ITU-T Telecommunication Standardization Sector	Document AVC-633R
Study Group 15	25 March 1994
Experts Group for Video Coding and Systems 
in ATM and Other Network Environments

Source:	RAPPORTEUR (Sakae OKUBO)	
Title:	REPORT OF THE FIFTEENTH EXPERTS GROUP MEETING IN PARIS 
(16-25 March 1994) - Part III	
Purpose:	Report	

-----------------------

Part I	General (see AVC-632R)
Part II	Sole sessions (see AVC-632R)
Part III	Joint sessions

Part III  Joint sessions in Paris

Contents

A. 	Introduction
B. 	REQUIREMENTS sub-group
C. 	VIDEO sub-group
D. 	SYSTEMS sub-group

A. Introduction

The joint sessions with MPEG were held at Le Conservatoire des Arts et Manufactures in Paris during 21-25 march 1994 at the kind invitation of AFNOR.  At the opening session on 21 March, Mr. Daniel Lecomte (Alcatel), French HoD for SC29, made a welcoming address on behalf of the hosting organization.  

The Experts Group appreciated the support and hospitality of the hosting organization.

A list of documents considered during the joint sessions is attached to this report as Annex 1.

B. REQUIREMENTS sub-group (by Sakae OKUBO)

B1. 	Introduction

The Requirements sub-group met four days (Monday, Tuesday Wednesday and Thursday) under chairmanship of Mr. Sakae Okubo.  The summary of the work was as follows:

Monday: 	All available input documents were reviewed briefly and discussion items were identified.  A direction to reorganize Chapter 8 and Annex E was agreed.

Tuesday: 	First draft of reorganized Chapter 8 and Annex was reviewed.  Second draft was worked out and reviewed.

Wednesday:	There was a joint meeting with Video sub-groups to exchange views on some technical issued raised through comments to CD.  There was also a joint meeting with Audio to find a way out from the quality issue.  Verification plan and amendment for additional profiles and levels were discussed.  Third draft of reorganized Chapter 8 and Annex was reviewed.

Thursday	DSM-CC documents were reviewed and suggestions were returned.  Clarification of scalable layers was worked out.

B2. 	Documents

Input documents
WG11 N0605	Req. sub-group	Verification test items and plan	
SC29 N710	Secretariat	Summary of voting on ISO/IEC CD 	
MPEG94/015	Okubo	Report of Ad-hoc group on procedure for introducing new Profiles and Levels	
MPEG94/044	ITU-T Japan	Comments for CD13818-2|Draft Rec. H.262	
MPEG94/072	Erdem, Sezan	Results of recent 10-bit experiments	
MPEG94/086	Eifrig,Tabatabai	Experiments with multigeneration MPEG 10 bit coding	
MPEG94/095	Puri, Haskell	A proposal for Temporal Profile in MPEG-2	
MPEG94/138	DSM sub-group	DSM-CC extension requirements document	

Output documents
WG11 N0702	Video Group	DIS 13818-2	
WG11 N0681	Video Group	Disposition of comments	

B3. 	Chapter 8 and Annex E of DIS 13818-2

B3.1	Reorganization of Chapter 8 and Annex E

In response to the comments from some National Bodies, we decided to reorganize Chapter 8 and Annex E so that it is easier to find profile and level dependent specifications in normative Chapter 8.  Specifications are now represented in table form with relevant tables in the previous Annex E moved to the current Chapter 8.  Mr. Chris Birch undertook this editing work.  Agreed texts have been included in the DIS document.  

The meeting also reviewed all the comments from National Bodies with getting help of VIDEO sub-group in some items and made disposition of comments relevant to Chapter 8 and Annex E.

The following is items of substance which we discussed;

•	handling of MPEG-1 D pictures by MPEG-2 decoders
•	scalable profile clarification
•	VBV buffer size
•	SP@ML capability of decoding MP@LL
•	bit rates and VBV buffer size allocation between layers
•	clarification of base layer bitstream decodability

B3.2 Clarification of scalable profiles

During the discussion with Video group, it was pointed out that we need clarification on permitted combinations for different layer characteristics of scalable profiles.  This was in the interest of ensuring interoperability.

B4. 	Verification test plan

We made a framework for the verification test items and plan at the Seoul meeting.  We discussed whether there remained further efforts of the Requirements sub-group, but reached a conclusion that what we need are commitments of participating members.  Mr. K. McCann advised the meeting of its plan to make complete hardware consisting of MPEG-2 Video (MP@ML), MPEG-1 Audio (basic stereo) and MPEG-2 Systems TS.  Mr. R. St. Girons also advised us of the US Grand Alliance plan to carry out MP@HL hardware tests before November 1994.

Provision of further testing plans and test results are awaited.

B5. 	Amendments for additional profiles and levels

B5.1 Amendment procedures (MPEG94/015)

As far as procedures are concerned, JTC1 Directives and Guide for ITU-T and ISO/IEC JTC1 Cooperation give us sufficient information.  The time necessary from an initial proposal to AMD may differ according to the nature of new profile; whether it is simply a new entry to the existing profile and level structure, or it needs definition of new coding tools.

B5.2 Possible additional profiles

During the meeting, the following three possible additions were indicated;

1) SNR Profile @ High-1440 Level (Swedish NB comments to CD 13818-2)

This requires only definition of a new conformance point in the current profile and level structure.  No new syntactic tools are necessary.

2) Temporal Scalability Profile (MPEG94/095)

This is intended as a profile of Main Profile plus temporal scalability tool which has already been defined in the current CD.  Though the decoder of this profile can decode Main Profile bitstreams, the "onion ring" relationship with the existing profiles are not maintained.  A new profile should be created for this purpose.

3) 10 bit amplitude resolution

This is intended for the use of editing and post production industries and thought to be advantageous in multiple generation characteristics.  It needs additional coding tools besides definition of a new profile.  There was a comment that in some countries there are multiple products which use 10 bits based on non-MPEG approaches, which may lead to an interoperability problem.

B5.3 Criteria for adding a new profile

We discussed what would be criteria for defining a new profile through the amendment procedures.  Since they may depend on the nature of new profile, we reached a conclusion that they can be stated in a general term as "the proposal should be convincing".  If we paraphrase "convincing", the proposal should at least be 

•	supported by more than one organization,
•	market driven or based on real applications.

B5.4 Amendment work

This should be initiated when proposals are made.  We should bear in mind a situation that there may be different proposals which require different time period to reach the AMD status.  The target date of AMD should be set with this consideration.

B6. 	DSM-CC requirements

Members of the REQUIREMENTS sub-group assisted the DSM sub-group by reviewing the drafts for call for contribution and attached list of requirements and provided some comments for improvement.

B7. 	Recommendations of the sub-group

REQUIREMENTS sub-group recommend that;

1) WG11 approve Chapter 8 and Annex E as an integral part of DIS 13818-2 and associated disposition of National Body comments.

2) WG11 request members to thoroughly review details on permitted combinations for different layer characteristics of High Profile as contained in WG11 N0702 which we believe correct and contributive to the interoperabiliy of base layer bitstreams.

3) WG11 request members to further contribute to materializing the verification test framework as agreed in Seoul and contained in WG11 N0605.

C. VIDEO sub-group (by Kiyoshi SAKAI)

C1.	Introduction

The video sub-group met for five days, but there was not a large attendance as before (20 to 30 people).  The group mainly worked to deal with the comments submitted to the national body ballot regarding to the proposed video DIS.  According to the comments, the group made editorial corrections, clarified the ambiguities (especially in scalable syntax) and also made fixes for a few remaining inadequacies.  As the result of the meeting, the group approved the revised proposal for DIS.

The main topics discussed during the meeting were as follows. 

C2.	Restriction for number of bits per MB

This issue was raised by UK and US national body in order to relax the decoder implementation complexity, mainly concerned with the necessary bandwidth at the external RAM interface.  The discussion point was how to accommodate an appropriate restriction in the standard to achieve good trade-off between encoder complexity and decoder complexity.  This limitation would introduce some additional feedback circuit in the encoder, in exchange for reducing the decoder hardware complexity.

Several alternative solutions were discussed and as a result the group decided to include a modification on the original proposal that strictly limits the number of bits per MB.  The compromised restriction was to limit the number of bits per MB to 12*(number of samples in a MB) for all MBs except any 2 MBs in each horizontal row.  The number "2" was introduced to account for the feedback delay in the encoder circuit.  On the other hand, restriction for the escape coding, which was defined to cope with the same problem, was deleted.  
 
C3.	IDCT mismatch control

It has been pointed out that two different compliant IDCTs can generate severe degradation in near still pictures, even though currently defined mismatch control algorithm is applied.  There were some contributions and proposals to reduce the degradation by applying some tricks at the encoder side, but no essential resolution or perfect mismatch control algorithm has been found.  So, the group finally decided to keep the current mismatch control algorithm as it is, but to add the following note to draw attention (see Note 2 in section 7.4.4 of DIS):

Warning.  Small non-zero inputs to the IDCT may result in zero output for compliant IDCTs.  If this occurs in an encoder, mismatch may occur in some pictures in a decoder that uses a different compliant IDCT.  An encoder can avoid this problem by checking the output of its own IDCT.

Related to this issue, it was reconfirmed that the sample domain coefficients for a block shall all take the value zero if its pattern_code is zero or the block is within a skipped MB.  This issue was raised by the fact that some compliant IDCTs generate non-zero data even its input data are all zero.

C4.	Conformance testing

The group spent several hours to discuss conformance issues in order to develop the MPEG-2 Video Conformance Working Draft.  As a result of discussion, the group decided to re-establish the ad-hoc group for conformance testing and to generate test bitstreams which will be useful for testing conformance of video decoders.  Toward the next meeting, members of the ad-hoc group are requested to generate and exchange 15 test bitstreams including the worst case bitstreams (so called, evil bitstreams), such as bitstreams with maximum number and concentration of coded bits, maximum number and concentration of run-level symbols, bursts concentrated at the beginning of a picture, etc.

D. SYSTEMS sub-group (by Stuart DUNSTAN)

D1.	Introduction

An interim version of the MPEG-2 Systems Committee Draft ISO/IEC JTC1/SC29/WG11 N0701 was submitted to the closing MPEG plenary in Paris.  This document reflects technical agreements and editing changes agreed to at the meeting, and includes some hand written attachments.

Further work is required before the DIS can be issued.  In addition no replies to national body comments on the CD have yet been written.  A WG11 meeting for the Systems group will be held from 8th - 10th June, in Atlanta, USA.  This will be preceded by a two day ad-hoc group meeting.

The contribution from the SG15 EG [1] was presented, however there was little time for discussion.

D2.	DSM-CC extension

The following MPEG 94 documents addressed DSM-CC extension: 038, 040, 083, 087, 089, 091, 110, 113.  These documents presented examples and concepts of what the future DSM-CC could look like.  The following documents, relating to DSM-CC, have been issued by the WG11 Convenor:

MPEG 94/150,  "Call for contributions for DSM-CC Extension"
MPEG 94/151,  "DSM-CC Extension Requirements Document".

D3.	Descriptors

Discussion on audio and video descriptors was initiated.  The intention is that a video descriptor, for example, would indicate the video profile and level.  The opinion was expressed that a video descriptor might also duplicate some parameters contained within the video elementary stream e.g. frame rate, allowing faster access to these parameters.  The format of these descriptors has not been finalised.

The format of the registration descriptor, which indicates coding formats in private streams, has been revised.  ASN.1 Basic Encoding Rules are used.  A copyright descriptor, which provides a globally unique label for elementary streams or programs, has also been added, and is coded similarly to the registration descriptor.

D4.	Transport Stream STD

The Transport Stream STD model has been changed to that of a leaky bucket.  The previous latency model was found to have problems under certain conditions.  For video the rate of removal of data from the TB buffer is defined as being proportional to , which is specified for each profile and level in table 8-12 of 13818-2.  The rate of removal for audio data is fixed at 1 Mbit/s.  The preliminary figure for systems is also 1 Mbit/s.

PES headers are placed in the buffer , and some space must be allocated for them.  For video the size of the buffer  is the size of the VBV, as defined in table 8-13 of 13818-2 for each profile and level, plus an amount to deal with remultiplex timing jitter, plus an amount to deal with PES packet headers.  The latter is proportional to the profile and level.  For audio the size of the buffer  is fixed.

In the Transport Stream STD the Network Information Tables, which are part of the PSI, do not enter .

D5.	PCR tolerance

The PCR in the Transport Stream is the output of a 42 bit binary counter clocked by the 27 MHz system clock.  The view was expressed that it is difficult in practice to place the PCR in a Transport Stream, and guarantee that all 42 bits represent the correct position of the appropriate PCR byte in the Transport Stream.  The agreement was that PCRs may have a maximum jitter of .  Additional semantics need also to be defined.

It is not clear whether the same tolerance will also apply to the SCR in the Program Stream.

6.	VBR Transport Stream

A statement exists in the interim CD that, with respect to Transport Streams and variable bit rate, only a single program Transport Stream will be supported, and that a variable bit rate multiple program Transport Stream is not possible.  Clarification of this statement may be required.

D7.	Remultiplexing support

Additional fields to support remultiplexing have been added in an adaptation field extension.  A legal time window specifies a time window in which the buffer  is guaranteed not to underflow or overflow.  A piece wise rate field specifies the bit rate of transport packets in a particular PID.

D8.	Splice point

The syntax and semantics dealing with splicing have been revised and refined.  A second adaptation field extension exists which contains fields expressing the splice type and the decoding time of the next access unit.

The definition of a discontinuity has been refined to ensure that Transport Stream packets of elementary streams with PTSs and DTSs, that refer to the system clock reference following a discontinuity, do not appear in the Transport Stream before the Transport Stream packet indicating the discontinuity, in the elementary stream carrying the program PCR, has occurred.

The semantics dealing with the Transport Stream continuity counter at a discontinuity have not been decided.

D9.	Other issues

The PSI tables allow a Transport Stream to carry programs or elementary streams with no associated PCR.

In line with 13818-2, the reference to skipped pictures has been replaced by the expression "VBV underflow".

The equations for PCR, SCR, ESCR, and OPCR have been revised to correctly express their intended meaning, though formal agreement, and inclusion in the interim CD, was not reached at the meeting.

At present only a few editorial comments made by national bodies have been incorporated in the interim CD.

D10.	Ad hoc groups

The following ad hoc group was formed to continue work towards the next meeting:

•	Ad Hoc Group to edit the Systems CD - chairman: Jan van der Meer

A meeting is planned in Atlanta, Georgia, USA, on 6 - 7th June 1994.

Reference

[1]	ISO/IEC MPEG 94/134,  "Report on ATM Network Adaptation in the H.32X terminal", ITU-TS SG15 Experts Group for Video Coding and Systems in ATM and Other Network Environments.

END