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DVD Chapter <strong>13</strong> 731<br />
2.7x CD drive. Many DVD drives list two speeds, one for reading DVD discs and another for reading<br />
CD discs. For example, a DVD-ROM drive listed as a 16x/40x would indicate the performance when<br />
reading DVD/CD discs, respectively.<br />
As with CDs, drive manufacturers began increasing the speeds of their drives by making them spin<br />
more quickly. A drive that spins twice as fast was called a 2x drive, drive that spins four times faster<br />
was 4x, and so on. At higher speeds, it became difficult to build motors that could change speeds<br />
(spin up or down) as quickly as needed when data was read from different parts of the disc. Because of<br />
this, most faster DVD drives spin the disc at a fixed rotational, rather than linear speed. This istermed<br />
constant angular velocity (CAV) because the angular velocity (or rotational speed) is what remains a<br />
constant.<br />
The faster drives are useful primarily for data, not video. Having a faster drive can reduce or eliminate<br />
the pause during layer changes when playing a DVD video disc, but having a faster drive has no effect<br />
on video quality.<br />
DVD-ROM drives have been available in speeds up to 20x or more, but because virtually all are CAV,<br />
they actually achieve the rated transfer speed only when reading the outer part of a disc. Table <strong>13</strong>.20<br />
shows the data rates for DVD drives reading DVD discs and how that rate compares to a CD-ROM<br />
drive.<br />
Column 8 Column 9 Column 10 Column 11 Column 12 Column <strong>13</strong><br />
Single Layer Usual<br />
Average Maximum Maximum Rot. Speed Single Layer Transfer<br />
Transfer Linear Linear Min. if CLV Rot. Speed Rate When<br />
Rate if CAV Speed Speed Max. if CAV Max. if CLV Reading<br />
(Bytes/sec) (m/sec) (mph) (rpm) (rpm) CD-ROMs<br />
969,231 3.5 7.8 570 1,515 2.7x<br />
1,938,462 7.0 15.6 1,<strong>13</strong>9 3,030 5.4x<br />
3,946,154 14.0 31.2 2,279 6,059 11x<br />
5,884,615 20.9 46.8 3,418 9,089 16x<br />
7,823,077 27.9 62.5 4,558 12,119 21x<br />
9,761,538 34.9 78.1 5,697 15,149 27x<br />
11,769,231 41.9 93.7 6,836 18,178 32x<br />
15,646,154 55.8 124.9 9,115 24,238 43x<br />
19,592,308 69.8 156.1 11,394 30,297 54x<br />
23,469,231 83.8 187.4 <strong>13</strong>,673 36,357 64x<br />
31,292,308 111.7 249.8 18,230 48,476 86x<br />
39,184,615 <strong>13</strong>9.6 312.3 22,788 60,595 107x<br />
47,007,692 167.5 374.7 27,345 72,714 129x<br />
48,946,154 174.5 390.3 28,485 75,743 <strong>13</strong>4x
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