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MP3 is a compression format. It provides a representation of
pulse-code modulation-encoded (PCM) audio data in a much smaller size
by discarding portions that are considered less important to human
hearing (similar to JPEG, a lossy compression for images).
A number of techniques are employed in MP3 to determine which portions
of the audio can be discarded, including psychoacoustics. MP3 audio
can be compressed with different bit rates, providing a range of
tradeoffs between data size and sound quality.
The MP3 format uses a hybrid transformation to transform a time domain
signal into a frequency domain signal: 32-band polyphase quadrature
filter
36 or 12 tap MDCT; size can be selected independent for sub-band 0...1
and 2...31
Aliasing reduction postprocessing
In terms of the MPEG specifications, AAC (Advanced audio coding) from
MPEG-4 is to be the successor of the MP3 format, although there has
been a significant movement to create and popularize other audio
formats. Nevertheless, any succession is not likely to happen for a
significant amount of time due to MP3's overwhelming popularity. MP3
enjoys extremely wide popularity and support, not just by end-users
and software but by hardware such as portable media players (referred
to as MP3 players) DVD and CD players.
Development
MPEG-1 Audio Layer 2 encoding began as the Digital Audio Broadcast
(DAB) project managed by Egon Meier-Engelen of the Deutsche Forschungs-
und Versuchsanstalt f?t- und Raumfahrt (later on called Deutsches
Zentrum f?t- und Raumfahrt, German Aerospace Center) in Germany. This
project was financed by the European Union as a part of the EUREKA
research program where it was commonly known as EU-147. EU-147 ran
from 1987 to 1994.
In 1991, there were two proposals available: Musicam (known as Layer
2), and ASPEC (Adaptive Spectral Perceptual Entropy Coding). The
Musicam technique, as proposed by Philips (The Netherlands), CCETT
(France) and Institut f?dfunktechnik (Germany) was chosen due to its
simplicity and error robustness, as well as its low computational
power associated to the encoding of high quality compressed audio. The
Musicam format, based on sub-band encoding, was a key to settle the
basis of the MPEG Audio compression format (sampling rates, structure
of frames, headers, number of samples per frame). Its technology and
ideas were fully incorporated into the definition of ISO MPEG Audio
Layer I and Layer II and further on of the Layer III (MP3) format.
Under the chairmanship of Professor Mussmann (University of Hannover)
the editing of the standard was made under the responsibilities of
Leon van de Kerkhof (Layer I) and Gerhard Stoll (Layer II).
A working group consisting of Leon Van de Kerkhof (The Netherlands),
Gerhard Stoll (Germany), Yves-Fran篩s Dehery (France), Karlheinz
Brandenburg (Germany) took ideas from Musicam and ASPEC, added some of
their own ideas and created MP3, which was designed to achieve the
same quality at 128 kbit/s as MP2 at 192 kbit/s.
All algorithms were approved in 1991, finalized in 1992 as part of
MPEG-1, the first standard suite by MPEG, which resulted in the
international standard ISO/IEC 11172-3, published in 1993. Further
work on MPEG audio was finalized in 1994 as part of the second suite
of MPEG standards, MPEG-2, more formally known as international
standard ISO/IEC 13818-3, originally published in 1995.
Compression efficiency of encoders is typically defined by the bit
rate because compression rate depends on the bit depth and sampling
rate of the input signal. Nevertheless, there are often published
compression rates that use the CD parameters as references (44.1 kHz,
2 channels at 16 bits per channel or 2x16 bit). Sometimes the Digital
Audio Tape (DAT) SP parameters are used (48 kHz, 2x16 bit).
Compression ratios with this reference are higher, which demonstrates
the problem of the term compression ratio for lossy encoders.
Karlheinz Brandenburg used a CD recording of Suzanne Vega's song Tom's
Diner to assess the MP3 compression algorithm. This song was chosen
because of its softness and simplicity, making it easier to hear
imperfections in the compression format during playbacks. Some have
taken to jokingly refer to Suzanne Vega as "The mother of MP3". Some
more serious and critical audio excerpts (glockenspiel, triangle,
accordion, ...) were taken from the EBU V3/SQAM reference compact disc
and have been used by professional sound engineers to assess the
subjective quality of the MPEG Audio formats.
MP3 goes public
A reference simulation software written in C language known as ISO
11172-5 was developed by the members of the ISO MPEG Audio committee
in order to produce bit compliant MPEG Audio files (Layer 1, Layer 2,
Layer 3). Working in non real time on a number of operating systems it
was able to demonstrate the first real time hardware decoding (DSP
based) of compressed audio. Some other real time implementation of
MPEG Audio encoders were available for the purpose of digital
broadcasting (radio DAB, television DVB) towards consumer receivers
and set top boxes.
Later on, on July 7, 1994 the Fraunhofer Society released the first
software MP3 encoder called l3enc. The filename extension .mp3 was
chosen by the Fraunhofer team on July 14, 1995 (previously, the files
had been named .bit). With the first real-time software MP3 player
Winplay3 (released September 9th, 1995) many people were able to
encode and playback MP3 files on their PCs. Because of the relatively
small hard drives back in that time (c.500 MB) the technology was
essential to store music for listening pleasure on a computer.
