Many of us have downloaded mp3 or iTune tracks off of the internet. Many of us have streamed audio from the internet. Most of us have experienced the power of Dolby Sound either at home or at the theatre. The common enabling thread of these and numerous other technologies that we unknowingly encounter on a daily basis is digital data reduction or compression. Few of us really understand the underlying principles that enable these technologies that many of us have so fully embraced. At Electronic Musician there is a great article on the subject of Digital Data Compression. The wide variety of compression standards presents a veritable alphabet soup of acronyms and trade names, Dolby AC-3, Sony Atrac, Roland R-DAC, DTS, MP3, MP4, etc. So what are they and why are they used. Fidelity in digital audio is a function of the sampling rate and resolution of the digitization process. However, as sampling rate and resolution increase the data rate and storage space also increase. Transmission of the highest quality audio without compression would require massive amounts of bandwidth. Likewise uncompressed 5.1 channel audio (or video) would quickly outstrip the capacity of a CD or DVD. Compression reduces the bandwith required to transmit (stream) the music data and the storage space required to contain it. That is why your little MP3 player can hold such vast archives of music.

Compression comes in two basic flavors, lossless and lossy. Lossless compression applies a mathematical algorithm that reduces the total amount of data in such a fashion that the process can be reversed and an exact bit for bit copy of the original uncompressed Data can be retrieved. Techniques such as Roland's R-DAC roughly triples the data that can be stored in their VS recorders compared to storing uncompressed audio data with no loss in audio quality. A 3:1 compression ratio is just about the limit for a lossless compression algorithm.

To achieve higher compression ratios one must resort to a lossy compression algorithm. These algorithms are not completely reversible. The inverse algorithm does not produce an exact copy of the original data. These algorithms are all based on the perceived audio quality of the recovered data. The question becomes, how much can you squeeze the data and still have it sound good on playback. Some compression algorithms even allow you to make a choice about quality versus size at encoding time. MP3 gives you a choice of effective bit rates when you encode your music. The higher the bit rate the bigger the file and the better the sound quality.

Many of these compression algorithms are patented and proprietary and and are used by license. This includes the various multichannel encoding schemes that are used for the sound tracks to movies and DVD's. For more specifics and a more en depth look at the technology of data compression visit this article at Electronic Musician.