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. 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 bit rates. MPEG Layer-3 is the most
powerful member of the MPEG audio coding family. For a given sound quality level,
it requires the lowest bit rate - or for a given bit rate, it achieves the highest
sound quality.
Sound Quality
Some typical performance
data of MPEG Layer-3 are: 2.5 kHz mono 8 kbps * 96:1
better than, short wave:
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)