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.
