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~ MP3 FAQ

  • History. In 1987, the IIS started to work on perceptual audio coding in the framework of the EUREKA project EU147, Digital Audio Broadcasting (DAB). In a joint cooperation with the University of Erlangen (Prof. Dieter Seitzer), the IIS finally devised a very powerful algorithm that is standardized as ISO-MPEG Audio Layer-3 (IS 11172-3 and IS 13818-3).
    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. Factors of 24 and even more still maintain a sound quality that is significantly better than what you get by just reducing the sampling rate and the resolution of your samples. Basically, this is realized by perceptual coding techniques addressing the perception of sound waves by the human ear.

    Using MPEG audio, one may achieve a typical data reduction of

1:4

by Layer 1 (corresponds with 384 kbps for a stereo signal),

1:6...1:8

by Layer 2 (corresponds with 256..192 kbps for a stereo signal),

1:10...1:12

by Layer 3 (corresponds with 128..112 kbps for a stereo signal),

still maintaining the original CD sound quality.

By exploiting stereo effects and by limiting the audio bandwidth, the coding schemes may achieve an acceptable sound quality at even lower bitrates. MPEG Layer-3 is the most powerful member of the MPEG audio coding family. For a given sound quality level, it requires the lowest bitrate - or for a given bitrate, it achieves the highest sound quality.

  • Sound Quality. Some typical performance data of MPEG Layer-3 are:

    sound quality bandwidth mode bitrate reduction ratio
    telephone sound

    2.5 kHz

    mono

    8 kbps *

    96:1

    better than shortwave

    4.5 kHz

    mono

    16 kbps

    48:1

    better than AM radio

    7.5 kHz

    mono

    32 kbps

    24:1

    similar to FM radio

    11 kHz

    stereo

    56...64 kbps

    26...24:1

    near-CD

    15 kHz

    stereo

    96 kbps

    16:1

    CD

    >15 kHz

    stereo

    112..128kbps

    14..12:1

    In all international listening tests, MPEG Layer-3 impressively proved its superior performance, maintaining the original sound quality at a data reduction of 1:12 (around 64 kbit/s per audio channel). If applications may tolerate a limited bandwidth of around 10 kHz, a reasonable sound quality for stereo signals can be achieved even at a reduction of 1:24.

    For the use of low bit-rate audio coding schemes in broadcast applications at bitrates of 60 kbit/s per audio channel, the ITU-R recommends MPEG Layer-3. (ITU-R doc. BS.1115)

  • What is mp3? MP3 stands for MPEG (Moving Pictures Experts Group) layer 3, which is a standard format for storing compressed music. The basic idea is that music digitized at CD quality has information that can be suppressed with little or no quality loss, as perceived by most people. Thanks to this and some mathematics, it is possible to compress a digitized track to about 8 percent of its original size, or a compression ratio of 12:1. MPEG has created its own homepage, providing information on the what, where, when and how of the standards.

    This means that, while a regular music CD can contain about one hour of music (74 minutes, actually), a similar CD used to store mp3 files can contain almost 12 hours of music.

    Until a couple of years ago, the MP3 format was not very interesting for regular users, since there was no affordable hardware to decompress and play MP3 music on the fly. With the arrival of Pentium processors, this changed completely. Today, inexpensive machines with successors of the Pentium processor can easily play MP3 files and still leave enough processing power for the user to perform regular tasks with their computer. Many people who spend several hours a day in front of a computer can also use it as a sound system.

  • Are MP3 files illegal? Mp3 files themselves are not illegal. You can sing "Happy Birthday", record it as a .wav file and then convert it to the mp3 format. The problem starts when you cut a track from an audio CD. For all we know, if you keep it to yourself and do not trade it with other people, it is not likely that anyone will prosecute you. If the music on the CD is copyrighted (most of it is) and you distribute it, in most countries you are violating copyright laws if you don't have express permission from the copyright owner.

  • How do I listen to mp3 files? Get a program to play mp3 files. If you are running Windows, I recommend Winamp. Of course, you need to have a sound card and speakers or headphones!


    • To download files its better to use special program, which could resume stopped downloads (what happens often), the examples of such program is GetRight.

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