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Optical 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. This section examines these specifications.

CD Data Transfer Rate

The data transfer rate for a CD 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 optical drives is the "x" speed, which is defined as a multiple of the particular standard base rate. For example, CD 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 today's optical drives supporting multiple disc formats, multiple read and write specifications are given for each form of media a drive supports.

CD Drive Speed

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 data is recorded using a technique called constant linear velocity (CLV).

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 2,568 rpm to 5,959 rpm 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 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 11.23 compares CLV and CAV.

Table 11.23. CLV Versus CAV Technology Quick Reference

CLV (Constant Linear Velocity)

CAV (Constant Angular Velocity)

Speed of CD rotation

Varies with data position on disc. Faster on inner tracks than on outer tracks.

Constant.

Data transfer rate

Constant.

Varies with data position on disc. 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. 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.

Table 11.24 contains data showing CD drive speeds along with transfer rates and other interesting data. This information also applies to DVD or BD drives when CDs are used.

Table 11.24. CD-ROM Drive Speeds and Transfer Rates

Advertised CD-ROM Speed (Max. if CAV)

Time to Read 74-Minute CD if CLV

Time to Read 80-Minute CD if CLV

Transfer Rate (Bps) (Max. if CAV)

Actual CD-ROM Speed Minimum in CAV

Minimum Transfer Rate if CAV (Bps)

Average CD-ROM Speed if CAV

Average Transfer Rate if CAV (Bps)

Maximum Linear Speed (mps)

Maximum Linear Speed (mph)

Rotational Speed Min. if CLV Max. if CAV (rpm)

Rotational Speed Max. if CLV (rpm)

1x

74.0

80.0

153,600

0.4x

61,440

0.7x

107,520

1.3

2.9

214

497

2x

37.0

40.0

307,200

0.9x

138,240

1.5x

222,720

2.6

5.8

428

993

4x

18.5

20.0

614,400

1.7x

261,120

2.9x

437,760

5.2

11.6

856

1,986

6x

12.3

13.3

921,600

2.6x

399,360

4.3x

660,480

7.8

17.4

1,284

2,979

8x

9.3

10.0

1,228,800

3.4x

522,240

5.7x

875,520

10.4

23.3

1,712

3,973

10x

7.4

8.0

1,536,000

4.3x

660,480

7.2x

1,098,240

13.0

29.1

2,140

4,966

12x

6.2

6.7

1,843,200

5.2x

798,720

8.6x

1,320,960

15.6

34.9

2,568

5,959

16x

4.6

5.0

2,457,600

6.9x

1,059,840

11.5x

1,758,720

20.8

46.5

3,425

7,945

20x

3.7

4.0

3,072,000

8.6x

1,320,960

14.3x

2,196,480

26.0

58.2

4,281

9,931

24x

3.1

3.3

3,686,400

10.3x

1,582,080

17.2x

2,634,240

31.2

69.8

5,137

11,918

32x

2.3

2.5

4,915,200

13.8x

2,119,680

22.9x

3,517,440

41.6

93.1

6,849

15,890

40x

1.9

2.0

6,144,000

17.2x

2,641,920

28.6x

4,392,960

52.0

116.3

8,561

19,863

48x

1.5

1.7

7,372,800

20.7x

3,179,520

34.4x

5,276,160

62.4

139.6

10,274

23,835

50x

1.5

1.6

7,680,000

21.6x

3,317,760

35.8x

5,498,880

65.0

145.4

10,702

24,828

52x

1.4

1.5

7,987,200

22.4x

3,440,640

37.2x

5,713,920

67.6

151.2

11,130

25,821

56x

1.3

1.4

8,601,600

24.1x

3,701,760

40.1x

6,151,680

72.8

162.8

11,986

27,808

Each of the columns in Table 11.24 is explained here. Column 1 indicates the advertised drive speed. This is a constant speed if the drive is CLV (most 12x and lower) or a maximum speed only if CAV.

Columns 2 and 3 indicate how long it would take to read a full disc if the drive was CLV. For CAV drives, those figures would be longer because the average read speed is less than the advertised speed. The fourth column indicates the data transfer rate, which for CAV drives would be a maximum figure only when reading the end of a disc.

Columns 3–6 indicate the actual minimum "x" speed for CAV drives, along with the minimum transfer speed (when reading the start of any disc) and an optimistic average speed (true only when reading a full disc; otherwise, it would be even lower) in both "x" and byte-per-second formats.

Columns 7–8 indicate the maximum linear speeds the drive will attain, in both meters per second and miles per hour. CLV drives maintain those speeds everywhere on the disc, whereas CAV drives reach those speeds only on the outer part of a disc.

Columns 9–12 indicate the rotational speeds of a drive. The first of these shows how fast the disc spins when being reading from the start; this applies to either CAV or CLV drives. For CAV drives, the figure is constant no matter what part of the disc is being read. The last column shows the maximum rotational speed if the drive were a CLV type. Because most drives over 12x are CAV, these figures are mostly theoretical for the 16x and faster drives.

Vibration problems can cause high-speed drives to drop to lower speeds to enable reliable reading. Your disc can become unbalanced, for example, if you apply a small paper label to its surface to identify the disc. For this reason, many of the faster optical 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.

Most recent optical drives use Z-CLV (zoned CLV) or P-CAV (partial CAV) designs, which help increase average performance while keeping rotational speeds under control.

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 3.84 mps 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 the inner track area is being read and more slowly when the outer track area is being read. The speed of rotation in a 1x drive (3.49 meters per second is considered 1x speed) varies from 1,515 rpm when reading the start (inner part) of the track down to 570 rpm when reading the end (outer part) of the track.

Single-speed (1x) DVD drives provide a data transfer rate of 1.385MBps, which means the data transfer rate from a DVD at 1x speed is roughly equivalent to a 9x CD (1x CD 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 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, for example, a DVD drive listed as a 16x/40x would indicate the performance when reading DVDs/CDs, respectively.

As with CD drives, DVD 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 speed rather than linear speed. This is termed 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 drives are 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 11.25 shows the data rates for DVD drives reading DVDs and how that rate compares to a CD drive.

Table 11.25. DVD Speeds and Transfer Rates

Advertised DVD-ROM Speed (Max. if CAV)

Time to Read Single-Layer DVD if CLV

Time to Read Dual Layer DVD if CLV

Transfer Rate (Bytes/sec) (Max. if CAV)

Actual DVD Speed Minimum in CAV

Minimum Transfer Rate if CAV (Bytes/sec)

Average DVD Speed if CAV

Average Transfer Rate if CAV (Bytes/sec)

Maximum Linear Speed (m/sec)

Maximum Linear Speed (rpm)

Single-Layer Rot. Speed Min. if CLV Max. if CAV (rpm)

Single Layer Rot. Speed Max. if CLV (rpm)

Usual Transfer Rate When Reading CD-ROMs

1x

56.5

51.4

1,384,615

0.4x

553,846

0.7x

969,231

3.5

7.8

570

1,515

2.7x

2x

28.3

25.7

2,769,231

0.8x

1,107,692

1.4x

1,938,462

7.0

15.6

1,139

3,030

5.4x

4x

14.1

12.8

5,538,462

1.7x

2,353,846

2.9x

3,946,154

14.0

31.2

2,279

6,059

11x

6x

9.4

8.6

8,307,692

2.5x

3,461,538

4.3x

5,884,615

20.9

46.8

3,418

9,089

16x

8x

7.1

6.4

11,076,923

3.3x

4,569,231

5.7x

7,823,077

27.9

62.5

4,558

12,119

21x

10x

5.7

5.1

13,846,154

4.1x

5,676,923

7.1x

9,761,538

34.9

78.1

5,697

15,149

27x

12x

4.7

4.3

16,615,385

5.0x

6,923,077

8.5x

11,769,231

41.9

93.7

6,836

18,178

32x

16x

3.5

3.2

22,153,846

6.6x

9,138,462

11.3

15,646,154

55.8

124.9

9,115

24,238

43x

20x

2.8

2.6

27,692,308

8.3x

11,492,308

14.2

19,592,308

69.8

156.1

11,394

30,297

54x

24x

2.4

2.1

33,230,769

9.9x

13,707,692

17.0

23,469,231

83.8

187.4

13,673

36,357

64x

32x

1.8

1.6

44,307,692

13.2x

18,276,923

22.6

31,292,308

111.7

249.8

18,230

48,476

86x

40x

1.4

1.3

55,384,615

16.6x

22,984,615

28.3

39,184,615

139.6

312.3

22,788

60,595

107x

48x

1.2

1.1

66,461,538

19.9x

27,553,846

34.0

47,007,692

167.5

374.7

27,345

72,714

129x

50x

1.1

1.0

69,230,769

20.7x

28,661,538

35.4

48,946,154

174.5

390.3

28,485

75,743

134

Each of the columns in Table 11.25 is explained here

Column 1 indicates the advertised drive speed. This is a constant speed if the drive is CLV or a maximum speed only if CAV (most DVD drives are CAV).

Columns 2 and 3 indicate how long it would take to read a full disc (single- or dual-layer) if the drive were CLV. For CAV drives, those figures are longer because the average read speed is less than the advertised speed. The fourth column indicates the data transfer rate, which for CAV drives is a maximum figure seen only when reading the end of a disc.

Columns 4–8 indicate the actual minimum "x" speed for CAV drives, along with the minimum transfer speed (when reading the start of any disc) and an optimistic average speed (true only when reading a full disc; otherwise, it's even lower) in both "x" and byte-per-second formats.

Columns 9 and 10 indicate the maximum linear speeds the drive attains, in both meters per second and miles per hour. CLV drives maintain those speeds everywhere on the disc, whereas CAV drives reach those speeds only on the outer part of a disc.

Columns 11 and 12 indicate the rotational speeds of a drive. The first of these shows how quickly the disc spins when being read from the start. This applies to either CAV or CLV drives. For CAV drives, the figure is constant no matter what part of the disc is being read. The second of these two columns shows the maximum rotational speed if the drive were a CLV type. Because most faster drives are CAV, these figures are mostly theoretical for the faster drives.

Column 13 shows the speed the drive would be rated if it were a CD drive. This is based on the rotational speed, not the transfer rate. In other words, a 12x DVD drive would perform as a 32x CD drive when reading CDs. Most DVD drives list their speeds when reading CDs in the specifications. Due to the use of PCAV (Partial CAV) designs, some might have higher CD performances than the table indicates.

Access Time

The access time for an optical 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. Access rates quoted by many manufacturers are an average taken by calculating a series of random reads from a disc.

Buffer/Cache

Most optical 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 can range from 2MB up to 8MB or more (depending on the drive). Generally, faster rewritable drives come with more buffer memory to handle the higher transfer rates.

Direct Memory Access and Ultra-DMA

Busmastering PATA controllers use Direct Memory Access (DMA) or Ultra-DMA transfers to improve performance and reduce CPU utilization. Virtually all modern PATA drives support Ultra-DMA utilization.

To determine whether your system has this feature enabled, open the Device Manager and check the properties sheet for the controller to view its capabilities.

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.

If your drive is a parallel ATA model that supports any of the Ultra-DMA (also called Ultra-ATA) modes, you need to use an 80-conductor cable. Most 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 Serial ATA (SATA) models, these cabling issues do not apply.

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. 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 you shouldn't minimize its importance. Four types of interfaces are normally used for attaching an optical drive to your system:

  • SATA (Serial ATA)—The SATA interface is the same interface used by most recent computers for connecting their hard disk drives. With many recent systems featuring support for as little as one PATA (Parallel ATA) drive, but support for eight or more SATA drives, most optical drive vendors are now producing SATA versions of their drives.Compared to similar PATA optical drives, SATA drives feature equal performance, but are easier to install because it is not necessary to jumper the drive for master/slave or cable select.
  • PATA (Parallel AT Attachment)—The PATA interface is the same interface most older computers use to connect to their hard disk drives. PATA is sometimes also referred to as ATA (AT Attachment) or IDE (Integrated Drive Electronics).
  • USB port—Universal serial bus (USB) is normally used for external drives, and provides benefits such as hot-swappability, which is the capability to be plugged in or unplugged without removing the power or rebooting the system. USB 2.0 is the most common, but USB 3.0 drives might be available in the future.
  • FireWire (IEEE 1394)—A few external optical drives are available with a FireWire (also called IEEE 1394 or i.LINK) interface instead of, or in addition to USB 2.0.
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See "Universal Serial Bus (USB)," p. 692 and "IEEE 1394 (FireWire or i.LINK)," p. 707 (Chapter 14, "External I/O Interfaces").

Some older drives were available in external versions using SCSI/ASPI (Small Computer System Interface/Advanced SCSI Programming Interface) or parallel printer port interfaces, but these are obsolete.

Loading Mechanism

Three distinctly different mechanisms exist for loading a disc into an optical drive: the tray, caddy, and slot.

Most current 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.

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 cost, and that is a big factor. Most drives today use the tray mechanism for handling discs.

Caddy systems have been used on several types of optical drives. The caddy system requires that you place the disc 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.

The drawbacks to the caddy system include the expense and the inconvenience of having to put the discs into the caddies. Caddy-loaded drives were popular in early CD drives, but few were made or sold after 1994.

Some drives use a slot-loading mechanism, identical to that used in most automotive players. This is convenient because you just slip the disc into the slot, where the mechanism grabs it and draws it inside. Some drives can load several discs 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, such as DualDisc.

Other Drive Features

Although drive specifications are of the utmost importance, you should consider other factors and features when evaluating optical drives. Besides quality of construction, the presence of drive sealing or self-cleaning lenses bears scrutiny when you are making a purchasing decision.

Dirt is your 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.

To determine whether a particular drive is sealed, you may need to view FAQ or support questions considering drive cleaning; this information may not always be listed on the drives' spec sheet.

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.

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