CHAPTER 13

730 Chapter 13 Optical Storage
Note that although no more than 10 0s are allowed in the modulation generated by EFM+, the sync
bits added when physical sectors are written can have up to 13 0s, meaning a time period of up to
14T between 1s written on the disc and pits or lands up to 14T intervals or bit cells in length.
DVD Drive Speed
As with CDs, DVDs rotate counterclockwise (as viewed from the reading laser) and typically are
recorded at a constant data ratecalled 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 3.49 meters per second
(or 3.84m/sec 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 1.385MB/second, 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 153.6KB/s, or 0.1536MB/s). 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
Table 13.20 DVD Speeds and Transfer Rates
Column 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7
Time Time Transfer Actual Minimum
Advertised to Read to Read Rate DVD Speed Transfer Average
DVD-ROM Speed Single Layer Dual Layer (Bytes/sec) Minimum Rate if CAV DVD Speed
(Max. if CAV) DVD if CLV DVD if CLV (Max. if CAV) if CAV (Bytes/sec) if CAV
1x 56.5 51.4 1,384,615 0.4x 553,846 0.7x
2x 28.3 25.7 2,769,231 0.8x 1,107,692 1.4x
4x 14.1 12.8 5,538,462 1.7x 2,353,846 2.9x
6x 9.4 8.6 8,307,692 2.5x 3,461,538 4.3x
8x 7.1 6.4 11,076,923 3.3x 4,569,231 5.7x
10x 5.7 5.1 13,846,154 4.1x 5,676,923 7.1x
12x 4.7 4.3 16,615,385 5.0x 6,923,077 8.5x
16x 3.5 3.2 22,153,846 6.6x 9,138,462 11.3
20x 2.8 2.6 27,692,308 8.3x 11,492,308 14.2
24x 2.4 2.1 33,230,769 9.9x 13,707,692 17.0
32x 1.8 1.6 44,307,692 13.2x 18,276,923 22.6
40x 1.4 1.3 55,384,615 16.6x 22,984,615 28.3
48x 1.2 1.1 66,461,538 19.9x 27,553,846 34.0
50x 1.1 1.0 69,230,769 20.7x 28,661,538 35.4