History
In 1987, the Fraunhofer 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
Fraunhofer 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 Discs). 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 to 384 kbps for a stereo signal), |
1:6...1:8 |
by Layer 2 (corresponds to 256..192 kbps for a stereo signal), |
1:10...1:12 |
by Layer 3 (corresponds to 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 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 |
*) Fraunhofer IIS uses a non-ISO extension of MPEG Layer-3
for enhanced performance ("MPEG 2.5")
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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)
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