Remember when writable CDs first came out? You could write a whopping 650MB of data on a small disc. This allowed more than an hour of music to be stored on a disc in the simple CD-Audio format, even without compressing the audio. Alas, our data storage requirements have continued to grow, especially for video: DV-formatted video from a digital camcorder (at a data rate of 25 Mbps) requires around 11GB of storage per hour of video, and MPEG video compressed to 6 Mbps still requires 2.3GB per hour.
One of the design requirements for the DVD format, therefore, was to have enough capacity to hold at least a two-hour movie. This requirement led to the standard DVD size of 4.7GB.
On the surface, CDs and DVDs look very similar. In fact, if you get out your ruler, you’ll see that they’re identical in size: 120 millimeters in diameter, 1.2 millimeters in depth, with a 15 mm diameter hole in the center. The intent was that DVD players and burners would be backward-compatible with CDs. And that’s exactly what happened: almost all set-top DVD players also play music on CD and Video CD discs, and desktop DVD burners can play and burn both CD and DVD formats.
The similarities don’t stop there. With both CDs and DVDs, an extremely accurate laser is used to read (detect) or write (burn) depressions, called pits, in a substrate material sandwiched between two clear, protective plastic layers. CDs consist of only a single layer on the bottom side; DVDs, on the other hand, can have up to two layers on each side, for a total of four layers.
Pits on both CDs and DVDs are organized in a running track that continuously spirals around the DVD, like grooves in a record player. Between the pits is the original, unaltered substrate, which is called a land. In the first layer of a DVD, the spiral starts at the center and expands outward to the edge. In the second layer, however, the path can start at either the inside of the disc and spiral outward like the first, or start at the outside of the disc and spiral inward. The latter is called Opposite Track Path (OTP), and is frequently used to keep video playing continuously (moving the heads back to the interior of the disc can cause older players to pause anywhere from 1 to 3 seconds). The 1s and 0s happen based on the basis of the pits and lands. A 1 occurs when you switch from a pit to a land or back. A 0 occurs wherever there is no change—either inside a pit, or along a land.
With DVDs, the spirals are smaller and denser (i.e., more tightly packed along the radius) than on CDs. In addition, the data pit length value is interpreted as half that of a CD. For example, the minimum pit length of a CD is .83 micrometers. The minimum pit length of a DVD, on the other hand, is around .4 micrometers—that’s half the size!
Switching to a smaller laser wavelength (780 to 650 nm) permitted engineers to significantly reduce the size of each pit on a DVD (0.834 to 0.4 microns) and tighten the pitch between the pits (1.6 to 0.74 microns). See Table 2-1 for a rundown on the differences between a CD and a DVD.
So, what does this mean? It means that DVDs can squeeze around seven times more data on the same physical size disc by packing the data more tightly and by using the space more efficiently. See Figure 2-3.
Like CD, DVD supports a base read-only format, called DVD-ROM. The DVD-ROM format specifies the DVD physical format and characteristics we saw earlier, as well as standardizing on the use of the popular Universal Disc Format (UDF) filesystem to store data. As with CD-ROM, DVD-ROM is a manufactured format—discs are stamped from a master that has been cut with the data pits.
However, DVD-ROM is another one of those multiuse terms. It is used to describe both manufactured discs containing computer data (such as software applications or encyclopedias), and the DVD hardware drives that read (but do not record) DVD discs. Remember that formats such as DVD-Video and DVD-Audio are application formats, because they describe the format of the data stored on a disc that allows players to decode and display video and audio. So, if you buy a movie on DVD, it uses the DVD-Video format. However, the physical format used to store the data is still the base DVD-ROM format. In fact, you can write DVD-Video material to a recordable disc such as DVD-R, or copy it to a CD (if it is short enough). You can even save it to a folder on hard disk and play it from there, with no optical disc involved at all!
DVD gets even more interesting when it goes beyond read-only DVD-ROM to writable formats. As with CD, DVD supports both write-once recordable (such as CD-R) discs and rewritable discs (such as CD-RW). However, DVD has more disc types to choose from.
The original DVD formats, including DVD-ROM and DVD-Video, were defined by the DVD Forum, which we mentioned at the opening of the chapter. However, the DVD Forum is a voluntary industry consortium and not an international standards body. The Forum was established in April 1997 to bring together both the consumer electronic and computer industries to establish common standards, and was very successful in developing the series of standards that led to today’s 4.7GB formats, as well as the continued spectacular growth of DVD as a consumer electronics product and computer application and peripheral.
However, a second industry consortium, the DVD+RW Alliance (http://www.dvdrw.com), has since developed a competing "plus” format (compared to the original "dash” format). This group now includes Dell, HP, Mitsubishi/ Verbatim, Philips, Ricoh, Sony, Thompson, and Yamaha—some of the same companies that were part of the original DVD Forum. The Alliance originally developed just a rewritable DVD+RW format, and then added a write-once DVD+R format in 2002 (see Figure 2-4).
With major companies lined up behind both camps, the dash versus plus “format war” may be a long, drawnout affair. The competition continues, with advances in recording speed and the announcement of dual-layer DVD recording in 2004. The good news is that, with both sides in full production, there should not be major differences between them in pricing or compatibility, and any technical differences in the capabilities of the formats are heavily dependent on the DVD hardware and/or software used to record on them.
CDs were designed for streaming playback of music, and not for random access by a computer (as with a hard drive). While a CD-R is writeonly, you can write to it in several sessions. That is, you can start burning at one sitting, then append more data later, eventually filling up the disc. CD-RW adds the ability to bulk-erase the disc so that it once again appears empty and can be reused, but again, there isn’t any support for random-access writing.
DVD-R and DVD-RW were designed for many of the same purposes, especially to support continuously streaming a movie from the disc for two hours. However, DVD-RW and especially DVD+RW were designed for data use as well, and provide the ability to write and rewrite blocks of data—albeit not as efficiently as a hard disk. However, manufacturers have found this useful for creating consumer electronics devices such as set-top DVD recorders and even DVD camcorders that can erase and reuse previously recorded video clips, and can dynamically update the DVD menus.
Although the RW formats do support use as data discs, the DVD Forum also defined another recordable format, DVD-RAM, which was explicitly designed for data usage. DVD-RAM was designed for enterprise data storage applications, with built-in error correction and defect management, and with the ability to be written more than 100,000 times, compared to 1000 times for DVD-RW. In addition, DVD-RAM discs are stored in a caddy to protect the disc surface.
DVD-RAM is a significantly different optical disc format from the rest of the DVD family, and therefore is not compatible with general DVD drives and players. You cannot expect a DVD drive to read or write the DVD-RAM format unless it is explicitly marked with a DVD-RAM logo (or a DVD-Multi label signifying support for R, RW, and RAM). Meanwhile, DVD-RAM also has found a perfect niche in consumer DVD recorders and camcorders, where the ability to access data in the same way as a hard disk does simplify operation and editing.
Unfortunately, we’re not done with DVD disc formats yet; there still are more ways to fit more data onto a disc. DVD discs can be double-sided, with a full 4.7GB of data on each side for a total of 9.4GB. DVD discs also can be dual-layer, with a semitransparent top layer that the laser can focus through to read the second layer. As a result, dual-layer discs have a slightly smaller capacity of 8.5GB. Many commercial movies are now manufactured on dual-layer discs, which have room for the movie, multiple formats, and additional features, all on one side, leaving room for a printed label on the other side.
The DVD industry uses numeric shorthand to refer to these different manufactured formats, based on creative rounding up of the disc capacity (see Table 2-2). Until recently, recordable discs were available only in single-layer format, but dual-layer recordable discs started becoming available in 2004.
One other note: like CDs, DVD discs are now also available in a smaller form factor, roughly half-size or 8 cm versus the standard 12 cm disc diameter. Sometimes called “Mini-DVD,” today’s 8 cm discs can store 1.4GB per side, or 2.8GB on a double-sided disc. These smaller discs are particularly handy for DVD-RAM based camcorders.