INTERNATIONAL ORGANISATION FOR STANDARDISATION
ORGANISATION INTERNATIONALE DE NORMALISATION
CODING OF MOVING PICTURES AND ASSOCIATED AUDIO
ISO/IEC JTC1/SC29/WG11 N0707
25 March 1994
|Source:||Leonardo Chiariglione - Convenor|
|Status:||Approved at 26th meeting|
The 26th meeting of ISO/IEC JTC1/SC29/WG11 (MPEG) was held in Paris this week, hosted by AFNOR, the French National Body of ISO at Conservatoire National des Arts et Métiers and has produced a set of results that represents an important contribution to the establishment of interoperable digital audio-visual services and applications on a world-wide scale.
The results include the promotion of part 4 of ISO/IEC 11172-4 (MPEG-1 Conformance Testing) to the status of DIS (Draft International Standard), the promotion of ISO/IEC 11172-5 (Software for MPEG-1 coding) to CD (Committee Draft) status, the promotion of ISO/IEC 13818-2 (MPEG-2 Video) and ISO/IEC 13818-3 (MPEG-2 Audio) to DIS status, the official start of the so-called DSM-CC (Digital Storage Media Command and Control) extension for wider functionalities compared to those existing in ISO/IEC CD 13818-1 (MPEG-2 Systems), start of the 10-bit extension of MPEG-2 Video, official start of the so-called Audio NBC (non backwards compatible mode), assessment of the first MPEG-2 Video quality verification tests and preparation of the second round of video quality verification tests. Finally the activity carried out to date on the New Work Item on Very Low Bitrate Audio-Visual Coding (so-called MPEG-4) has led to the production of a first draft of the Requirements document that will be at the basis of the technical development of the new standard.
The promotion of MPEG-2 Systems to DIS status will occur at the next MPEG meeting planned to be held on 8 to 10 June. MPEG can then confirm that all 3 parts of MPEG-2 will reach IS (International Standard) status at its 29th meeting in Singapore on 7 to 11 November 1994.
ISO/IEC 11172 (a.k.a.MPEG-1)
MPEG-1 is a three-part standard (Systems, Video and Audio) that was published in August 1993. A number of products available today in the market make use of this standard. A fourth part of the standard, on conformance testing, was raised to DIS status at the Paris meeting. The purpose of the conformance testing part is the specification of methodologies for testing bitstreams and decoders for conformance with parts 1, 2 and 3. The attainment of DIS status at the Paris meeting reassures the users that the methodologies identified by ISO/IEC DIS 11172-4 can be used with high reliability.
The full software implementation of the three parts of MPEG-1 contained in part 5 of MPEG-1 is an aid to implementors and novices providing an example encoder and decoder for Systems, Video, and Audio. This software has also a wide domain of applicability thanks to the ever increasing processing capability of computers that promise to make possible software implementation of the standard under certain conditions.
The promotion of MPEG-2 to DIS for its Video and Audio part (Systems will follow in about 2 months) is a very important signal for a variety of technical and business communities which are poised to introduce this technology for a variety of applications and services.
The completion of the work related to the video part of the standard, developed jointly with ITU-T, has prompted the execution of a campaign of subjective tests of the Main Profile, thought to be the one to be used in largest scale, at the resolution of today's television. This work has been carried out in collaboration with EBU, ITU-R and SMPTE. A variety of test materials have been selected and used as input to a large number of software encoders with different degrees of optimisation. The pictures produced were subjectively assessed by over 250 observers from 3 continents.
The results proved that MPEG-2 video has the capability to comply with the identified requirements, up to the highest quality if an adequate bit rate is selected. For all test material 9 Mbit/s coding provides near transparent quality while at 6 Mbit/s the achieved quality for all test material equals or exceeds those of conventional TV systems. At lower bit rates, such as 4 Mbit/s high quality pictures will be obtained on most picture material.
Association with professional contribution links raises no significant problems.
A similar set of subjective tests were carried out to validate the current performance of the MPEG-2 multichannel audio coding standard, which is backwards-compatible with the two-channel MPEG-1 Audio coding standard. Two additional non backwards compatible codecs from AT&T and Dolby were also added to provide evidence on the need to develop an unconstrained multichannel audio coding standard.
The tests were carried out at Telekom and BBC during the period January to March 1994 to evaluate the audio quality on a representative range of test material. The loudspeaker arrangement, the listening conditions and the methodology were those recommended by the ITU-R, viz. three frontal loudspeakers and two surround loudspeakers all of the same high quality. The subjective test listeners were all experienced listeners, from professional audio production facilities and/or with prior experience of critical audition tests. More than 20 listeners took part at each test site but only one listener at a time took part in the tests and had personal control over which stimulus he/she auditioned at any instance.
The programme material was selected from 64 items provided internationally for these tests. These were pre-auditioned by a selection panel after coding and decoding to supply the necessary ten critical test items used for the tests.
At the low bitrates (320 and 384 kbit/s) selected for the tests (operation at 384 kbit/s means a compression ratio of 10) the average performance of most codecs was satisfactory, but all codecs showed deviations from transparency for some of the items.
One conclusion drawn from the tests was that, at the current state of encoder optimisation, some applications making use of the MPEG-2 Multichannel Audio coding standard may need to use a higher bitrate than used for the tests in order to reach the desired quality level. Encoder optimisation, at present in a rather immature stage, is expected subsequently to permit reduction of these bitrates with comparable quality.
Work on a number of features was started at the meeting that will complement ththose on the current draft standard. The first is the Digital Storage Media Command and Control (DSM-CC), a protocol intended to provide support to interaction between set-top terminals and video servers, interaction with business video databases, CD-based MPEG video games, and a number of others. A comprehensive set of technical requirements aimed at providing a highly functional and interoperable environment for storage and network based MPEG applications were produced and an aggressive work schedule is being pursued designed to bring the work to Working Draft level by the end of 1994 and IS status in July 1996. A general call for contributions requesting technical proposals to satisfy portions or all of the group’s requirements was issued. These contributions will be assessed and merged during the July MPEG meeting in Grimstad, Norway.
The second extension kicked-off in Paris is the support of 10-bit video coding, a higher input video sample precision required by some applications. The procedure to request the formal activation of this extension was initiated in Paris.
Non backwards compatible audio is the third extension decided in Paris. The results of the audio validation tests confirmed that backwards compatibility, an important feature in the progressive introduction of digital audio-visual services, does carry some performance penalty. Thus it was agreed that MPEG-2 will have an NBC extension. A call for proposals was issued. Submissions will be considered at the November 1994 meeting with a workplan leading to IS status in March 1997.
MPEG-4 is the work item formally approved in July 1993 which is attracting a considerable number of new members in the group . The standard will find applications in cases of very low bitrate transmission channels or reduced storage capacity. The relevant applications and operational environments are studied by a special subgroup within MPEG.In Paris a first draft of the MPEG-4 Requirements document was produced based on lists of applications and categories of requirements previously developed. This will be at the basis of the subsequent technical work on compression algorithm development.