A CD stores music in the uncompressed .wav format. A .wav file plays music at 1,410 kilobits per second (kbps), which gives it high dynamic range. These digital files are also quite large, making them inconvenient for portable music players. Compression formats shrink the file size. MP3 is a lossy compression format, meaning it shrinks the file size by throwing out bits of the data, typically very quiet or loud sounds, which are determined by the MP3 algorithm. WMA is also a lossy compression format that works in the same fashion, but it has a lossless rate as well.
File size is determined by bit rate. Most MP3s come in a variety of bit rates, from 128 kbps to 320 kbps, which is substantially less than CD quality. An uncompressed three-minute .wav file can be 30 to 40 MB in size; the lossy compression of MP3 reduces that by a factor of 10, to about 3 to 4 MB, which enables greater storage on a portable media player.
The WMA lossless format is similar in concept to a .zip file, in that the file is shrunk without losing any of the original data. To encode a CD file in WMA lossless format, you need to download the Windows Media encoder from the Microsoft website. WMA lossless encodes audio of bit rates between 470 kbps and 940 kbps, so the files sizes can be half to a third of the CD file size. Not all portable media players support WMA lossless playback.
MP3 can support audio sample rates of 48 kHz; a CD is sampled at 44.1 kHz. WMA supports higher-quality audio used in DVD-Audio formats; it can compress up to six channels of audio and supports the professional audio sample rate of 24 bits at 96 kHz.