Hardware Reference
In-Depth Information
A single-sided, single-layer DVD holds 4.7 GB; when you double the layers, that amount
approximately doubles, and when you double the sides on which the disc can be written,
you double the capacity once again. Table 2.4 provides a quick summary
A double-layer disc has two sets of optical media on top of each other. The
upper layer is semi-translucent; that is, it will let through a certain level of
light without altering its data, so that two sets of data can coexist in the
same surface area. Double-layer discs are common in mass-produced
video DVDs because they enable longer movies to be placed on a single
DVD. Many PC DVD drives can read and write double-layer discs too.
TABLE 2.4
DVD capacity
Single Sided
Double Sided
Single Layer
Double Layer
Capacity (Gigabytes)
4.7
8.54
9.4
17.08
Aside from storing more data, the data-transfer rates are greater for DVD than for CD. For
each rotation, the drive reads from a DVD about nine times the amount of data that it reads
from a CD. Table 2.5 illustrates how DVD rotational speeds translate to data transfer rates.
TABLE 2.5
DVD speeds
Speed
Maximum Data Rate (Megabits per Second)
1x
10.8
2x
21.6
4x
43.2
10x
108
20x
216
24x
259.2
DVD writeable and rewriteable blanks are available in two formats: DVD+ and DVD-.
The writeable-once versions are DVD+R and DVD-R; the rewriteable versions are DVD+RW
