994 Atlanta MPEG Systems meeting, a "stream type" was reserved in ISO/IEC 13838-1 table 2-35 (p. 56) for "ITU-T auxiliary data." Stream type value 0x09 identifies streams that carry control-type information such as ITU-T control and indication messages, closed-caption data, etc.  Stream_type values are used in the TS_program_map_section() and program_stream_map() to identify elementary stream types within Transport Stream and Program Stream PSI tables.

The execution or use of some ITU-T auxiliary information must be synchronized with the presentation of audiovisual data [1, 2, 3].  This contribution suggests that the ITU-T define syntax for ITU-T auxiliary information that uses the MPEG-2 PES Packet structure to achieve this synchronization.  A benefit of this approach is that it supports the transmission of ITU-T auxiliary information in Program Streams as well as in Transport Steams.

We propose that ISO/IEC and ITU-T allocate one stream_id in ITU-T H.222.0 | ISO/IEC 13818-1 table 2-18 (p. 30) for ITU-T auxiliary information: the suggested codeword is the currently-reserved value "1111 0100".  (This proposal is an alternate to that suggested in document AVC-656|MPEG94/214, which proposes the allocation of multiple stream_id's to indicate the type of ITU-T channel: control and indication, data, etc.).  This table entry should include a note that refers to ITU H.222.1, will define the syntax of elementary streams with this stream_id value.  Suggested text for the note is, "This stream_id only is allowed in streams with stream_type = 0x09 (table 2-35)."

It would be possible to carry ITU-T auxiliary information in elementary streams that use stream_id values of 1011 1101 or 1011 1111 (private_stream_1 or private_stream_2), but the use of a separate stream_id value helps ensure that decoders recognize the type of such elementary streams.  Further, the availability of a unique stream_id value for ITU-T auxiliary data allows decoders to distinguish between elementary streams that carry true control/indication information from ITU-T auxiliary streams that carry ITU-T audiovisual data such as H.261 video or G.711 audio.  To enable the transmission of ITU-T encoded audio and video data in elementary streams with stream_type = 0x09, we recommend that:

•	the description of stream_id 110x xxxx be changed to read, "ISO/IEC 13818-3 or ISO/IEC 11172-3 or ITU-T audio stream number x xxxx"
•	the description of stream_id 1110 xxxx be changed to read, "ITU-T H.262 | ISO/IEC 13818-2 or ISO/IEC 11172-2 or ITU-T video stream number xxxx"

3.	ITU-T Auxiliary Data Syntax

We propose that H.222.1 define ITU-T auxiliary elementary streams to follow the same PES packet format as private_stream_1.  This syntax allows the execution of ITU-T control and indication messages to be synchronized with the presentation of audiovisual data, by use of the PTS and DTS fields.  We recommend that in ITU-T auxiliary elementary streams, the following fields in the PES packet header be set to fixed values:

•	data_alignment = 0
•	copyright = 0
•	original_or_copy = 1
•	DSM_trick_mode = 0
•	additional_copyright_info = 0

The syntax and semantics for the PES packet payloads in an ITU-T auxiliary elementary stream will be determined by the elementary stream descriptors used to describe the elementary stream.  (Section 2.6 of ISO/IEC 13818-1 explains elementary stream descriptors.) ITU-T H.222.1 should define descriptors for ITU-T auxiliary elementary streams.  H.222.1-defined descriptors will use "user private" descriptor tag values (ISO/IEC 13818-1 table 2-39).  These descriptors will not be used in the ISO/IEC 13818-1 program_stream_map() or TS_program_map_section() to describe entire programs, but only will be used to describe elementary streams.

Benefits of this proposed syntax include:

•	minimal modifications are needed to the ITU-T H.222.0 | ISO/IEC 13818-1 DIS,
•	the existing PTS/DTS mechanism is used for synchronization of ITU-T auxiliary information with audiovisual data,
•	the format of auxiliary data as its own stream allows ITU-T auxiliary streams to be added and dropped from ITU-T H.222.0 | ISO/IEC 13818-1 programs easily, solely via entries in the TS_program_map_section() or program_stream_map(),
•	much of this syntax can be parsed in the same way as other components of ITU-T H.222.0 | ISO/IEC 13818-1 streams.

Drawbacks of this method include:

•	Possible inefficient use of bits to transmit auxiliary information.  However, H.222.1 could define "packed" formats that combine several control and indication messages within a single PES packet.

Although it is beyond the scope of this proposal to define the ITU-T auxiliary elementary stream descriptors and the syntax of ITU-T auxiliary elementary stream data, we feel that the adoption of the syntax presented above would allow decoders to know where within the ITU-T H.222.0 | ISO/IEC 13818-1 syntax to look for auxiliary information.

4.	Example of ITU-T Auxiliary Information

H.222.1 could define a video_control_descriptor, following the syntax of ITU-T H.222.0 | ISO/IEC 13818-1 section 2.6.  This descriptor, applied to an elementary stream, indicates that the elementary stream carries video display control signals.  An elementary stream referenced by this descriptor could carry directives such as "freeze_frame_release" in its PES packet payloads.  The time at which a directive should be implemented is carried in the PTS field of the header of the PES packet that carries the directive.

5.	Summary of Suggested Changes to DIS 13818-1

•	In table 2-35, the description corresponding to value 0x09 should read: "ITU-T auxiliary".
•	In table 2-18, stream_id value 1111 0100 should be allocated for ITU-T auxiliary data.  A note attached to this table entry should read, "This stream_id only is allowed in streams with stream_type = 0x09 (table 2-35)."
•	In table 2-18, the description of stream_id 110x xxxx be changed to read, "ISO/IEC 13818-3 or ISO/IEC 11172-3 or ITU-T audio stream number x xxxx"
•	In table 2-18, the description of stream_id 1110 xxxx be changed to read, "ITU-T H.262 | ISO/IEC 13818-2 or ISO/IEC 11172-2 or ITU-T video stream number xxxx"

6.	References

[1]	AVC-656 | MPEG94/214	Necessary provision for applying H.222.0 to the H.32X terminals (Japan)
[2]	AVC-668	Video conferencing Control and Indication signals for H.262 (AT&T)
[3]	AVC-665 | MPEG94/220	Closed caption in ITU-T H.262|ISO/IEC 13818-2 (Bellcore)