If you place a CD, a DVD, and a Blu-ray Disc™ next to each other, they look nearly identical. But if you try to play a DVD in your CD player, or a Blu-ray Disc in your DVD player, it won't read it. It can't because each type of disc has a different physical structure, data format, and error correction system. Below we'll discuss what makes a DVD different from a CD. And on page 2, we'll talk about what makes Blu-ray unique.

DVD

Where a CD is a single 1.2-millimeter-thick disc, all DVDs use a "sandwich" design — two 0.6-millimeter discs bonded together. This sandwich construction allows DVD discs to have information on both sides and on one or two layers per side. This design is also more structurally stable and resistant to disc warping.

Compared to CD, DVD uses smaller data pits and more closely-spaced pit rows of "tracks," as illustrated above. This increased data density makes it possible for each layer on a DVD to hold more than six times the amount of data on a CD. DVD players use a red laser in place of the infrared laser found in CD players. The red laser's shorter wavelength, combined with a special lens design, result in a narrower, more tightly focused laser beam that can easily read the smaller pits.

DVD disc types: It's about sides and layers

In order to fit multiple movie formats, plus the common extra material like interviews, commentaries, and outtakes, movie studios are making greater use of DVD's dual-sided and dual-layered capabilities. All DVD players can play all of these different disc types. Some newer players may be a little quicker at handling layer changes on dual-layer discs.

Single-sided, single-layer: Even DVD discs with a single information layer can hold the complete audio and video for a full-length movie, including Dolby® Digital 5.1 soundtracks in three different languages. Total capacity: 4.4 gigabytes (over 2 hours of video).

Double-sided, single-layer: Most discs of this type include a movie version formatted for a standard TV screen on one side, and a widescreen version on the other side. Only a few titles spread a single version over two sides. In these instances, you'll need to manually flip the DVD over when it reaches the end of side 1 (only a few recent mega changers are able to change DVD sides). Total capacity: 8.75 gigabytes (about 4.5 hours of video).

Single-sided, dual-layer: This disc type has two information layers, providing nearly twice the data capacity of a single-layer DVD. The layer closer to the player's laser pickup has a semi-transparent coating. The laser is able to shine through this layer to read the deeper layer beneath it, then re-focus to read the semi-transparent layer. One way to tell if a DVD is dual-layer is to look at the disc's playing surface — single-layer discs are silver while dual-layer discs are gold. Total capacity: 8 gigabytes (about 4 hours of video).

Double-sided, dual-layer: This disc type provides the maximum data capacity. It's basically two single-sided, dual-layer discs bonded together. So far, only a few movie transfers have been in this format (Ben-Hur is one). As with all double-sided DVDs, unless you own one of the dual-side-play mega changers mentioned above, you'll have to flip the DVD over yourself. Total capacity: 15.9 gigabytes (over 8 hours of video).