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MPEG Around the time the first CDs were being shipped, proposals for what has become modern audio coding were greeted with suspicion and disbelief. There was widespread agreement that it would simply not be possible to satisfy ?golden ears? with only around 10% of the original digital audio data. Furthermore, MPEG ? Moving Pictures Experts Group, true to its name, was focused almost exclusively on video compression projects. But the audio coding pioneers were persistent and an audio group was formed within MPEG. Since 1988, they have been working on the standardization of high quality audio coding. Today almost all agree not only that audio bitrate reduction is effective and useful, but that the MPEG process has been successful at picking the best technology and encouraging compatibility across a wide variety of equipment. Researchers who have decided to work within MPEG are dedicated to creating standard, widely usable, top-quality audio and video encoders and decoders, preempting what may become an unmanageable tangle of formats. It seems to be working. Despite persistent attempts to lock users into proprietary schemes, by far the most popular high fidelity audio codecs are developed and offered as standard under the MPEG umbrella. The main reason MPEG has been effective in finding the best technology is that the process is open and competitive. A committee of industry representatives and researchers meet to determine goals for target bitrate, quality levels, application areas, testing procedures, etc. Interested developers that have something to contribute are invited to submit their best work. A careful double-blind listening test series is then conducted to determine which of the entrant's technologies delivers the highest performance. The subjective listening evaluations are carried out at various volunteer organizations around the world that have access to both experienced and inexperienced test subjects. Broadcasters are the common participants, with many of the important test series conducted at the BBC in England, the CBC and CRC (Communications Research Centre) in Canada, and NHK in Japan. In 1992, under MPEG-1, this process resulted in the selection of three related audio coding methods, each targeted to different bitrates and applications. These are the famous layers: 1, 2 and 3. As the layer number goes up, so does performance and implementation complexity. Layer 1 is not much used. Layer 2 is widely used for DAB in Europe, audio for video, and broadcast delivery systems. Layer 3 is widely used in broadcast codecs and has gone on to significant Internet and consumer electronics fame under the name derived from the file extension, MP3. (Forgive, please, a moment?s lapse of modesty. Telos was the first to license and use MP3 commercially and some have credited (or blamed) us with getting the whole MP3 thing rolling through our promotion and our posting of an early PC-based player on the Zephyr website. At one time, we were getting more than 10,000 downloads per day!) MPEG-2 opened the door for new work, and some minor enhancements were added to both Layers 2 and 3. In 1997, the first in the AAC family was added to the MPEG-2 standard. MPEG-4 audio, finalized in late 1999, makes some enhancements to AAC and adds the new AAC-LD codec. MPEG-7 work is underway now. MPEGs-3, 5, and 6 have been skipped for rather strange reasons.1 There has been a lot of confusion regarding the naming of MPEG codecs. For one, the word layer probably made sense to the developers because the codecs under MPEG-1 & 2 are layered in the sense that the higher-numbered layers build upon the previous ones. But to users, the naming is certainly a little strange ? perhaps levels would have been better. And then there is the confusion resulting from the conflation of MPEG-2 with Layer 2. All of the layers are subsets of MPEG-1 & 2, so the full correct names are MPEG-2 Layer 2 audio and MPEG-2 Layer 3 audio. The latter is the same as MP3. Already, some people are referring to MPEG-2 AAC as MP4. Guess the logic here is that it is the next step up for Internet audio from MP3, and it is part of MPEG-4? 1 Many people have wondered about the strange numbering system of MPEG standards: 1, 2, 4, and, on the horizon, 7. Here is the story. Work was begun on an MPEG-3 standard for high-definition television, but it became clear that the tools needed were very similar to those in MPEG-2, so MPEG-3 was quickly abandoned, and HDTV support was included in MPEG-2. When the latest work item was started, the first question taken up was what number to use. One participant recalled that the conversation was something like, "Shall the number for the next job be 5, which follows 4, or should it be 8, attractive in its own binary way, to follow 1, 2 and 4? After some thought, MPEG members decided that their new work item was so different from what had gone before that they threw both ideas overboard and chose 7 as the lucky number.? | |
