What is Digital Compression ?

Audio Compact Disks store the audio data in files on the disk. The audio data is in a PCM (Pulse Code Modulation) format. Each minute of recording time consumes about 9 MB (Megabytes) of file storage space. A three minute song would occupy about 27 MB of file storage space, and a 5 minute song would occupy about 45 MB of file storage space. Not really that bad, a 650 Megabyte Compact disc can contain up to 74 minutes of PCM audio.

With file space at a premium and very slow modems the norm, the sheer size of PCM audio data files made them un popular as a storage medium for audio on a computer. Digital compression came along and changed all that. Digital compression offered a huge reduction in the amount of file space required to store audio data files. This reduction in file size also reduced the time required to transmit them electronically dramatically. 

Audio compression can be one of two categories, lossless or lossy. Lossless digital compression is commonly used to reduce the size of computer files for electronic transmission. In order for the files to be useable on a computer the files that are extracted from a compressed data file must be identical to the original file (before it was compressed). Lossless compression is great because it makes perfect copies but it doesn't yield very high compression ratios. That means it doesn't save huge amounts of disk storage space. ZIP, ARC, TAR, and  SIT are some of the acronyms or formats of Lossless Compression commonly used on computers.

Lossy compression algorithms offers much higher compression ratios than lossless algorithms but in order to achieve this they need to discard some of the original data. Lossy compression is only suitable for use on audio or graphical data. The audio or graphics are reproduced but at a lower overall quality than they had before they were compressed. In some cases the difference is difficult to perceive. The compression ratio can usually be adjusted so the quality level can vary widely. Audio that is compressed at a 20:1 or 10:1 ratio will certainly sound inferior to audio that was compressed at a 2:1 ratio. MPEG, MP3, AAC, RA, WMF, JPEG, QT, and DivX are some of the acronyms or formats of Lossy compression commonly used for audio and video.

With the increase in inexpensive mass storage and the increases in connection speeds we are rapidly approaching a point in time when compression of audio may not be as important of an issue as it was in the past. Lossless compressors specifically for digital audio provide the best of both worlds, a reduction in the amount of file storage space as well as no degradation in the quality of the audio or video data. One example is the totally lossless .shn audio format.  The biggest proponent of Lossless Compression is Etree.org a music distribution hub but not for audio that has been compressed in a lossy fashion like MP3 or AAC but for audio that has been compressed in a lossless way. Etree.org uses the Shorten (.shn) audio compression format because it creates an exact clone of the original source.