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The hundreds or thousands of megabytes on your CDs and DVDs would do you no good without a means for reading them. That means is the optical disc drive. A CD drive or CD-ROM reader gives you access only to CDs, be they audio or data discs. All DVD drives can read both CD and DVD discs, although early drives often cannot read discs written with CD-R and CD-RW drives.

With the price difference between CD and DVD drives disappearing, there's less reason than ever to consider a CD drive. A DVD drive will do everything the CD drive does (but likely better). Performance and compatibility are no longer issues. Consequently, in any new computer, a DVD drive is the preferred option.

All optical disc drives in computers can play back audio CDs. The multimedia standards require that CD-ROM drives on multimedia computers have both a front panel headphone jack and volume control. Of course, the multimedia PC standards date back to a time when decent speakers connected to a computer were as rare as bagpipes in a symphony. Today's drives get around needing the extra jacks because all drives have an audio connector on their rear panels for linking to your computer's sound system. This connector provides analog audio—that is, the sound from a disc that has already been decoded from its original digital format. Drives also have standard mass storage interface jacks (typically AT Attachment or SCSI) to send digital data to your computer.

In theory, you can recover digital audio through this connection. Drives differ in their ability to deliver usable audio in digital form, however, a feature called digital audio extraction (DAE). For example, if you wanted to capture a track from an audio CD to convert into an MP3 file, you would need a drive that supports digital audio extraction. All current drives support DAE. Older CD drives with the AT Attachment interface often had difficulty with DAE (if they supported the feature at all). You may encounter problems if you try to use one of these older drives to capture audio.

Dedicated home DVD players have video jacks but the DVD drives in computers do not. At one time, decent video playback in your computer required that the DVD reader plug into an MPEG decoder that slid into an expansion slot and directly linked to your computer's video system. As the microprocessors and video systems in computers gained speed, however, the complex decoding job was readily assumed by software, a special MPEG video codec.


When Philips and Sony originally propounded the Compact Disc system, memory was expensive and computers were only for hobbyists. Players had to read audio data from each disc at the same rate as it was to be played. Higher speeds were irrelevant. Once computers adopted the CD, however, audio speed became a horrendous holdup. Audio CD speed was lower than a modern floppy disk, and floppy disks were too slow.

Data, unlike music, need not abide by the constraints of real-time playback. It wasn't long before computer engineers took advantage of a simple expedient to speed up the CD drives connected to computers. They made the discs spin faster…and faster…and faster. From humble beginnings of only twice as fast as ordinary audio CDs, they have revved up to as high as 50 times faster.

Drive-makers describe the speed of their products using the x-factor. The base speed of an audio CD drive is 1x. A 2x drive spins twice as fast. A 12x drive spins 12 times faster, and so on.

After about 12x, the rotation rates for CDs become awesome. When reading from the inside of the disk, a 12x rate amounts to about 3600 RPM. At the outer edge, the disc spins at half that rate. Higher speed factors (for example, 20x, 36x, or 50x) are comparably faster. Not only is spinning the disc at these rates a challenge, altering the spin speed from the inner to outer reaches of the disc is tricky and time consuming. The need to change speeds can really slow down random access.

In data applications, a constant data rate is unnecessary. With that gem of wisdom in mind, drive-makers developed CD players with a constant spin rate that, with a logic peculiar to the computer market, are known as variable-speed drives. Although the spin rate is constant, the data rate the drives deliver is not. It is twice as high at the outer edge of the disc as it is at the inner edge. The x-factor of the drive varies with the data rate.

As any cynic would expect, drive manufacturers invariably quote the fastest speed that variable-speed drives can possibly deliver. In practical use, drives almost never achieve this rate. Because these "multispeed" drives actually spin their discs at a constant rate (that is, at a constant angular velocity), at smaller diameters, the data rate (and the x-factor) is lower than at larger diameters near the outside edge of the disc. But the data on optical discs starts at the inside of the track spiral and continues to the outside. Consequently, variable-speed drives read (or write) at their top speed only at the very end of a completely full disc. Most discs aren't filled to capacity, so the highest speed areas of the disc aren't often used—and variable speed drives rarely achieve the high speed at which they are rated. A few more reputable computer-makers describe variable-speed drives by listing both their slowest and fastest speed, such as 12/24x.

On the other hand, variable-speed drives have an advantage in random access. Because they spin your discs at a constant rate, you don't have to wait for the drive to alter its speed to match the rate required at a particular place on the disc.

Note, too, that high-speed drives impose an access penalty. Drives with faster speed ratings require more time to spin your discs up to the proper speed. That means you have to wait longer after you slide a disc into the drive before you can read it. The problem is particularly severe with portable computers, which stop their discs from spinning after a minute or so to conserve power. Each time the drive powers down, you have to wait for it to spin the disc back up to speed before you can read from it.

The x-factor for DVD drives is not the same as for CD drives. As with the CD, the DVD speed ratings are pegged to the base data rate of the original video-only DVD system. The linear velocity of the DVD system is about 2.98 times faster than the linear velocity of the CD. But drive electronics don't care about linear speed. They lock onto the data rate. The data format of the CD has larger features than those of DVDs. The minimum pit length, for example, is 0.83 microns for CDs and only 0.4 for DVDs. For a given data rate, the CD must spin about twice as fast as a DVD to produce the same data rate, the 26.16Mbps basic read rate of DVDs. Add the increased linear velocity to the change required by feature size and read rate, and the result is that a 1x DVD drive best reads data at about six times the base CD rate. Consequently, every DVD "x" is worth about six "x" from a CD drive. Table 18.3 gives a comparison of some common CD and DVD speeds (assuming a constant data rate).

Table 18.3. Comparison of CD and DVD Drive Speed Factors
CD Factor DVD Factor Raw Data Rate
1x 0.17x 4.32Mbits/sec
2x 0.33x 8.64Mbits/sec
4x 0.67x 17.3Mbits/sec
6x 1x 26.2Mbits/sec
12x 2x 52.4Mbits/sec
18x 3x 78.6Mbits/sec
20x 3.3x 86.4Mbits/sec
24x 4x 105Mbits/sec
30x 5x 131Mbits/sec
36x 6x 157Mbits/sec
40x 6.7x 173Mbits/sec

Because the coding system used by DVDs is more efficient than that used by CDs, a DVD drive extracts more information at a given raw data rate. The user data rate from a 1x DVD drive reading a DVD is about nine times that of a 1x CD drive reading a CD.

In general, a faster drive is a better drive—providing you can put up with the extended spin-up times of really quick disc drives. Some software makes particular speed demands from CD drives. The minimum speed requirement usually is in the range of 4x to 6x. All CD drives in new computers deliver at least 12x, so you should encounter no problem with software speed compatibility. Similarly, the base DVD rate is fast enough for most applications, although you may prefer the quicker load times afforded by a 4x or faster DVD drive.

When you play audio from CDs or movies from DVDs, high drive speeds and x-factors make no difference. A faster drive will not produce a higher-quality signal from these isosynchronous sources. Only when you read computer data from disc do the x-factors matter. For example, play a CD so you can listen to it, and your drive will operate at 1x. When you attempt to extract digital data from the same audio CD to create an MP3 file, however, the drive may shift to a higher speed—if your computer can process the data fast enough. Extraction need not occur in real time.

Access Time

Compared to magnetic hard disks, all optical disc readers are laggardly beasts. Mass is the reason. The read head of an optical drive is substantially more massive than the flyweight mechanisms used in hard disks. Instead of a delicate read/write head, the CD or DVD drive has a large optical assembly that typically moves on a track. The assembly has more mass to move, which translates into a longer wait for the head to settle into place. Optical drives consequently have hefty access times—where the typical hard disk drive now has an average access time of 9 milliseconds or less, a quick optical drive is about one-tenth the speed (with an average access time about 90 to 100 milliseconds).

As noted earlier, the constant linear velocity system used by some drives also slows the access speed. Because the spin rate of the disc platter varies depending on how far the read/write head is located from the center of the disc, as the head moves from track to track, the spin rate of the disc changes. With music, which is normally played sequentially, that's no problem. The speed difference between tracks is tiny, and the drive can quickly adjust for it. Make the optical drive into a random-access mechanism, and suddenly speed changes become a big issue. The drive might have to move its head from the innermost to outermost track, requiring a drastic speed change. The inertia of the disc spin guarantees a wait while the disc spins up or down.


Nearly all optical disc drives have an eject button on the front panel. Pressing this button causes the drive to spin down the disc inside the drive (if there is one) and then slide out the tray or pop out the caddy should one be used. Pressing the button again slides the tray back in. Usually pressing on the tray open will also trigger the drive to slide it all the way in.

The various Multimedia PC (MPC) specifications require a volume control on the front panel of any CD-ROM drive you have in your multimedia computer. This control is useful if you decide to use your drive for playing back music while you work. You can plug headphones into the jack on the front of the drive (also required by the MPC standards) and use the volume control to adjust the loudness of the playback independent of the CD control software you run on your computer. The front panel volume control usually does not control the output on the audio connector on the rear of the drive.

Some CD and DVD drives have extended control panels, usually with the standard motion controls as found on dedicated disc players: stop, play, fast forward, rewind, track forward, and track back. Although these controls are not required, they can be handy when you use the DVD player in your computer for playing video through an external monitor.

With the exception of the volume control, all the front panel control functions (including eject) can be operated through suitable software.


DVD drives are required to be able to play back audio CDs made under the Red Book standard. Although as a digital system that can read CD-ROM data, DVD could be compatible with any CD, this is not always the case. Most DVD systems cannot handle interactive Green Book CDs. The dye used in many CD-R media is invisible to DVD wavelengths, so early DVD drives may be unable to read the CD-R discs you make. Drive-makers have adopted a number of strategies to enable their newer DVD drives to read CD-Rs properly. Of course, different CD-R media use different dyes, so you might find your DVD drive works with some CD-Rs and not with others. CD-RW media complicate matters because the medium is not as reflective as that used by prerecorded CDs and DVDs. Manufacturers have developed a "MultiRead" label to assure you that the drive will read CD-ROM, CD-R, and CD-RW media. DVD drives are not required to be able to read CD-Video disc (White Book), but most can—it takes no great technical feat because the DVD drive can read the CD medium, and the MPEG-2 circuitry in the DVD drive also handles the MPEG-1 of CD-Video. DVD players can read data from Enhanced CDs made under the Blue Book standard, including music CDs.

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