Telos Zephyr Xstream - Manual (Page 126)

USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 114
MPEG
By far, the most popular high fidelity audio coders rely upon techniques developed under the
MPEG umbrella. MPEG stands for Motion Pictures Expert Group, a Joint Committee of the
International Standards Organisation (ISO) and the International Electrotechnical Commission.
Over a decade ago, when the CD had just been introduced, the first proposals for audio coding
were greeted with suspicion and disbelief. There was widespread agreement that it would
simply not be possible to satisfy golden ears while deleting 80% or more of the digital audio
data. But the audio coding pioneers were persistent and the MPEG audio group was formed.
Since 1988, they have been working on the standardization of high quality audio coding. Today,
almost all agree not only that audio bit rate 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.
The MPEG process is open and competitive. A committee of industry representatives and
researchers meet to determine goals for target bit rate, quality levels, application areas, testing
procedures, etc. Interested organizations 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 done at various volunteer organizations around the world that have
access to both experienced and inexperienced test subjects. Broadcasters are the most
common participants with many of the important test series conducted at the BBC in England,
the CBC in Canada, and NHK in Japan. Finally, results are tabulated, a report is drafted and
ultimately a standard is issued.
In 1992, under MPEG1 (the first of the MPEG standards), this process resulted in the selection of
three related audio coding methods, each targeted to different bit rates 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 playout systems. Layer‐3 – which Telos was the first to use in the
Zephyr – is widely used in broadcast codecs and has gone on to significant Internet and
consumer electronics fame under the moniker derived from the file extension: MP3. MPEG2
opened the door for new work, and some minor improvements were added to both Layers 2
and 3. In 1997, the first in the AAC family was added to the MPEG2 standard. MPEG4 is ongoing
now, but it has already been decided that AAC will be the “general audio” coder under this
umbrella. (MPEG3 was skipped for reasons unknown.)
MPEG2 AAC (Advanced Audio Coding)
The MPEG2 AAC system is the newest audio coding method selected by MPEG and became an
International standard in April 1997. It is a fully state‐of‐the‐art audio compression tool that
provides performance superior to any known approach at bit rates greater than 64 kbps and
excellent performance relative to the alternatives at bit rates reaching as low as 16 kbps.
The idea that led to AAC was not only to start fresh, but also to combine the best work from the
world’s leading audio coding laboratories. Fraunhofer, Dolby, Sony, and AT&T were the primary
collaborators that offered components for AAC. The hoped for result was ITU (International
Tele‐communications Union) “indistinguishable quality “at 64 kbps per mono channel. That is,