Overview about the ISO-MPEG Standard - or: What is MPEG all about?

Q: What is "MPEG"?
A: MPEG is the "Moving Picture Experts Group", working under the joint direction of the International Standards Organization (ISO) and the International Electro-Technical Commission (IEC). This group works on standards for the coding of moving pictures and audio. MPEG has created its own homepage, providing information on the what, where, when and how of the standards.


Q: What is MPEG-1, -2, and so on?
A: MPEG approaches the growing need for multimedia standards step-by-step. Today, three main "steps" are defined (MPEG-1, MPEG-2, MPEG-4). MPEG-1: "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s" MPEG-2: "Generic Coding of Moving Pictures and Associated Audio Information" MPEG-3: originally planned mainly for HDTV applications; later on, it was merged into MPEG-2 MPEG-4: "Coding of Audio-Visual Objects"


Q: Are MPEG-3 and Layer-3 the same thing?
A: No! Layer-3 is a powerful audio coding scheme which certainly is part of the MPEG standard. Layer-3 is defined within the audio part of both existing international standards, MPEG-1 and MPEG-2. So please do not mix audio layers and MPEG standards!


Q: What is the status of MPEG-1?
A: Work on MPEG-1 is finished. The first three parts are standardized since 1992. MPEG-1 consists of five parts: IS-11172-1 ("System") describes synchronization and multiplexing of video and audio signals. IS-11172-2 ("Video") describes compression of video signals, focussing on progressive scan video (and mainly aiming at "Video-on-CD" applications). IS-11172-3 ("Audio") describes a generic audio coding family, with three hierarchically compatible members (called "Layer-1", "Layer-2" and "Layer-3"). IS-11172-4 ("Compliance Testing") describes procedures for determining the characteristics of coded bitstreams and the decoding process and for testing compliance with the requirements stated in the other parts. DTR-11172-5 ("Software Simulation") is a technical report about a full software implementation of the first three parts of MPEG-1.


Q: What is the status of MPEG-2?
A: MPEG-2 currently consists of nine parts. The first three parts are standardized since 1994, with some amendments included later on. Other parts are at different levels of completion. IS-13818-1 ("System") describes synchronization and multiplexing of video and audio signals; it is also standardised by ITU-T as H.222. IS-13818-2 ("Video") describes a generic video coding tool set, supporting interlaced scan; it is also standardised by ITU-T as H.262. IS-13818-3 ("Audio") describes a backward compatible extension of MPEG-1 for multichannel audio coding ("surround sound", "multilingual sound") and a non-backward compatible extension to lower sample rates, to support sound applications with limited audio bandwidth requirements. IS-13818-4 ("Conformance Testing") describes procedures for determining the characteristics of coded bitstreams and the decoding process and for testing compliance with the requirements stated in the other parts. DTR-13818-5 ("Software Simulation") is a technical report about a full software implementation of the first three parts of MPEG-2. IS-13818-6 ("System Extensions - Digital Storage Media Command and Control (DSM-CC))" describes a set of protocols for client-server applications CD-13818-7 ("Audio, Non-Backwards-Compatible (NBC) - Coding") describes an improved audio coding scheme for mono- and stereophonic signals as well as for multichannel sound 13818-8 ("Video, extension to 10-bit input samples") has been withdrawn, due to insufficient interest. IS-13818-9 ("Real-Time Interface Specification for Low-Jitter Applications") defines timing constraints on the real-time delivery of MPEG-2 transport bitstreams. WD-13818-10 ("Conformance Extensions - DSM-CC") describes the addendum to IS 13818-4 for DSM-CC


Q: "NBC audio"?" What is the motivation for this working group? What are the results?
A: Well, during the work for multichannel audio coding (IS-13818-3), it turned out that backwards compatible (BC) schemes suffer from the matrixing process. Matrixing is required to allow a MPEG-1 decoder to playback all surround channels via its two stereophonic channels. Unfortunately, some of the introduced quantisation noise may become audible after dematrixing. All in all, during an ISO listening test in spring 1994, BC multichannel coding performed poorer, compared to non-ISO coding schemes (e.g., Dolby�s AC-3). So the NBC working group currently develops a new audio coding scheme. NBC audio achieves a significant better performance, not only for multichannel surround sound, but even for monophonic signals (here targeting "true transparency" at 64 kbps). In spring 1996, ISO performed a listening test for 5-channel surround sound, and NBC audio using a total bit-rate of 320 kbps scored better than Layer-2 BC at a bit-rate of 640 kbps. NBC audio will also become one of the MPEG-4 audio coding algorithms.


Q: How do I get the MPEG documents?
A: Well, you may contact ISO, or you order it from your national standards body. E.g., in Germany, please contact DIN.


Q: Is some public C source available?
A: Well, there is "public C source" available on various sites, e.g. at ftp://ftp.fhg.de/pub/layer3/ or at ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg2/public_software/ . This code has been written mainly for explanation purposes, so do not expect too much performance.