A Compact Disc Audio or CDA file holds a small bit of data that serves as an index for tracks on an audio CD. Each CDA file takes the form of Track xxxx.cda, and includes the location of the beginning and end of the named track, but not the actual audio stream. Some ripping programs, however, will assign a .cda extension to tracks when ripped (copied) to a computer.
The actual audio format used on CDs is pulse-code modulation or PCM. PCM captures analog waveforms using an uncompressed, digital sampling technique that results in a very "true" digital representation of the original analog wave. A track ripped to a computer as an audio CDA file is actually a PCM file assigned a .cda extension.
Although a CDA file containing a PCM wave stream is extremely high quality, it also takes up a lot of space and will not be recognized by most portable digital players designed for use with compressed files. Once on the computer, it is easy to convert the PCM or CDA file to a compressed format. Some ripping software will allow the user to choose a format before ripping, eliminating the need of converting the files afterward. Many people like to archive music in an uncompressed state, however, then make compressed files from these high-quality originals.
There are two classes of compressed files: lossless and lossy. The first category includes formats that compress files without loss of quality. These files are still rather large and many portable audio players do not support them. Lossless compressed formats include FLAC, Monkey's Audio (APE), Apple® Lossless, Windows® Media Audio Lossless (WMA Lossless) and others.
Lossy formats sacrifice a little quality for a much smaller footprint. Unlike the ripped CDA file that carries a full-bodied PCM wave stream, a lossy format leaves out some data to achieve its much smaller file size. Formats include the familiar MP3 format, Vorbis, AAC, WMA (lossy version), and others. The loss of quality is not too noticeable on portable devices that rely on earbuds or small portable speakers for output. Audiophiles aside, many people cannot tell the difference between a lossy file and its PCM or ripped CDA file counterpart when played on a portable device.
The advantage of using compressed files on portable devices is that many more tracks will fit into the limited memory. Uncompressed files, however, are still recommended for burning CDs or audio DVDs for car stereos, home stereos and surround sound systems.