Introduction - or: What is "MPEG Audio Layer-3"?

Today, efficient coding techniques are a must for cost-effective processing of digital audio and video data by computers. Data reduction of moving pictures and sound is a key technology for any application with limited transmission or storage capacity. In the recent years, a lot of progress has been achieved. While there (still) exist several proprietary formats for audio and video coding, the ISO/IEC standardisation body has released an international standard ("MPEG") for powerful audio and video coding tools.

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 1400 kbit 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

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. 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.