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DVD Chapter <strong>13</strong> 721<br />
The DVD disc’s pits and lands are much smaller and closer together than those on a CD, allowing the<br />
same physical-sized platter to hold much more information. Figure <strong>13</strong>.7 shows how the grooved<br />
tracks with pits and lands are just over four times as dense on a DVD as compared to a CD.<br />
DVD CD<br />
Figure <strong>13</strong>.7 DVD data markings (pits and lands) versus those of a standard CD.<br />
DVD drives use a shorter wavelength laser to read these smaller pits and lands. DVD can have nearly<br />
double the initial capacity by using two separate layers on one side of a disc and double it again by<br />
using both sides of the disc. The second data layer is written to a separate substrate below the first<br />
layer, which is then made semireflective to enable the laser to penetrate to the substrate beneath it.<br />
By focusing the laser on one of the two layers, the drive can read roughly twice the amount of data<br />
from the same surface area.<br />
DVD Tracks and Sectors<br />
The pits are stamped into a single spiral track (per layer) with a spacing of 0.74 microns between<br />
turns, corresponding to a track density of 1,351 turns per millimeter or 34,324 turns per inch. This<br />
equates to a total of 49,324 turns and a total track length of 11.8km or 7.35 miles in length. The track<br />
is comprised of sectors, with each sector containing 2,048 bytes of data. The disc is divided into four<br />
main areas:<br />
■ Hub clamping area. The hub clamp area is just that, a part of the disc where the hub mechanism<br />
in the drive can grip the disc. No data or information is stored in that area.<br />
■ Lead-in zone. The lead-in zone contains buffer zones, reference code, and mainly a control data<br />
zone with information about the disc. The control data zone consists of 16 sectors of information<br />
repeated 192 times for a total of 3,072 sectors. Contained in the 16 (repeated) sectors is<br />
information about the disc, including disc category and version number, disc size and maximum<br />
transfer rate, disc structure, recording density, and data zone allocation. The entire lead-in<br />
zone takes up to 196,607 (2FFFFh) sectors on the disc. Unlike CDs, the basic structure of all sectors<br />
on a DVD are the same. The buffer zone sectors in the lead-in zone have all 00h (zero hex)<br />
recorded for data.<br />
■ Data zone. The data zone contains the video, audio, or other data on the disc and starts at sector<br />
number 196,608 (30000h). The total number of sectors in the data zone can be up to 2,292,897<br />
per layer for single layer discs.<br />
■ Lead-out (or middle) zone. The lead-out zone marks the end of the data zone. The sectors in the<br />
lead-out zone all contain zero (00h) for data. This is called the middle zone if the disc is dual<br />
layer and is recorded in opposite track path (OPT) mode, in which the second layer starts from<br />
the outside of the disc and is read in the opposite direction from the first layer.
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