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).

Blu-ray raises disc technology to a new level

The DVD format was certainly a huge leap compared to VHS tapes, but it has major shortcomings as an entertainment medium for the high-definition era. DVD's compression scheme and disc structure were designed for standard-definition video. As TVs have grown bigger and better, the limitations of the DVD format have become more apparent. When watching DVDs on some of the better 1080p HDTVs with screens of 50" or larger, compression noise and artifacts are sometimes noticeable. Blu-ray, on the other hand, offers 1080p resolution for an incredibly smooth, detailed picture.

HD's much higher level of picture detail requires much more information. So, any high-definition format requires much higher data storage capacity (measured in gigabytes). Here's an example: a digital recorder with a 250GB hard drive can store about 200 hours of standard-definition video, but only about 30 hours of HD video. HD's superior picture quality also requires much faster data transfer rates (often called "bit rates") from the player to your TV (measured in megabits per second — Mbps). If the flow of information from a DVD player to a TV could be characterized as a babbling brook, the flow from a high-definition player would be a roaring river.

As the chart makes clear, Blu-ray discs provide much greater data storage capacity and faster bit rates than standard DVD. Translation: much improved picture and sound quality.

Fitting more data on the disc

Even though high-definition video requires so much more data, high-def discs can easily hold even the longest movies on a single disc. Blu-ray discs can hold multiple hours of HD content, with plenty of room to spare for the bonus features you may have grown accustomed to with DVD. The developers of Blu-ray couldn't make the disc physically larger, so in order to significantly increase the information storage capacity, they increased the data density. The information pits got smaller, and the spacing of the pit rows got tighter (see illustration below). The discs also have a super-thin transparent protective coating, which places the data layer closer to the disc's surface and thus closer to the laser. In order to read these much smaller data pits, Blu-ray players use a blue-violet laser, which has a shorter wavelength and a smaller "beam spot" than the red laser used in DVD players. The players also spin the discs at higher speeds for even faster data transfer.

Putting high-definition video on a disc requires much higher storage capacity than DVDs allow. Compared to DVD and HD DVD, Blu-ray discs have smaller data pits and more closely spaced pit rows.

Storing high-definition video requires much higher data density than standard DVDs allow. Blu-ray discs have smaller data "pits" and more closely spaced pit rows compared to DVDs and HD DVDs. Blu-ray players require a blue laser to read these smaller pits. In Blu-ray players, the laser's higher "numerical aperture" (NA) allows the beam to be focused to create a tighter spot for reading smaller pits.

Codecs
At the core of all recent digital entertainment forms is the concept of data "compression." Compression is needed to squeeze digital content so that it takes up a minimum of storage space. Compression is what made video formats like DVD and HDTV possible, as well as audio formats like Dolby Digital and MP3. The digital data is compressed for transmission or encoding on a disc, and then decompressed by your player. These compression/decompression technologies are often referred to as "codecs" for short.

MPEG-2 is the video codec used for DVDs and current HDTV content, including broadcast, cable and most satellite TV. Blu-ray also uses MPEG-2, as well as two newer, higher-efficiency codecs: AVC MPEG-4 and VC-1 (based on Windows Media Video 9). Because Blu-ray employs such high bit rates (54Mbps, compared to 19.2Mbps for over-the-air HDTV), the picture quality of Blu-ray discs is exceptionally clean, with fewer visible compression artifacts.

The expanded storage capacity of Blu-ray also makes it possible for these discs to offer dramatically improved sound quality. The fact that Dolby Digital sounds as good as it does is remarkable considering how aggressive the compression is for DVDs. High-definition discs have much more space available for soundtracks, and often feature new, higher-quality codecs from Dolby and DTS. One of Dolby's new formats, Dolby Digital Plus, offers up to 7.1-channel surround sound for even more enveloping audio than standard 5.1-channel Dolby Digital. There are even "lossless" options, Dolby TrueHD and DTS-HD Master Audio, which deliver the closest possible reproduction of the movie studio's original master. Many Blu-ray titles feature multichannel LPCM soundtracks — uncompressed audio that should also match the quality of the studio master.