DVD Discs: Recording Technology

Place a CD next to a DVD and the discs look nearly identical. And if you put a CD in a DVD player, it will play just fine (in fact, it'll sound great). But if you try to play a DVD in your CD player, it won't read.

It can't because a DVD disc has a different physical structure, data format, and error correction system than a CD. 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.

The illustrations below show how hours of theater-quality pictures and sound can be squeezed onto a disc you can hold in your hand.

Compact Disc                                                DVD

Compared to CD, DVD uses smaller data pits and more closely-spaced pit rows of "tracks." 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.

Formatted to fit your screen...or taste
DVD is the first home entertainment medium that gives viewers so much control over how movies, concerts, and other programs are presented. The available options will vary from disc to disc (the disc producers make those decisions). On-screen menus make it easy to select the type of presentation that best matches your A/V equipment and personal preferences:

• Aspect ratio (screen shape): Standard, Widescreen (anamorphic), or Letterbox.
• Soundtrack audio: Dolby® Digital 5.1 surround (DTS® 5.1 soundtracks are featured on a few titles), Pro Logic® surround, or stereo.
• Soundtrack language: Often English and French for Region #1 (USA and Canada), but other languages may be included as well. Subtitle languages: DVD discs can provide up to 32 subtitle tracks.

In order to fit multiple movie formats (not to mention the increasingly common extra material like interviews, commentaries, outtakes, etc.), 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.

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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).

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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).

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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).

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Double-sided, dual-layer: This disc type provides the maximum data capacity. It is 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).

DVD Disc Layout & Formats

DVD discs come in a range of different physical formats with capacities from 4.7 GB to 17.1 GB. DVD discs have the same physical dimensions as CDs, but each DVD comprises two substrates each 0.6mm thick and bonded together like a sandwich.

This offers the possibility of discs with up to two sides and up to two layers offering four possible read-only formats:

Name	Capacity (GB)	Layers	Sides	Comments
DVD-5	4.7	1	1	Single side/layer
DVD-9	8.54	2	1	Dual layer
DVD-10	9.4	1	2	Double sided
DVD-18*	17.08	2	2	Dual layer/side

*DVD-18 is difficult to manufacture and, as it is a double-sided format, there is little room for a label and the disc needs to be turned over to play both sides.

Note that 1 GB = 1,000,000,000 bytes.

DVD Disc Layout

Each layer of a DVD disc contains lead-in, data area and lead-out like a CD.  For dual layer (DVD-9) discs the two layers comprise a single volume, but can be organized in two different ways depending on the application:

• Parallel Track Path PTP), where the two layers both start at the inside diameter (ID) and end at the OD with the lead-out.  This is suitable for DVD-ROM.
• Opposite Track Path (OTP), where layer 0 starts at the ID and layer 1 starts where layer 0 ends. For such discs there is one lead-in (on layer 0), one lead-out (on layer 1) and two middle areas.  This is suitable for DVD-Video where a single movie can occupy both layers with a layer break in the middle. Using OTP allows a near seamless jump from layer 0 to layer 1.