Hardware Reference
In-Depth Information
creased, which increases the length of each pit and land. When reading a dual-layer disc,
the drive spins slightly faster to compensate, resulting in the same data rate. However, be-
cause the distance on the track is covered more quickly, less overall data can be stored.
Besides the standard four capacities listed here, a double-sided disc with one layer on one
side and two layers on the other can be produced. This would be called a DVD-14 and
have a capacity of 13.2GB, or about 6 hours and 15 minutes of MPEG-2 video. Addition-
ally, 80mm discs, which store less data in each configuration than the standard 120mm
discs, can be produced.
Because of the manufacturing difficulties and the extra expense of double-sided
discs—and the fact that they must be ejected and flipped to play both sides—most DVDs
are configured as either a DVD-5 (single-sided, single-layer) or a DVD-9 (single-sided,
dual-layer), which allows up to 8.5GB of data or 242 minutes of uninterrupted MPEG-2
video to be played. The 133-minute capacity of DVD-5 video discs accommodates 95%
or more of the movies ever made.
Note
When you view a dual-layer DVD movie, you will see a momentary pause onscreen when
the player starts to play the second layer. This is normal, and it takes so little time that if you
blink, you might miss it.
Data Encoding on the DVD Disc
As with CDs, the pits and lands themselves do not determine the bits; instead, the trans-
itions (changes in reflectivity) from pit to land and land to pit determine the actual bits
on the disc. The disc track is divided into bit cells or time intervals (T), and a pit or land
used to represent data is required to be a minimum of 3T or a maximum of 11T intervals
(cells) long. A 3T long pit or land represents a 1001, and a 11T long pit or land represents
a 100000000001.
Data is stored using eight to sixteen modulation, which is a modified version of the eight
to fourteen modulation (EFM) used on CDs. Because of this, eight to sixteen modulation
is sometimes called EFM+. This modulation takes each byte (8 bits) and converts it in-
to a 16-bit value for storage. The 16-bit conversion codes are designed so that there are
never fewer than two or more than ten adjacent 0 bits (resulting in no fewer than three
or no more than eleven time intervals between 1s). EFM+ is a form of RLL encoding
called RLL 2,10 (RLL x,y, where x equals the minimum and y equals the maximum run
of 0s). This is designed to prevent long strings of 0s, which could more easily be misread
due to clocks becoming out of sync, as well as to limit the minimum and maximum fre-
quency of transitions actually placed on the recording media. Unlike CDs, no merge bits
