Some digital audio formats are open, which means that the specifications of the format--how the data is structured, the algorithms used to encode the data--are freely available, and use of the format is free of legal restrictions. Usually open formats are maintained by a national or international standards organization. Advocates argue that use of open formats will help guarantee long-term access to data and encourage cooperative development of the formats.
Other formats are proprietary; for these, a private concern--usually a commercial enterprise--maintains control over the format and the release of details on its structure, encoding, and decoding. In many cases, the owner of the format will release information on the structure of file and how it is encoded but retain rights over the decoding algorithm. Owners of proprietary formats are interested in promoting use of their format and often take actions to discourage the use of competing formats.
Some proprietary formats are actually based on open formats. Apple, for example, sells tracks on its iTunes Music Store in a proprietary format that uses AAC-encoded audio (an open format) with a proprietary digital rights management wrapper that restricts use of the file.
Many popular, well-established formats are proprietary, and librarians often choose to base their digital audio services on proprietary formats because they are familiar to patrons, and software to play back the files is readily available--sometimes even packaged with the computer's operating system. There are some risks, however, in basing audio services on proprietary formats. Support can be very good until the sponsoring company abandons or alters the format. Companies often promote their own proprietary audio formats to the detriment of others with the hope of securing a greater market share, and they make adoption of their format attractive by offering convenient tools for encoding sound in the formats. Often proprietary formats are developed for specific hardware and software, which will place limits on the playback options for listeners. For these reasons, a proprietary format that works well on one operating system may present problems for another.