The Test Model served as a cook book for creating bitstreams during the collobrative co-exerpimental phase of MPEG-2 video. The many documented experiments were an attempt to verify the usefulness of various proposed coding techniques. Each participant would follow the recipe given in the TM document to create an encoder. If the proposal met the criteria (coding gain, implementation complexity, robustness)--it survived. In the end, only a handful made their way into the MPEG syntax.
The last major update of the Test Model document, version 5, took place at the Sydney, Australia meeting of the MPEG working group (WG11) in March 1993. Since the MPEG-2 main profile syntax froze at this meeting, only limited scalability experiments continued past March 1993. A small two page delta document describing Temporal Scalability experiments, affectionately called "TM 6", was produced at the New York City meeting in July 1993. From then on, direct bitstream exchanges among participants of the working group helped to resolve ambiguities in the official MPEG working draft document which ultimately became the official standard document (ISO/IEC 13818-2, ITU-T H.262) we all know and love today.
The Test Model evolved in parallel with the MPEG video working draft. The TM series was a joint effort between ITU-T SG15.1 (known then as CCITT SG XV, Working Party XV/1, Experts Group on ATM Video Coding) and ISO/IEC JTC1/SC29 WG11 (MPEG).
Version Meeting location Date MPEG doc. no. ITU-T SG15 doc no. ------- ---------------- ------ ------------- ------------------ TM 0 Singapore Jan 92 [ known as Video Working Draft 0 (to keep Cesar happy) ] TM 1 Haifa, Israel Mar 92 MPEG 92/160 AVC-260 TM 2 Rio, Brazil Jul 92 MPEG 92/245 AVC-323 TM 2.2 Tarrytown, NY Oct 92 MPEG 92/535 AVC-356 TM 3 London Dec 92 WG11 92/328 AVC-400 TM 4 Rome Feb 93 MPEG 93/225b AVC-445b TM 5 Sydney Apr 93 MPEG 93/457 AVC-491 TM 5b Sydney Apr 93 WG11 93/400 AVC-491b "TM 6" New York City Jul 93 - -See Leonardo's nostalgic meeting record for a complete list of meetings.
The first drafts of the coding models inherited much of the coding methods and documentation style from previous standardization efforts.
MPEG-1: final document was SM-3 (Simulation Model version 3) H.261: final document was RM-8 (Reference Model version 8)
Sections 1-8, 10, Appendix B, C, K and L of Test Model 5.0 apply to the Main Profile syntax of today. Of course, corresponding sections in the official MPEG document (IS 13818-2 / ITU-T H.262) always supersede those in the Test Model. Section 9, Appendix D through L are mostly scalability sections, many of which no longer even apply to the scalability syntax of today.
The following is a list of comments on those sections which DO apply to Main Profile:
Although chroma subsampling is a display process which falls outside the official scope (normative) of the MPEG standard, it should be done properly for quality reasons. The Encoder's chroma decimator should anticipate the behaviour of the Decoder's chroma interpolator. The MPEG bitstream header element chroma_420_type was meant to convey which of two methods (interlaced or progressive) a decoder should use to upsample (interpolate) the decoded chroma output. By November 1994, chroma_420_type became synonymous (and therefore redundant) with the progressive_frame header element. chroma_420_type was kept in the syntax for compatibility reasons. After all, the syntax was frozen over 1 year earlier!
Test Model 5 gives two sinc conversion filters suitable for intra-field vertical chroma subsampling in interlaced video:
{-29,0,88,138,88,0,-29}/256 for field 1
{1,7,7,1}/16 for field 2
4:2:2 4:2:0
Odd
Even X
Odd
Even X
Odd
Even X
Odd
An inter-field (intra-frame) filter would be used for progressive
video sequences which renders the subsampled chroma components centered
between the spatial positions of the original components.
4:2:2 4:2:0
Odd
X
Even
Odd
X
Even
Odd
X
Even
The progressive vs. interlaced vertical chroma filter was later resolved at the November 1993 meeting in Seoul, Korea. Look at section 6.1.1.8 of the final MPEG-2 video document (IS 13818-2 / ITU-T H.262) for a description of interlaced and progressive chroma sample siting.
The official MPEG video document (IS 13818-2 section 6) does an excellent job of describing the layred structure of video data.
See the modified Chapter 5.
See the modified Chapter 6.
See the modified Chapter 7.
See the modified Chapter 8.
- An additional informative example of encoding is described in Appendix D of the MPEG-1 video document (IS 11172-2).
The following comments point out irrelevant sections with respect to today's Scalable MPEG-2 syntax:
Section 9 never discusses general or Main Profile syntax.
- frequency scalability was dropped from the MPEG toolkit.
- the syntax and semantics for scalability in the MPEG-2 video document supersede those present in this section of TM 5. The are quite different today than they were in April 1993.
See the modified Chapter 10.
See the modified Appendix A.
This section is extremely redundant with Appendix B of IS 13818-2. All the official VLC tables are listed there.
The TM only referred to the video specification Working Draft... and rightly so!
- again, frequency scalability was dropped from the MPEG toolkit.
- this experiment could be generic enough and mostly independent of scalability to apply to Main Profile, although it does refer to jettisoned items such as leaky prediction.
- leaky prediction was dropped from the MPEG toolkit.
- data partitioning is still a part of the MPEG syntax (toolkit), but is yet not enabled in any MPEG Profile.
- the weights for spatial scalablity were modified several times before the final November 1994 video standard draft.
- the only items from Appendix G which would still apply to the scalability syntax are those high-level parameters which specify bitrate, M and N factors, picture types, etc.
- low delay coding is still very much a part of the Main Profile syntax, though is not commonly used. See section D.5 in the MPEG-2 video document for a description.
- again, leaky prediction is not a part of the MPEG toolkit.
- again, frequency scalability was dropped from the MPEG toolkit.
- again, no "Unified Field Coding" for scalability was reached. Maybe MPEG-4 will do this. (supressing urge to comment)
- intra_slice is still a legitimate trick mode technique (and about the only one viable today).
- again, data partition is still a part of the MPEG toolkit (cf section 7.10 in IS 13818-2 / ITU-T H.262), but yet not enabled by any Profile and/or Level combination. This section does however demonstrate one of Data Partitioning's primary intended uses.
- see comments above
- syntax for data partitioning has changed. In particular, there is no data_partitioning_flag, intra_pbp in the sequence and picture headers, respectively. Instead, Data partitioning mode would be signaled by the scalable_mode element (if a tag were defined for it). The pbp element in Appendix L of the TM 5 document is now called priority_breakpoint in the MPEG-2 video document.
- The 8x1 DCT mode was an attempt to solve the ringing artifacts present around "text" and other sharp edges within reconstructed pictures. It is not a part of the MPEG toolkit. Careful coding using existing syntax tools (macroblock quantizer_scale) is recommended.