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

• The drive's performance specifications

• The interface the drive requires for connection to your PC

• The physical disc-handling system the drive uses

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.

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.

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.

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.

DMA 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.

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.