Digital Audio Compression is a form of data encoding used in audio industry. The purpose of audio compression is to deliver smaller audio files. This is achieved by encoding raw audio data at lower bit rates, the lower the bit rates the smaller the audio file. The trade-off for smaller audio files is lower audio quality. There are two techniques of audio compression, Lossless and Lossy.

Lossless audio compression retains the same sound quality as the original file but achieves far lower compression ratios than the lossy audio compression technique. This results too much larger audio files and therefore is mostly used for professional use.

Lossy audio compression eliminates the sounds that the human ear can not recognize. The disadvantage of using these formats is that they lead to lower audio quality, still they are widely used by digital audio players since they can deliver much smaller audio files. The most common lossy audio compression formats are the MP3, AAC and WMA.

Each one of the above competitive formats can deliver equal audio quality but at different bit rates. The audio quality for a compressed audio file is determined by two factors, the audio format and its bit rate. Below it is described the bit rate at which each audio format can achieve near CD quality sound, standard for each audio format, and real CD quality sound.

Audio format	Standard Kbps	CD quality Kbps
MP3	128	256
AAC	96	192
WMA	64	128

Below is showed how many MB is needed to compress 1 minute of music at different bit rates. The increase of the file’s size is proportional to the increase of the bit rate, if the bit rate doubles the file’s size will also double.

Bit rate (Kbps)	File size (MB per minute of music)
256	2
192	1.4
128 (MP3)	1
96 (AAC)	0.7
64 (WMA)	0.5

Generally speaking 1MB equals to 1 minute of music encoded at 128 Kbps, which is the MP3’s standard bit rate. It is important to notice that a song based on the WMA format needs 50 % less storage capacity, than one based on the MP3 format, and still attain equal audio quality. Or the WMA format can use the regular bit rate of the MP3 format, 128 Kbps, for achieving CD quality sound.

Another way for comparing the above audio formats is by audio compression ratio. It indicates how many times an original audio file from an audio CD, with a standard bit rate of 1411 Kbps, can be compressed. The below listed audio compression formats deliver equal sound quality, near CD quality, through different compression ratios.

Format	Bit Rate	Compression Ratio
MP3	128	11
AAC	96	15
WMA	64	22

The higher the compression ratio the better, since it delivers smaller audio files. A table over the number of songs, music minutes and the number of CDs that each memory card capacity can hold, for each of the above audio formats, can be found at digital audio storage capacity.

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