What is an MP3???

MP3 is one of the best, easiest and most popular way to compress audio data for your computer or portable player such as Rio, Nomad, etc. MP3 is a simple word that is now well known around the world, in fact MPEG-1 Layer 3 was reduced to MP3 due to file extensions that are mostly three characters (Layer 1 and 2 were called mp1; mp2). Most of the time, MP3 compression is used to reduce dramatically the size of CD tracks for backup purposes.

The MPEG 1 Layer 3 algorithm is based on a very complicated psycho-acoustic model. This model is based on the capability of eliminating those frequencies which the human ear is unable to hear. This compression algorithm can't be compared to ZIP because it destructs some audio parts which will never be reconstructed, that's why MP3 can't reach exact CD quality.

In fact, according to Fraunhofer institute, "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)." This same institute has devised MP3 algorythm that was previously standardized by ISO (ISO 11172-3 and 13818-3) and developed a non-ISO extension "MPEG 2.5".  1:12 means that you can get a file around 12 times smaller than uncompressed CD quality without major noticeable difference for your ears.

MP3 are divided into two bitrate categories: constant and variable (aka VBR; variable bitrate). Using constant bitrate, it's easy to predict the final file length. For example, a 60 minutes encoded at 128kbps would take about 57mb. Xing introduced variable bitrate not so long ago which is a part of the ISO standard, but wasn't used by any other encoder. The only way to know how much space your file will take with VBR is to encode it and wait for the final result.