Compressed audio optical disc

Did You Know?

• This technology is most commonly used in audiobooks new on CD since 2000 or so, especially for unabridged audiobooks that run many hours.
• The Consumer Electronics Association and Audio Publishers Association have published standards CEA-2003-C and CEA-2004 for digital audiobook file format and player compatibility.

Description

Many modern CD and DVD players support compressed audio files such as MP3, Ogg Vorbis, AAC, Windows Media Audio, and ATRAC. Because the audio is compressed, the disc does not need to spin continuously, which can save battery power, though decompression demands more processor time. Audio is buffered in random-access memory, offering protection against skipping. The number of files a disc can hold depends on encoding and audio length; a 650-MB data CD with mid-quality (128-kb/s) audio can store about 11 hours or roughly 225 songs.

Some players can display ID3 tags stored in the files and can search directories on the disc. There is no official standard for how audio files are stored on such discs, leading to incompatibilities due to filename length limits, sub-directory limits, number-of-files limits, and special-character bugs. Occasionally, pressed CDs containing MP3 are used, as some CD-ROM video games function as MP3 CDs. Older classic CD-ROM games often use WAV files, which are compatible with CD players that support Yellow Book CD-ROM.

Disadvantages compared to other means to play compressed digital audio

Most disadvantages of compressed audio optical discs are those of CD and DVD in general. Optical discs are either non-rewritable (CD-R/DVD-R) or can be rewritten only about 120 times, sometimes requiring erasure of the entire volume before rewriting; flash memory supports over 100,000 write/erase cycles while retaining old data, though this may be moot for less demanding uses. Rewritable discs have less compatibility with older players, though most CD and DVD players that support compressed audio handle RW discs easily.

The moving parts in optical disc players reduce battery life and overall service life compared to solid-state portable media players; an optical disc player can fail in less than one month of uninterrupted spinning, whereas a solid-state player can run up to six months without failure. A Yellow Book disc with compressed audio may free up as many as 11 Red Book audio CDs, but still demands more shelf space than external hard drives or flash memory.

Optical discs have a reflective surface that can be damaged by relatively low levels of surface damage, and repeated handling exposes them to dust and permanent damage. Some CD and DVD discs have defective aluminum layers that can flake, causing natural deterioration. Archival-grade discs such as gold CD or M-DISC may last far longer than hard drives or flash memory, according to manufacturer claims. For multi-gigabyte collections, repeated insertion and removal of discs may be required; copying files to a hard drive is a possible workaround for faster access.