HOW IT WORKS How it Works (CD) explains how patterns of bumps on a mirrored surface can be interpreted as bits. These bits can be assembled into bytes and then played back through an analog-to-digital converter to create music. A CD can hold about 650 megabytes of information or about 75 minutes of music. A DVD works exactly the same way, but it can hold a lot more information -- about 4.7 gigabytes (about seven times as much as a CD). DVDs can hold more data than CDs because the bumps are smaller and the tracks are closer together, giving DVDs more storage space. Here are the typical contents of a movie stored on a DVD: Up to 133 minutes of high-resolution video in letterbox or pan-and-scan format, at 720 dots of horizontal resolution (The video compression ratio is typically 40:1 under MPEG-2.) Soundtrack presented in up to eight languages using 5.1 channel Dolby digital surround sound Subtitles in up to 32 languages You can also use DVDs to store music. If you do, you can store almost eight hours of music per side! MPEG-2 compression is important to the whole scheme, because without a good compression algorithm there's no way a movie could fit on a DVD. See mpeg.org for more information on MPEG.