Prior to the development of compact discs and digital files, analog audio cassettes were the most common format used for music and other audio distribution, including audiobooks. Even after more advanced technologies became available, audio cassettes remained a popular format for "books on tape." Cassette manufacturers have produced tapes capable of holding anywhere from 15 minutes to an hour of recorded material. A cassette comprises the tape itself and a separate housing that protects and contains it.
One of the most prolific formats used for both music and audiobooks, compact discs (commonly referred to as CDs) are capable of holding a significantly greater amount of recorded audio than the cassettes that preceded them. CDs are round discs a few inches in diameter made from a synthetic material called polycarbonate. In addition to capacity, the other notable difference between CDs and cassettes is the form of the recorded information. Unlike cassettes, CDs record and play back audio as digital data.
A compact disc that has been recorded with files in the MP3 format is called an MP3-CD. Recording a CD using MP3 files allows the disc to hold nearly ten times the amount of audio that it would otherwise. Most CD players were being manufactured to support and play MP3 formatted CDs by the mid-2000s. As music became available on MP3-CDs, so did audiobooks. The increased data storage capacity allowed longer books to be recorded on fewer discs, saving money and materials.
Audiobooks have also become available for digital download direct from the retailer to the customer's computer or other media device, such as an MP3 player. Digital downloads forgo the use of any type of physical packaging and provide nothing other than the audio data itself. The main advantage of downloading audiobooks is the speed and ease with which it can be done. Downloading can be done instantly on the Internet, whereas CDs and cassettes must be purchased in person or shipped through the mail.