CHAPTER 13

722 Chapter 13 Optical Storage
The center hole in a DVD is 15mm in diameter, so it has a radius of 7.5mm from the center of the
disc. From the edge of the center hole to a point at a radius of 16.5mm is the hub clamp area. The
lead-in zone starts at a radius of 22mm from the center of the disc. The data zone starts at a radius of
24mm from the center and is followed by the lead-out (or middle) zone at 58mm. The disc track officially
ends at 58.5mm, which is followed by a 1.5mm blank area to the edge of the disc. Figure 13.8
shows these zones in actual relative scale as they appear on a DVD.
Figure 13.8 Areas on a DVD (side view).
Officially, the spiral track of a standard DVD starts with the lead-in zone and ends at the finish of the
lead-out zone. This single spiral track is about 11.84 kilometers or 7.35 miles long. An interesting fact
is that in a 20x CAV drive, when reading the outer part of the track, the data moves at an actual speed
of 156 miles per hour (251km/h) past the laser. What is more amazing is that even when the data is
traveling at that speed, the laser pickup can accurately read bits (pit/land transitions) spaced as little
as only 0.4 microns or 15.75 millionths of an inch apart!
DVDs come in both single- and dual-layer as well as single- and double-sided versions. The doublesided
discs are essentially the same as two single-sided discs glued together back to back, but subtle
differences do exist between the single- and dual-layer discs. Table 13.17 shows some of the basic
information about DVD technology, including single- and dual-layer DVDs. The dual-layer versions
are recorded with slightly longer pits, resulting in slightly less information being stored in each layer.
Table 13.17 DVD Technical Parameters
DVD Type: Single-Layer Dual-Layer
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1x read speed (m/sec) 3.49 3.84
Laser wavelength (nm) 650 650
Media refractive index 1.55 1.55
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Track (turn) spacing (um) 0.74 0.74
Turns per mm 1,351 1,351
Turns per inch 34,324 34,324
Total track length (m) 11,836 11,836
Total track length (feet) 38,832 38,832
Total track length (miles) 7.35 7.35
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