R&D Information
International Standardization of Lossless Coding
Technology for Audio Signals

A specification for lossless coding technology foraudio signals was officially approved as a standard
called MPEG-4ALS* at the end of 2005 (ALS: audio
lossless coding). MPEG (moving picture experts
group) is a working group of ISO/IEC (InternationalStandards Organization, International Electrotechnical Commission). NTT Communication Science Laboratories made a large contribution in MPEG toward
the standardization of this technology, which
includes a number of elementary technologies proposed by NTT.
Some standard audio coding schemes such as MP3and AAC (advanced audio coding) already exist andare in use. They are all based on perceptual coding,which offers a high compression ratio in exchange forminor waveform distortion at the decoder. That is, thewaveform is different from the original, even though
it is carefully controlled and perceptually very close
to the original. In contrast, lossless coding enables
perfect reconstruction of the waveform. This is very
important for applications such as waveform editingor the storage of high-quality audio signals with, forexample, high sampling rates, high word resolution,and multiple channels. The cost of perfect reconstruction is a limited compression ratio: the compressed file size varies from 15 to 70% of the original
depending on the statistical properties of the original
waveform. Nevertheless, the compression performance of MPEG-4ALS, exceeds that of ZIP, a general-purpose lossless compression tool, and the otheravailable compression tools for audio signals. Thus,
it offers a wide range of flexibility in selecting theoperation mode at the encoder, such as a very fastencoding mode with lower compression performanceor a very high compression mode at the cost of slow
encoding and decoding.
MPEG-4ALS accepts a wide range of input for* The official name is MPEG-4 audio 3rd edition amendment 2, but it
is usually called ALS.

Vol. 4 No. 4 Apr. 2006

mats:
• Sampling rates of up to 192 kHz (44.1 kHz for acompact disc (CD))• Various integer PCM (pulse code modulation)formats up to 32 bits per sample (16 bits forCD)• 32-bit floating point data in the IEEE754 format(integer for CD)• Up to 65,536 channels (2 channels for CD).
Moreover, it is very useful for downloading and storing compressed files because file compression
enables significantly smaller archive files and shorter
downloading times and the decoding is generally
very fast, at least ten times faster than the playback
time of the music.
NTT developed and contributed several elementary
technologies used in the standardized specification.
For example, the time domain linear prediction isbased on PARCOR coefficients invented by NTT
while the multi-channel coding and long-term prediction were developed in collaborative work between
NTT and the University of Tokyo.
NTT Communication Science Laboratories willcontinue to support the standardization of the conformance and reference software and the enhancementof the encoder performance, and NTT Communications will design and provide integrated delivery andarchiving systems using practical software that complies with the standard. NTT group companies will
also work with partners or license others to produce
applications, including professional audio editing
tools.
For further information, please contact:
E. Tamechika or H. Motai
Planning Division
NTT Science and Core Technology Laboratory
Group
Tel: 046-240-5152
http://www.ntt.co.jp/sclab/contact_e/55