Articles > About MPEG Audio Layer
What does MP3 stand for? MP3 stands for MPEG audio Layer-3 developed by Fraunhofer Institute. And what is MPEG? MPEG is the acronym for Moving Picture Experts Group. This group has developed compression systems used for video data. For example, DVD movies, HDTV broadcasts and DSS satellite systems use MPEG compression to fit video and movie data into smaller spaces. The MPEG compression system includes a subsystem to compress sound. It describes the compression of audio signals using high performance perceptual coding schemes. It specifies a family of three audio coding schemes, simply called Layer-1, Layer-2, and Layer-3.It is this MPEG audio Layer-3 that we often called MP3.
Technical Information
Without data reduction, digital audio signals typically consist of 16 bit samples recorded at a sampling rate more than twice the actual audio bandwidth (e.g. 44.1 kHz for Compact Disks). So you end up with more than 1.400 Mbit to represent just one second of stereo music in CD quality. By using MPEG audio coding, you may shrink down the original sound data from a CD by a factor of 12, without losing sound quality. The MP3 standard divides the frequency spectrum into 576 frequency bands and compresses each band independently.
To get such reduction of the amount of data, the MP3 uses a few techniques and tricks. Some of these techniques are: -
1. The minimal audition threshold of the ear is not linear. It is represented, according to the law of Fletcher and Munson, by a curve dug between 2Khz and 5Khz. It is not therefore necessary to code sounds situated under this threshold, because they will not be perceived.
2. The masking effect: In audio, During strong sounds, you do not hear the weakest sounds. It is not therefore necessary to code all the sounds. For this, the MP3 encoder uses a psychoacoustic (the study of how the human brain perceives sound) model of the behavior of the human ear.
3. The reservoir of bytes: Often, some passages of a musical piece can not be coded to a given rate without altering the musical quality. MP3 uses a short reservoir of bytes that acts by using passages that can be coded to an inferior rate to the given flow as a buffer.
4. The Joint Stereo: Indeed under a given frequency the human ear is no longer able to locate the spatial origin of sounds. The mp3 format can therefore (in option) use such a trick by using what is called joint stereo. Some frequencies are then recorded as a monophonic signal followed by a few information in order to restore a minimum of spatialisation.
5. Huffman coding: The MP3 also uses the classic Huffman algorithm at the end of the compression to code information. This is not therefore itself a compression algorithm but rather a coding method. Huffmann coding creates variable length codes based on probability of the code. Huffman codes have the property of a unique prefix, they can therefore be decoded correctly in spite of their variable length. The decoding step is very fast (via a correspondence table). This kind of coding allows to save on the average a bit less than 20% of space.
Information on MP3 files:
An MP3 file - they use the file extension .mp3 - can also contain information about the file itself in a tag. The tag can contain things like the artist's name, a graphic (usually the CD cover art), a URL for further information, the song's lyrics, the genre, etc. MP3 files typically achieve very high rate of compression. A three minute song in a CD requires 30 megabytes of data space on your computer. Downloading MP3s takes comparatively less time because they are smaller in size. A typical 4-minute CD track in MP3 format can be stored in a file of between 3.5MB and 5MB. The corresponding reduction in bandwiddth sure helps.
How to make MP3 files :
Most MP3 files will have been produced from material originating on an audio CD. This is a two-stage process, the first involving the conversion of tracks from the CD-DA digital audio format to WAV format. This step is crucially important, and unavoidable. There are some programs that can produce an MP3 directly from CD audio, but they accomplish this by performing an audio extraction from the CD as the initial step in the process. The task is performed by specialised programs known as CD-Rippers. The CD-Ripper reads the tracks of an audio CD digitally and writes them to hard disk as .wav files. A 4-minute track occupies around 40-50MB in WAV format, so the conversion of an entire CD requires a large amount of hard disk space.
The second stage in the process is to convert the .wav files to .mp3 format. This step also involves the use of specialised software, and the programs that perform this task are known as MP3 Encoders. MP3 files can be produced using a variety of compression rates, allowing users to choose their optimal mix of quantity and quality. Typically, the following options are available:
1. 'CD quality' - compressed at 12:1 at rates of between 128Kbit/s to 192Kbit/s
2. 'Near-CD quality' - compressed at around 18:1
3. 'FM Radio Quality' (Real Audio) compressed 70:1 at a rate of 64Kbit/s.
The majority of the MP3 files available on the Internet are encoded at 44kHz and 128Kbit/s - a bitrate which results in a good quality/size ratio MP3 file. Encoding at 192Kbit/s will produce a superquality result - but at the cost of a considerably larger file size. Tracks recorded at 64Kbit/s and below are sampled at 22kHz. The reverse process - converting MP3 files to CD audio tracks - also involves two discrete stages. The decoding of an MP3 file to a WAV file is performed by a specialised program known as an MP3 Decoder. Getting the WAV file to CD is a function of the various specialised applications that exist for creating CDRs or CD-RWs.
Limitations of MP3 :
The MPEG 1 Layer 3 algorithm is based on a very complicated psycho-acoustic model. This model is based on the capability of eliminating those frequencies which the human ear is unable to hear. This compression algorithm can't be compared to ZIP because it destroys some audio parts which will never be reconstructed, that is why MP3 can't reach exact CD quality. For pure voice signals, MPEG Layer-3 is not as superior as compared to some Advanced speech coding schemes (eg. CS-ACELP [LD-CELP]).
MP3 and piracy
MP3 files are not illegal. What is illegal is taking copyrighted music from an artist's CD, encoding it in the MP3 format to make it smaller and then giving that music away for free on the Internet without the artist's permission. That violates copyright laws and also cheats the artist.
There are many legal tracks in MP3 format available freely on the Internet that have the permission of their copyright holders - mostly by unknown artists looking for free publicity. However, it is generally accepted that the vast majority of MP3 music files are illegal - they are unlicensed recordings of copyrighted work - and though there are somemajor artists who have sought to promote their music over the Internet using MP3, this has invariably been met with opposition.
Future of MP3 :
MP3 may be a phenomenon of the late 1990s, but it will almost certainly be eclipsed early in the new millennium. Since its development in 1995 a number of new formats have emerged - many of which give even better compression and comparable quality. AAC (Advanced Audio Compression), for example, can produce files that are 30% to 40% smaller than MP3 files, whilst retaining their level of quality. MPEG-2 AAC is the latest MPEG standard perceptual audio coding. MPEG-2 AAC is the Data Compression for the 21st Century and AAC shall be incorporated in MP4.
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