Hardware Reference
In-Depth Information
Figure 11.9. DVD data markings (pits and lands) versus those of a standard CD.
DVD drives use a shorter wavelength laser (650nm) to read these smaller pits and lands. A DVD can
have nearly double the initial capacity by using two separate layers on one side of a disc and double
it again by using both sides of the disc. The second data layer is written to a separate substrate below
the first layer, which is then made semi-reflective to enable the laser to penetrate to the substrate
beneath it. By focusing the laser on one of the two layers, the drive can read roughly twice the amount
of data from the same surface area.
DVD Tracks and Sectors
The pits are stamped into a single spiral track (per layer) with a spacing of 0.74 microns between
turns, corresponding to a track density of 1,351 turns per millimeter or 34,324 turns per inch. This
equates to a total of 49,324 turns and a total track length of 11.8km or 7.35 miles in length. The track
is composed of sectors, with each sector containing 2,048 bytes of data. The disc is divided into four
main areas:
•
Hub clamping area
—The hub clamp area is just that: a part of the disc where the hub
mechanism in the drive can grip the disc. No data or information is stored in that area.
•
Lead-in zone
—The lead-in zone contains buffer zones, reference code, and mainly a control
data zone with information about the disc. The control data zone consists of 16 sectors of
information repeated 192 times, for a total of 3,072 sectors. Contained in the 16 (repeated)
sectors is information about the disc, including disc category and version number, disc size and
maximum transfer rate, disc structure, recording density, and data zone allocation. The entire
lead-in zone takes up to 196,607 (2FFFFh) sectors on the disc. Unlike CDs, the basic structure
of all sectors on a DVD is the same. The buffer zone sectors in the lead-in zone have all 00h
(zero hex) recorded for data.
•
Data zone
—The data zone contains the video, audio, or other data on the disc and starts at
sector number 196,608 (30000h). The total number of sectors in the data zone can be up to
2,292,897 per layer for single-layer discs.
•
Lead-out (or middle) zone
—The lead-out zone marks the end of the data zone. All the sectors
in the lead-out zone contain zero (00h) for data. This is called the middle zone if the disc is
dual-layer and is recorded in opposite track path (OPT) mode, in which the second layer starts
from the outside of the disc and is read in the opposite direction from the first layer.
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
