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By far, the most popular perceptual coders rely
upon techniques developed under the MPEG umbrella. 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 not be possible to
satisfy golden ear listeners while deleting 80% or more of the digital audio
data. In response, the MPEG (Motion Pictures Experts Group) was formed, and
since 1988 the group has been working on the standardization of high quality low
bit rate audio coding.
Three standards have been completed: MPEG-1 (coding
of mono and stereo signals at sampling rates of 32, 44.1 and 48 kHz), MPEG-2
(ISO/MPEG IS-11172: coding of 5+1 multi-channel sound signals and low bit rate
coding of mono and stereo audio at sampling rates of 16, 22.05 and 24 kHz) and
the latest standard, MPEG-4 (ISO/IEC 14496). 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.
In 1992, 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. In 1997, another algorithm, Advanced
Audio Coding (AAC) was added to the MPEG standard.
All of the MPEG codecs rely upon the celebrated acoustic
masking principle—an amazing property of the human aural perception
system. When a tone—called a masker—is presented at a particular
frequency, we are unable to perceive audio at nearby frequencies that are
sufficiently low in volume. As a result, it is not necessary to use precious
bits to encode these inaudible, masked frequencies. In perceptual coders, a
filter bank divides the audio into multiple bands. When audio in a particular
band falls below the masking threshold, few or no bits are devoted to encoding
that signal, resulting in a conservation of bits that can then be used for the
bands where they are needed.
Layer 3
Technical Description
Layer 3
(popularly known as MP3) implements a unique combination of methods to attain
high compression ratios while preserving audio quality. Telos
Systems' Layer 3 implementation was developed in close collaboration
with its developer, Fraunhofer-Gesellschaft.
Psychoacoustic
masking:
The audio in Layer 3 is divided into 576 frequency bands.
First, a polyphase filter bank performs a division into the 32
"main" bands, which correspond in frequency to those
used by Layer 2. Filters are then used to further subdivide
each main band 18 times. At the 32kHz sampling rate, the
resulting bandwidth is 27.78Hz (compared to 500Hz for Layer 2)
allowing very accurate calculation of the masking threshold
values.
Redundancy reduction:
Redundancy reduction is accomplished by a Huffman (entropy)
coding process to take advantage of the statistical properties
of the signal output from the psychoacoustic stage. This
lossless redundancy reduction process is the ideal supplement to
psychoacoustic masking. In general, maskers with high tonality
have more redundancy but allow less masking, while noise-like
signals have low redundancy and high masking effect.
Bit reservoir buffering:
Often, there are some critical parts in a piece of music that
cannot be encoded at a given data rate without
"softening" of the transients. Layer 3 uses a short
time "bit reservoir" buffer to address that need. If a
critical part occurs, the encoder can use the saved bits to code
this part with a higher data rate.
Joint-stereo: Layer 3's joint stereo mode takes advantage of the redundancy
in stereo program material. The Telos
Zephyr Xstream encoder switches from
discrete L/R to a matrixed L+R/L-R mode on a per frame (24/36ms)
basis. The intensity coding scheme used in Layer 2 codecs
combines audio above about 6kHz to mono and pans it to seven
fixed positions across the stereo stage.
Layer 2
Technical Description
The ISO/MPEG-1 Audio Layer 2 compression
algorithm, commonly called Layer 2, has become an internationally acknowledged standard for high-quality digital delivery of audio around the
world since 1992. Its audio coding scheme is used in a wide variety of broadcasting, storage, and telecommunications applications. This industry benchmark was developed by CCETT
(Centre Commun d'Etudes de Telecommunications et de Telediffusion) in Rennes, France; the IRT in Munich, Germany; and Philips in
Eindhoven, Netherlands, who solely or jointly hold patent rights on the technology.
The ISO/MPEG-1 Audio Layer 2 standard defines the bit stream syntax and the decoder specifications. The encoder's open architecture allows continuous improvements and easily accommodates application-specific requirements. Because it is a worldwide standard, licenses are issued on a nondiscriminatory and fair basis.
MUSICAM or Layer 2: What's in A Name?
The licensors, CCETT, Philips, and the IRT, initially dubbed their ISO/MPEG Layer 2
audio coding scheme MUSICAM, an acronym for Masking pattern adapted Universal Subband Integrated Coding And Multiplexing. The three used the name MUSICAM before the algorithm was selected by the ISO/MPEG as
Layer 2 of the MPEG-1 audio standard.
Throughout much of the world, the protected name MUSICAM is held by the French broadcasting organization TDF and Thomson Brandt. However, in the United States the trademark, MUSICAM, is protected by one of the licensees of
Layer 2.
To avoid confusion in the marketplace and enhance user awareness of compatibility world wide, many of the algorithm's implementers now describe the coding scheme as MPEG Audio
Layer 2 or just Layer 2.
Evidence of Layer 2's value and performance is provided by:
- The outcome of critical international listening tests conducted by prestigious organizations including ISO/IEC, ITU-R, EUREKA, CRC and the Electronics Industries Association
(EIA).
- The wide range of applications.
- The production of Layer 2 decoder chips by the world's key semiconductor companies.
The Layer 2 implementation in Zephyr Xstream
meets the MPEG audio world-wide coding standard. We have worked closely with the Institut fur
Rundfunktechnik, co-developer of Layer 2, to incorporate the latest technological improvements for superior audio quality. Our
Layer 2 is fully compatible with all other Layer 2 implementations, included those marketed as
MUSICAM.
MPEG4 AAC Technical Description
For an overview of MPEG4 AAC, please click
here.
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