Build Your Skills: A comparison between DVD and CD-ROM

Explanation of the inner workings of the DVD drive

If you are curious about the inner workings of your computer’s drives, you’ve come to the right place. Here’s another Daily Drill Down in my “All About” series, which sheds some light on the specifications and usage of various pieces of hardware. Earlier pieces have tackled IDE ("All about IDE"), CD-ROM ("All about CD-ROM standards"), and CD-R ("All about CD-R and CD-RW"), and now it’s time to take on the Digital Versatile Disk, or DVD.

How does DVD compare to CD-ROM?
Let’s start with something easy—comparing a regular DVD drive (nonwritable) to a regular CD-ROM drive (also nonwritable). DVD is basically a souped-up version of CD-ROM, technologically speaking. As you learned in my "All About CD-R and CD-RW" Daily Drill Down, CD-ROM data is stored in patterns of pitted and unpitted (“land”) areas on a disc. DVD data is stored the same way, except the pits are much smaller and closer together. That’s why CD-ROMs top out at about 650-700 MB of data, while DVDs can hold multiple gigabytes of data. Since the pits are more tightly written on a DVD disc, the mechanism in the drive for reading them also has to be finer and more precise. That’s the main reason that DVD drives cost more than CD-ROM drives. DVD drives can also read regular CD-ROM discs with no problem, since the difference is mostly in the spacing.

How much data are we talking about on a DVD? Well, with a single-sided, single-layer disc of the same thickness as a normal CD-ROM (120 mm), it’s 4.7 GB of data or 135 minutes of video. (That’s the DVD-5 standard, by the way.) If the disc is double-sided (DVD-10), you can double that capacity to get a 9.4-GB or 270-minute capacity.

The DVD standard-makers also have figured out another way to squeeze more data onto a disc—they record it in two layers. The top layer is semi-reflective, so the read laser can pass through it to read the second layer of data beneath it. This scheme doesn’t exactly double the amount of data, but almost: A two-layer disc (DVD-9) can hold 8.5 GB. Combine the two methods for a double-sided, double-layer disk (DVD-14), and the capacity tops out at 13.24 GB.

What about DVD-1 through 4?
You might be wondering what happened to DVD-1 through DVD-4. Those were earlier standards that used a disk of different thickness (80 mm as opposed to the normal 120 mm). DVD-1 (single-sided, single-layer) held 1.4 GB or a half-hour of video, while DVD-4 (double-sided, double-layer) held 5.3 GB or 2.5 hours of video.

How do DVD speeds correspond to CD-ROM speeds?
DVD drives spin the disk faster and read the data faster than a normal CD-ROM drive, so you can’t fairly compare the speeds using X ratings alone. Recall from my Daily Drill Down “All About CD-ROM” that CD-ROM speed is measured in relation to the original 1X standard of 150-Kbps data transfer rate. Because the data is stored on a CD at a fixed number of bits per area, you can determine the drive’s raw speed at reading data from the speed at which the disc rotates. Therefore, a 2X drive would have to spin twice as fast as a 1X drive to double the data transfer rate to 300 Kbps.

With a DVD drive, since the data on a DVD disc is stored so much more compactly, a drive spinning at 1X speed can transfer approximately 1.4MBps, which makes it roughly equivalent to a regular 9X CD-ROM. The rates increase from there, with a 16X DVD drive topping out at about 22 MBps, or faster than a 140X CD-ROM (if there were such a drive).

However, you can’t just look at DVD transfer rates when evaluating a DVD drive for your system because in reality you will probably use it mostly for regular CD-ROM discs; with those, the transfer rate drops back down somewhat. A 1X DVD drive spins about three times the speed of a 1X CD-ROM drive, so you can approximately triple the advertised X speed on a DVD drive to determine how it will perform with regular CDs. For example, a 16X DVD drive would be equivalent to a 48X CD-ROM drive when reading from regular CD-ROMs.

Can DVD drives read homemade CD-R and CD-RW?
The earliest DVD drives could not read CD-R and CD-RW discs because of the difference in the light wavelength used for reading the discs. Regular CD-ROM drives use a wavelength of 780 nanometers (nm) to read discs, the same wavelength required by CD-R and CD-RW discs. DVD uses a wavelength of 650 nm. Regular mass-produced CD-ROMs are very reflective and can be read at a variety of wavelengths, so the 650 nm is not a problem for them. However, CD-R and CD-RW discs are not as powerfully reflective at other wavelengths, and a 650 nm laser has trouble reading them.

Drive manufacturers struggled for a while to overcome this problem and finally hit upon a two-laser system that blasts each disc with lasers at both 780 nm and 650 nm at once. That way, no matter what type of disc it is, some data will bounce back from the laser that the drive can interpret. Such drives are designated MultiRead. Almost all DVD drives manufactured today are MultiRead-capable.

When different types of DVD became available (including writable DVDs), the standard underwent a revision, and the result was MultiRead2. A MultiRead2-capable drive can read all CD-ROM formats (including all writable CD-ROMs) and all DVD formats, including DVD-ROM, DVD-Audio, DVD-Video, and DVD-RAM.

Are MPEG decoders required?
Many people are confused when they buy a DVD drive (or when one comes with their system) about whether an MPEG decoder card is required. The answer is that it all depends on what you are planning to use the drive for. If you want to use your DVD drive in your PC to play DVD movies, you will need MPEG decoding capability. If you are just using the DVD drive for data, you won’t need it. MPEG decoding processes the audio and video data and allows it to play on your PC. When you are reading data from a DVD data disc, it plays no role.

One type of MPEG decoder is a separate circuit card you install in your PC. You plug your monitor’s VGA plug into it rather than into your video card, and then you run a pass-through cable from the decoder card to your video card input. Some variants let you connect the monitor directly to your video card, but then an internal cable connects the video card to the MPEG decoder card. While this system takes up system resources (a PCI slot, an IRQ, and so on), it does result in good audio and video playback performance. It also takes most of the processing workload off your main system when playing movies.

An alternative to the separate MPEG decoder card is a video card with MPEG decoding built in. This is more convenient because a separate card and separate resources are not required, but the performance can lag behind that of a separate decoder card. In addition, more of your main system resources are consumed when playing a movie, and if you don’t have a fast system with lots of available RAM, playback performance can suffer. Note that some new DVD drives include MPEG-2 software decoding. In theory, these can substitute for a hardware-based decoding solution. However, unless your PC is lightning-fast, you may encounter performance problems.

What is DIVX?
Digital Video Express (DIVX) was a movie distribution format, pushed mainly by Circuit City, a few years ago. DIVX required a special player that connected by modem to a billing system. You would get the discs from Circuit City and then register for a 48-hour viewing period using your DIVX player and its modem. After 48 hours had expired, the disc would not play anymore unless you agreed to be billed for additional time or to buy the movie outright.

DIVX is now obsolete. DIVX movies are no longer being distributed by Circuit City (or anyone else, for that matter). If you have a DIVX player, don’t toss it, because it also plays regular DVD movies.

How do writable DVD systems work?
If you read my Daily Drill Down "All About CD-R and CD-RW," you’ve already got a good handle on how writable and rewritable CDs work. Writable DVDs work using essentially the same technology, but there are some minor differences. First, let’s look at what’s the same. Both technologies use a writable disc that’s coated with an organic dye (for write-once) or a metal composite (for rewritable). The laser heats an area of the disc, changing the properties of that area so that it reflects light differently when the read laser hits it.

Writable DVD discs, unlike CD-ROM, have predefined grooves or “tracks” in the blanks. The grooves are wavy, rather than straight like on a phonograph record (if you remember what those are). The interval of the waves helps keep the timing correct when playing back the disc on different drives that spin at different speeds. Depending on the writing technology, data can be written either in the grooves only, or both in the grooves and in the “land” areas between them.

The other details of writable technology aren’t nearly as clear-cut with DVD as with CD-ROMs. There are a variety of writable DVD standards at the moment, and the jury is still out as to which will prevail. Some standards are write-once, like CD-R; others are rewritable, like CD-RW. Some require the disc to stay in a protective caddy at all times (which means you can’t record videos and then play them on a DVD video player), while others allow a disc to be “loose.” And each technology has its own proprietary blank discs that it writes to.

Competing standards
One of the main competitors in write-once DVD is DVD-RAM. A DVD-RAM blank costs about $25 and can hold about 2.6 GB per side, writing both in the grooves and on the land. This technology requires a disc caddy, so you can’t use its discs in most regular DVD drives, either computer or video. Its main purpose is mass data storage.

The principal competitor to DVD-RAM is DVD-R. It’s a lot like CD-R and uses a similar technology for recording. It writes data only in the grooves on the blank (not on the land) but requires less overhead recording, so it can actually store more data per side than DVD-RAM (about 4 GB). Blanks cost about $40. The discs don’t require a caddy, and most regular DVD drives and players can read them.

On the rewritable front, there are two competing standards: DVD-RW and DVD+RW. These are still fairly new and battling it out. Both are based on the same technology as CD-RW, but they vary in their backward compatibility with other standards. In the next year or two, as the prices begin to fall for rewritable DVDs, expect this rivalry to heat up and a victor to emerge.

Most new PCs today come with DVD—evidence that DVD is here to stay. Standards for DVD players and discs are fairly stable now, to the point that you can buy without “Betamax” anxiety. However, the world of writable and rewritable DVD is still in a state of flux, and early adopters are taking a risk that the standard they choose will not become the definitive standard.

Further reading
Here are some additional resources about DVDs:
  • "Who will win the DVD standards fight?": This now-dated, but very informative, article outlines the major writable DVD standards and tells which manufacturers have gotten behind which technologies.
  • Competing rewritable DVD standards: This chart neatly summarizes the differences among the emerging writable DVD standards.
  • "DVD-RAM standards update": This article from the ever-reliable PC Magazine explains some of the standards and provides links to reviews of many different writable drives, both CD and DVD.


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