CD/DVD Read-Only Drives and Specifications / Upgrading and Repairing PCs

When evaluating a CD-ROM or DVD-ROM drive for your PC, you should consider three distinct sets of criteria, as follows:

These criteria will affect how fast the drive operates, how it is connected to your system, and how convenient (or inconvenient) it is to use. These same criteria, plus media and speed issues, are also important considerations when selecting a rewritable drive.

Performance Specifications

Many factors in a drive can affect performance, and several specifications are involved. Typical performance figures published by manufacturers are the data transfer rate, the access time, the internal cache or buffers (if any), and the interface the drive uses. The following sections examine these specifications.

CD Data Transfer Rate

The data transfer rate for a CD-ROM or CD-RW drive tells you how quickly the drive can read from the disc and transfer to the host computer. Normally, transfer rates indicate the drive's capability for reading large, sequential streams of data.

Transfer speed is measured two ways. The one most commonly quoted with CD/DVD drives is the "x" speed, which is defined as a multiple of the particular standard base rate. For example, CD-ROM drives transfer at 153.6KBps according to the original standard. Drives that transfer twice that are 2x, 40 times that are 40x, and so on. DVD drives transfer at 1,385KBps at the base rate, whereas drives that are 20 times faster than that are listed as 20x. Note that because almost all faster drives feature CAV, the "x" speed usually indicated is a maximum that is seen only when reading data near the outside (end) of a disc. The speed near the beginning of the disc might be as little as half that, and of course, average speeds are somewhere in the middle.

With recordable CD drives, the speed is reported for various modes. CD-R drives have two speeds listed (one for writing, the other for reading), CD-RW drives have three, and CD-RW/DVD-ROM drives have four. On a CD-RW drive, the speeds are in the form A/B/C, where A is the speed when writing CD-Rs, B is the speed when writing CD-RWs, and C is the speed when reading. The first CD-RW drive on the market was 2/2/6, with versions up to 54/24/52 available today. CD-RW/DVD-ROM drives use the format A/B/C-D, where the fourth number represents the DVD-ROM reading speed. Some of the fastest CD-RW/DVD-ROM drives today report 20/10/40-12.

CD Drive Speed

When a drive seeks out a specific data sector or musical track on the disc, it looks up the address of the data from a table of contents contained in the lead-in area and positions itself near the beginning of this data across the spiral, waiting for the right string of bits to flow past the laser beam.

Because CDs originally were designed to record audio, the speed at which the drive reads the data had to be constant. To maintain this constant flow, CD-ROM data is recorded using a technique called constant linear velocity (CLV). This means that the track (and thus the data) is always moving past the read laser at the same speed, which originally was defined as 1.3 meters per second. Because the track is a spiral that is wound more tightly near the center of the disc, the disc must spin at various rates to maintain the same track linear speed. In other words, to maintain a CLV, the disk must spin more quickly when reading the inner track area than when reading the outer track area. The speed of rotation in a 1x drive (1.3 meters per second is considered 1x speed) varies from 540rpm when reading the start (inner part) of the track down to 212rpm when reading the end (outer part) of the track.

In the quest for greater performance, drive manufacturers began increasing the speeds of their drives by making them spin more quickly. A drive that spins twice as fast was called a 2x drive, one that spins four times faster was called 4x, and so on. This was fine until about the 12x point, where drives were spinning discs at rates from 2,568rpm to 5,959rpm to maintain a constant data rate. At higher speeds than this, it became difficult to build motors that could change speeds (spin up or down) as quickly as necessary when data was read from different parts of the disc. Because of this, most drives rated faster than 12x spin the disc at a fixed rotational, rather than linear speed. This is termed constant angular velocity (CAV) because the angular velocity (or rotational speed) remains a constant.

CAV drives are also generally quieter than CLV drives because the motors don't have to try to accelerate or decelerate as quickly. A drive (such as most rewritables) that combines CLV and CAV technologies is referred to as Partial-CAV or P-CAV. Most writable drives, for example, function in CLV mode when burning the disc and in CAV mode when reading. Table 13.23 compares CLV and CAV.

Table 13.23. CLV Versus CAV Technology Quick Reference
	CLV (Constant Linear Velocity)	CAV (Constant Angular Velocity)
Speed of CD rotation	Varies with data position on CD—faster on inner tracks than on outer tracks	Constant
Data transfer rate	Constant	Varies with data position on CD—faster on outer tracks than on inner tracks
Average noise level	Higher	Lower

CD-ROM drives have been available in speeds from 1x up to 52x and beyond. Most nonrewritable drives up to 12x were CLV; most drives from 16x and up are CAV. With CAV drives, the disc spins at a constant speed, so track data moves past the read laser at various speeds, depending on where the data is physically located on the CD (near the inner or outer part of the track). This also means that CAV drives read the data at the outer edge (end) of the disk more quickly than data near the center (beginning). This allows for some misleading advertising. For example, a 12x CLV drive reads data at 1.84MBps no matter where that data is on the disc. On the other hand, a 16x CAV drive reads data at speeds up to 16x (2.46MBps) on the outer part of the disc, but it also reads at a much lower speed of only 6.9x (1.06MBps) when reading the inner part of the disc (that is the part they don't tell you). On average, this would be only 11.5x, or about 1.76MBps. In fact, the average is actually overly optimistic because discs are read from the inside (slower part) out, and an average would relate only to reading completely full discs. The real-world average could be much less than that.

What this all means is that on average the 12x CLV drive would be noticeably faster than the 16x drive, and faster than even a 20x drive! Remember that all advertised speeds on CAV drives are only the maximum transfer speed the drive can achieve, and it can achieve that only when reading the very outer (end) part of the disc.

Table 13.24 contains data showing CD-ROM drive speeds along with transfer rates and other interesting data.

Vibration problems can cause high-speed drives to drop to lower speeds to enable reliable reading of CD-ROMs. Your CD-ROM can become unbalanced, for example, if you apply a small paper label to its surface to identify the CD or affix its serial number or code for easy reinstallation. For this reason, many of the faster CD and DVD drives come with autobalancing or vibration-control mechanisms to overcome these problems. The only drawback is that if they detect a vibration, they slow down the disc, thereby reducing the transfer rate performance.

A company called Zen Research developed a technology called TrueX in the late 1990s, which used multiple laser beams to achieve constant high transfer rates with lower spin rates. Unfortunately, TrueX drives made by vendors such as Kenwood didn't live up to the promised performance of TrueX technology and suffered from many reliability problems. Zen Research closed in mid-2002, and TrueX drives are no longer sold. Instead of TrueX, most are now moving to Z-CLV (zoned CLV) or P-CAV (partial CAV) designs, which help increase average performance while keeping rotational speeds under control.

Table 13.24. CD-ROM Drive Speeds and Transfer Rates

DVD Drive Speed

As with CDs, DVDs rotate counterclockwise (as viewed from the reading laser) and typically are recorded at a constant data rate called CLV. Therefore, the track (and thus the data) is always moving past the read laser at the same speed, which originally was defined as 3.49 meters per second (or 3.84mps on dual-layer discs). Because the track is a spiral that is wound more tightly near the center of the disc, the disc must spin at varying rates to maintain the same track linear speed. In other words, to maintain a CLV, the disk must spin more quickly when reading the inner track area and more slowly when reading the outer track area. The speed of rotation in a 1x drive (3.49 meters per second is considered 1x speed) varies from 1,515rpm when reading the start (inner part) of the track down to 570rpm when reading the end (outer part) of the track.

Single-speed (1x) DVD-ROM drives provide a data transfer rate of 1.385MBps, which means the data transfer rate from a DVD-ROM at 1x speed is roughly equivalent to a 9x CD-ROM (1x CD-ROM data transfer rate is 153.6KBps, or 0.1536MBps). This does not mean, however, that a 1x DVD drive can read CDs at 9x rates: DVD drives actually spin at a rate that is just under three times faster than a CD-ROM drive of the same speed. So, a 1x DVD drive spins at about the same rotational speed as a 2.7x CD drive. Many DVD drives list two speeds, one for reading DVDs and another for reading CDs. For example, a DVD-ROM drive listed as a 16x/40x would indicate the performance when reading DVDs/CDs, respectively.

As with CDs, drive manufacturers began increasing the speeds of their drives by making them spin more quickly. A drive that spins twice as fast was called a 2x drive, a drive that spins four times as fast was 4x, and so on. At higher speeds, it became difficult to build motors that could change speeds (spin up or down) as quickly as needed when data was read from different parts of the disc. Because of this, faster DVD drives spin the disc at a fixed rotational, rather than linear speed. This is termed constant angular velocity (CAV) because the angular velocity (or rotational speed) remains a constant.

The faster drives are useful primarily for data, not video. Having a faster drive can reduce or eliminate the pause during layer changes when playing a DVD video disc, but having a faster drive has no effect on video quality.

DVD-ROM drives have been available in speeds up to 20x or more, but because virtually all are CAV, they actually achieve the rated transfer speed only when reading the outer part of a disc. Table 13.25 shows the data rates for DVD drives reading DVDs and how that rate compares to a CD-ROM drive.

Table 13.25. DVD Speeds and Transfer Rates

Access Time

The access time for a CD or DVD drive is measured the same way as for PC hard disk drives. In other words, the access time is the delay between the drive receiving the command to read and its actual first reading of a bit of data. The time is recorded in milliseconds; a typical manufacturer's rating would be listed as 95ms. This is an average access rate; the true access rate depends entirely on where the data is located on the disc. When the read mechanism is positioned to a portion of the disc nearer to the narrower center, the access rate is faster than when it is positioned at the wider outer perimeter. Access rates quoted by many manufacturers are an average taken by calculating a series of random reads from a disc.

Obviously, a faster (that is, a lower) average access rate is desirable, especially when you rely on the drive to locate and pull up data quickly. Access times for CD and DVD drives have been steadily improving, and the advancements are discussed later in this chapter. Note that these average times are significantly slower than PC hard drives, ranging from 200ms to below 100ms, compared to the 8ms access time of a typical hard disk drive. Most of the speed difference lies in the construction of the drive itself. Hard drives have multiple-read heads that range over a smaller surface area of the medium; CD/DVD drives have only one laser pickup, and it must be capable of accessing the entire range of the disc. In addition, the data on a CD is organized in a single long spiral. When the drive positions its head to read a track, it must estimate the distance into the disc and skip forward or backward to the appropriate point in the spiral. Reading off the outer edge requires a longer access time than the inner segments, unless you have a CAV drive, which spins at a constant rate so the access time to the outer tracks is equal to that of the inner tracks.

Access times have fallen a great deal since the original single-speed drives came out. However, recently a plateau seems to have been reached with most CD/DVD drives hovering right around the 100ms area, with some as low as 80ms. With each increase in data transfer speed, you usually see an improvement in access time as well. But as you can see in Table 13.26, these improvements are much less significant because of the physical limitation of the drive's single-read mechanism design.

Table 13.26. Typical CD-ROM Drive Access Times
Drive Speed	Access Time (ms)
1x	400
2x	300
3x	200
4x	150
6x	150
8x–12x	100
16x–24x	90
32x–52x or greater	85 or less

The times listed here are typical examples for good drives; within each speed category some drives are faster and some are slower. Because of the additional positioning accuracy required and the overall longer track, DVD drives usually report two access speeds—one when reading DVDs and the other when reading CDs. The DVD access times typically run 10ms–20ms slower than when reading CDs.

Buffer/Cache

Most CD/DVD drives include internal buffers or caches of memory installed onboard. These buffers are actual memory chips installed on the drive's circuit board that enable it to stage or store data in larger segments before sending it to the PC. A typical buffer for a CD/DVD drive is 128KB, although drives are available that have either more or less (more is usually better). Recordable CD or DVD drives typically have much larger buffers of 2MB–8MB or more to prevent buffer underrun problems and to smooth writing operations. Generally, faster drives come with more buffer memory to handle the higher transfer rates.

Having buffer or cache memory for the CD/DVD drive offers a number of advantages. Buffers can ensure that the PC receives data at a constant rate; when an application requests data from the drive, the data can be found in files scattered across different segments of the disc. Because the drive has a relatively slow access time, the pauses between data reads can cause a drive to send data to the PC sporadically. You might not notice this in typical text applications, but on a drive with a slower access rate coupled with no data buffering, it is very noticeable—and even irritating—during the display of video or some audio segments. In addition, a drive's buffer, when under the control of sophisticated software, can read and have ready the disc's table of contents, speeding up the first request for data. A minimum size of 128KB for a built-in buffer or cache is recommended and is standard on many 24x and faster drives. For greater performance, look for drives with 256KB or larger buffers.

CPU Utilization

A once-neglected but very real issue in calculating computer performance is the impact that any piece of hardware or software has on the central processing unit (CPU). This "CPU utilization" factor refers to how much attention the CPU (such as Pentium III/4, Athlon, and so on) must provide to the hardware or software to help it work. A low CPU utilization percentage score is desirable because the less time a CPU spends on any given hardware or software process, the more time it has for other tasks and thus the greater the performance for your system. On CD-ROM drives, three factors influence CPU utilization: drive speed, drive buffer size, and interface type.

Drive buffer size can influence CPU utilization. For CD-ROM drives with similar performance ratings, the drive with a larger buffer is likely to require less CPU time (lower CPU utilization percentage) than the one with a smaller buffer.

Because drive speed and buffer size are more of a given, the most important factor influencing CPU utilization is the interface type. Traditionally, SCSI-interface CD-ROM drives have had far lower CPU utilization rates than ATAPI drives of similar ratings. One review of 12x drives done several years ago rated CPU utilization for ATAPI CD-ROM drives at 65%–80%, whereas SCSI CD-ROM drives checked in at less than 11%. By using DMA or Ultra-DMA modes with an ATA interface drive, near-SCSI levels of low CPU utilization can be realized. Using DMA or Ultra-DMA modes can cut CPU utilization down to the 10% or lower range, leaving the CPU free to run applications and other functions.

Direct Memory Access and Ultra-DMA

Busmastering ATA controllers use Direct Memory Access (DMA) or Ultra-DMA transfers to improve performance and reduce CPU utilization. Virtually all modern ATA drives support Ultra-DMA. With busmastering, CPU utilization for ATA/ATAPI and SCSI CD-ROM drives is about equal at around 11%. Thus, it's to your benefit to enable DMA access for your CD-ROM drives (and your ATA hard drives, too) if your system permits it.

Most recent ATA/ATAPI CD-ROM drives (12x and above) support DMA or Ultra-DMA transfers, as does Windows 95B and above and most recent Pentium-class or newer motherboards. To determine whether your Win9x, Me, or XP system has this feature enabled, check the System Properties' Device Manager tab and click the + mark next to Hard Disk Controllers. A drive interface capable of handling DMA transfers lists "Bus Master" in the name. Next, check the hard drive and CD-ROM information for your system. You can use the properties sheet for your system's CD-ROM drives under Windows 9x/Me and Windows 2000/XP to find this information; you might need to open the system to determine your hard drive brand and model. Hard disk drives and CD-ROM drives that support MultiWord DMA Mode 2 (16.6MBps), UltraDMA Mode 2 (33MBps), UltraDMA Mode 4 (66MBps), or faster can use DMA transfers. Check your product literature or the manufacturer's Web site for information.

To enable DMA transfers if your motherboard and drives support it, open the Device Manager and then open the Properties sheet for the controller or drive. Click the Settings or Advanced Settings tab, and make sure DMA is enabled if available. Depending on which version of Windows you are using, some have the DMA setting in the controller properties and others have it with the individual drives.

Repeat the same steps to enable DMA transfers for any additional hard drives and ATAPI CD-ROM drives in your computer. Restart your computer after making these changes.

Note

If your system hangs after you enable this feature, you must restart the system in Safe mode and uncheck the DMA box. Because DMA transfers bypass the CPU to achieve greater speed, DMA problems could result in data loss. Make backups first, instead of wishing you had later.

Also, if your drive is a parallel ATA model that supports any of the Ultra-DMA (also called Ultra-ATA) modes, you should upgrade your ATA cables to the 80-conductor style. Also be wary of using cables longer than the 18'' limit according to the ATA standard. Using these cables prevents noise and signal distortion that will occur if you try to use a standard 40-conductor cable with the Ultra-DMA modes. Most drives and motherboards refuse to enable Ultra-DMA modes faster than 33MBps if an 80-conductor cable is not detected. Note that these cabling issues affect only parallel ATA drives. If your drives are newer Serial ATA (SATA) models, these cabling issues do not apply.

Drive interfaces that don't mention busmastering either can't perform this speedup or need to have the correct driver installed. In some cases, depending on your Windows version and when your motherboard chipset was made, you must install chipset drivers to enable Windows to properly recognize the chipset and enable DMA modes. A good Web site for both Intel and non-Intel chipset support of this important feature is the Bus Mastering – Drivers and Links page at www.hardwaresite.net/drvbmide.html. Follow the links at this site to the motherboard chipset vendors, their technical notes (to determine whether your chipset supports busmastering), and the drivers you need to download. Virtually all motherboard chipsets produced since 1995 provide busmaster ATA support. Most of those produced since 1997 also provide UltraDMA support for up to 33MHz (Ultra-ATA/33) or 66MHz (Ultra-ATA/66) speed operation. Still, you should make sure that DMA is enabled to ensure you are benefiting from the performance it offers. Enabling DMA can dramatically improve DVD performance, for example.

Interface

The drive's interface is the physical connection of the drive to the PC's expansion bus. The interface is the data pipeline from the drive to the computer, and its importance shouldn't be minimized. Five types of interfaces are available for attaching a CD-ROM, CD-R, or CD-RW drive to your system:

SCSI/ASPI

SCSI (pronounced scuzzy), or the Small Computer System Interface, is a name given to a special interface bus that allows many types of peripherals to communicate.

A standard software interface called ASPI (Advanced SCSI Programming Interface) enables CD-ROM drives (and other SCSI peripherals) to communicate with the SCSI host adapter installed in the computer. SCSI offers the greatest flexibility and performance of the interfaces available for CD-ROM drives and can be used to connect many other types of peripherals to your system as well.

The SCSI bus enables computer users to string a group of devices along a chain from one SCSI host adapter, avoiding the complication of installing a separate adapter card into the PC bus slots for each new hardware device, such as a tape unit or additional CD-ROM drive added to the system. These traits make the SCSI interface preferable for connecting a peripheral such as a CD-ROM to your PC.

Not all SCSI adapters are created equal, however. Although they might share a common command set, they can implement these commands differently, depending on how the adapter's manufacturer designed the hardware. ASPI was created to eliminate these incompatibilities. ASPI was originally developed by Adaptec, Inc., a leader in the development of SCSI controller cards and adapters who originally named it the Adaptec SCSI Programming Interface before it became a de facto standard. ASPI consists of two main parts. The primary part is an ASPI-Manager program, which is a driver that functions between the operating system and the specific SCSI host adapter. The ASPI-Manager sets up the ASPI interface to the SCSI bus.

The second part of an ASPI system is the individual ASPI device drivers. For example, you would get an ASPI driver for your SCSI CD-ROM drive. You can also get ASPI drivers for your other SCSI peripherals, such as tape drives and scanners. The ASPI driver for the peripheral talks to the ASPI-Manager for the host adapter. This is what enables the devices to communicate together on the SCSI bus.

The bottom line is that if you are getting a SCSI interface CD-ROM, be sure it includes an ASPI driver that runs under your particular operating system. Also, be sure that your SCSI host adapter has the corresponding ASPI-Manager driver as well. Substantial differences exist between SCSI adapters because SCSI can be used for a wide variety of peripherals. Low-cost, SCSI-3-compliant ISA or PCI adapters can be used for CD-ROM interfacing. In contrast, higher-performance PCI adapters that support more advanced SCSI standards, such as Wide, Ultra, UltraWide, Ultra2Wide, and so on, can be used with both CD-ROM drives and other devices, such as CD-R/CD-RW drives, hard drives, scanners, and tape backups. To help you choose the appropriate SCSI adapter for both your CD-ROM drive and any other SCSI-based peripheral you're considering, visit Adaptec's Web site.

The SCSI interface offers the most powerful and flexible connection for CD-ROMs and other devices. It provides better performance, and seven or more drives can be connected to a single host adapter. The drawback is cost. If you do not need SCSI for other peripherals and intend to connect only one CD-ROM drive to the system, you will be spending a lot of money on unused potential. In that case, an ATAPI interface CD-ROM drive is a more cost-effective choice.

See "Small Computer System Interface," p. xxx. (Chapter 8)

ATA/ATAPI

The ATA/ATAPI (AT Attachment/AT Attachment Packet Interface) is an extension of the same ATA (AT Attachment) interface most computers use to connect to their hard disk drives. ATA is sometimes also referred to as IDE (Integrated Drive Electronics). ATAPI is an industry-standard ATA interface used for CD/DVD and other drives. ATAPI is a software interface that adapts the SCSI/ASPI commands to the ATA interface. This enables drive manufacturers to take their high-end CD/DVD drive products and quickly adapt them to the ATA interface. This also enables the ATA drives to remain compatible with the Extensions that provide the CD/DVD drive with a software interface with DOS. With Windows 9x and later, the CD-ROM extensions are contained in the CD file system (CDFS) VxD (virtual device) driver.

ATA/ATAPI drives are sometimes also called enhanced IDE (EIDE) drives because this is an extension of the original IDE (technically the ATA) interface. In most cases, an ATA drive connects to the system via a second ATA interface connector and channel, leaving the primary one for hard disk drives only. This is preferable because ATA does not share the single channel well and would cause a hard disk drive to wait for CD/DVD commands to complete and vice versa. SCSI does not have this problem because a SCSI host adapter can send commands to different devices without having to wait for each previous command to complete.

The ATA interface represents the most cost-effective and high-performance interface for CD-ROM drives. Most new systems that include a CD and/or DVD drive have it connected through ATA. You can connect up to two drives to the secondary ATA connector; for more than that, SCSI is your only choice and provides better performance as well.

Many systems on the market today can use the ATA/ATAPI CD/DVD drive as a bootable device, which allows the vendor to supply a recovery CD that can restore the computer's software to its factory-shipped condition. Later, you'll see how bootable CDs differ from ordinary CDs and how you can use low-cost CD-R/CD-RW drives, along with mastering and imaging software to make your own bootable CDs with your own preferred configuration.

See "ATA Features," p. 508.

See "Bootable CDs and DVDs—El Torito," p. 786.

Parallel Port

In the past, some external CD-ROM or CD-RW drives were available in versions that connected to a PC's parallel port. USB has for the most part replaced the parallel port for this type of use.

If you use a parallel port drive, for best performance you should set your printer port to use IEEE-1284 standards, such as ECP/EPP or ECP, before connecting your parallel-port CD-ROM. These are bidirectional, high-speed extensions to the normal Centronics parallel port standard and provide better performance for virtually any recent parallel device. If your operating system supports Plug and Play (PnP) (such as Windows 9x/Me or 2000/XP does), simply plugging a PnP drive into the parallel port enables the OS to detect the new hardware and load the appropriate driver automatically.

See "IEEE-1284 Parallel Port Standard," p. 971.

Note

Parallel port CD-ROM drives nearly always include a cable with a pass-through connector. This connector plugs into the parallel port and, if necessary, a printer cable can plug into the connector. This enables you to continue using the port to connect to your printer while sharing the interface with the CD-ROM drive. Note there might be performance problems when trying to print and read from the drive at the same time.

With the popularity, performance, and ease of use of USB, I recommend a USB external drive over a parallel port version. USB drives are much faster, more compatible, and easier to install and use. If you need parallel port support for some systems, look for drives that feature parallel and USB interfaces, such as the Backpack product line from Micro Solutions, Inc. See the Vendor list on the DVD for Web site and contact information.

USB Interface

Universal Serial Bus (USB) has proven to be extremely flexible and has been used for everything from keyboards and joysticks to CD/DVD drives from several vendors.

USB 1.1 and earlier drives provide read and write transfer rates that match the fastest rates possible with IEEE-1284 parallel ports, with read rates on typical 6x models ranging from 1,145KBps to 1,200KBps. USB 2.0 and later provide a transfer rate up to 60MBps, which is 40 times faster than USB 1.1 and yet is fully backward-compatible.

USB also provides benefits that no parallel port drive can match: for example, hot-swappability, which is the capability to be plugged in or unplugged without removing the power or rebooting the system. Additionally, USB devices are fully Plug and Play, allowing the device to be automatically recognized by the system and the drivers automatically installed.

For Windows 98/Me or Windows 2000/XP systems with USB ports, USB-based CD-RW drives are an excellent solution for backup and archiving of data onto low-cost, durable optical media.

FireWire

In addition, external CD/DVD drives are now available on the market with a FireWire (also called IEEE-1394 or iLink) interface. FireWire is a high-performance external interface designed mainly for video use. It evolved as an Apple standard and is used primarily on Macintosh systems. Because few PCs include FireWire ports as a standard item—whereas all PCs include USB—I usually recommend the more universally supported USB for external CD/DVD drives. Make sure any external drives you purchase use the faster USB 2.0 (also known as Hi-Speed USB), which is faster and far more readily available than FireWire versions. FireWire drives can be useful if you work in a two-platform environment (both PCs and Macs). However, because most Macs also support USB (and you can easily add a USB interface to those that don't), if your primary platform is the PC, I'd still recommend USB over FireWire.

If you do want to use a FireWire drive and your system does not include FireWire integrated into the motherboard, you can easily add a FireWire interface card to your PC. Additionally, some video and sound cards are available with FireWire ports as an option.

See "Universal Serial Bus," p. 947, and "IEEE-1394," p. 956.

Loading Mechanism

Three distinctly different mechanisms exist for loading a disc into a CD/DVD drive: the tray, caddy, and slot. Each one offers some benefits and features. Which type you select has a major impact on your use of the drive because you interact with this mechanism every time you load a disc.

Some drives on the market allow you to insert more than one disc at a time. Some of these use a special cartridge that you fill with discs, much like multidisc CD changers used in automobiles. Newer models are slot-loading, allowing you to push a button to select which internal cartridge slot you want to load with a CD/DVD. The drive's door opens and you slide in the CD, which the drive mechanism grabs and pulls into place. Typical capacities range from three to six discs or more, and these are available in both SCSI and ATA interfaces.

Tray

Most current SCSI and ATAPI CD/DVD drives use a tray-loading mechanism. This is similar to the mechanism used with a stereo system. Because you don't need to put each disc into a separate caddy, this mechanism is much less expensive overall. However, it also means that you must handle each disc every time you insert or remove it.

Tray loading is less expensive than a caddy system (see the section, "Caddy," later in this chapter) because you don't need a caddy. It is also more convenient, unless you have caddies for all your discs. However, this can make it much more difficult for young children or those who work in harsh environments to use the discs without smudging or damaging them due to excess handling.

The tray loader itself is also subject to damage. The trays can easily break if bumped or if something is dropped on them while they are extended. Also, any contamination you place on the tray or disc is brought right into the drive when the tray is retracted. Tray-loaded drives should not be used in a harsh environment, such as a commercial or an industrial application. Make sure both the tray and the data surface of the disc are clean whenever you use a tray-loading drive.

The tray mechanism also does not hold the disc as securely as the caddy. If you don't have the disc placed in the tray properly when it retracts, the disc or tray can be damaged. Even a slight misalignment can prevent the drive from reading the disc properly, forcing you to open the tray and reset the disc.

Some tray drives can't operate in a vertical (sideways) position because gravity prevents proper loading and operation. Check to see whether the drive tray has retaining clips that grab the hub of the disc or tabs that fold in or flip over from the outside of the tray to retain the disc. If so, you can run the drive in either a horizontal or vertical position.

The main advantage of the tray mechanism over the others is in cost, and that is a big factor. Most drives today use the tray mechanism for handling discs.

Caddy

At one time, the caddy system was used on most high-end CD-ROM drives as well as the early CD-R and DVD-RAM drives. The caddy system requires that you place the disc itself into a special caddy, which is a sealed container with a metal shutter. The caddy has a hinged lid you open to insert the disc, but after that the lid remains shut. When you insert the caddy containing the disc into the drive, the drive opens a metal shutter on the bottom of the caddy, allowing access to the disc by the laser.

When caddy-loaded drives were popular, they were extremely convenient to use if you had a caddy for each drive and extremely inconvenient if you shared a single caddy among all your media. The caddy was inserted into the drive, much the way you would insert a 3 1/2'' floppy disk. The caddy protected the CD from scratches, contamination, and careless handling.

The drawbacks to the caddy system included the expense and the inconvenience of having to put the discs into the caddies.

When DVD-RAM was first introduced, the disc had to remain in a caddy because the recordable surface is delicate. Since then, DVD-RAM drives have been made caddy-less, but especially with double-sided discs the information is at risk every time you handle the disc. Because of this fragility, as well as the general incompatibility of DVD-RAM with DVD-ROM, I recommend DVD+RW as the best solution for recordable DVD. No caddy is required with DVD+RW, and the format is fully two-way compatible with most recent DVD-Video players and DVD-ROM drives.

The caddy-loading system has declined in popularity because of the convenience of the tray. Today only a few drives on the market use caddies, and they are generally not for mainstream use (most have a SCSI interface, for example).

Slot

Some drives now use a slot-loading mechanism, identical to that used in most automotive CD players. This is very convenient because you just slip the disc into the slot, where the mechanism grabs it and draws it inside. Some drives can load several CDs at a time this way, holding them internally inside the drive and switching discs as access is required.

The primary drawback to this type of mechanism is that if a jam occurs, it can be much more difficult to repair because you might have to remove the drive to free the disc. Another drawback is that slot-loading drives usually can't handle the smaller 80mm discs, card-shaped discs, or other modified disc physical formats or shapes.

Other Drive Features

Although drive specifications are of the utmost importance, you should also consider other factors and features when evaluating CD-ROM drives. Besides quality of construction, the following criteria bear scrutiny when making a purchasing decision:

Drive Sealing

Dirt is your CD/DVD drive's biggest enemy. Dust or dirt, when it collects on the lens portion of the mechanism, can cause read errors or severe performance loss. Many manufacturers seal off the lens and internal components from the drive bay in airtight enclosures. Other drives, although not sealed, have double dust doors—one external and one internal—to keep dust from the inside of the drive. All these features help prolong the life of your drive.

Some drives are sealed, which means no air flows through the chamber in which the laser and lens reside. Always look for sealed drives in harsh industrial or commercial environments. In a standard office or home environment, it is probably not worth the extra expense.

Self-Cleaning Lenses

If the laser lens gets dirty, so does your data. The drive will spend a great deal of time seeking and reseeking or will finally give up. Lens-cleaning discs are available, but built-in cleaning mechanisms are now included on virtually all good-quality drives. This might be a feature you'll want to consider, particularly if you work in a less-than-pristine work environment or have trouble keeping your desk clean, let alone your drive laser lens. You can clean the lens manually, but it is generally a delicate operation requiring that you partially disassemble the drive. Also, damaging the lens mechanism by using too much force is pretty easy to do. Because of the risks involved, in most cases I do not recommend the average person disassemble and try to manually clean the laser lens.

Internal Versus External Drives

When deciding whether you want an internal or external drive, think about where and how you're going to use your drive. What about the future expansion of your system? Both types of drives have advantages and disadvantages, such as the following:

Upgrading and Repairing PCs