Hardware Reference
In-Depth Information
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 transitions (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 into 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 frequency
of transitions actually placed on the recording media. Unlike CDs, no merge bits exist between codes.
The 16-bit modulation codes are designed so that they will not violate the RLL 2,10 form without
needing merge bits. Because the EFM used on CDs really requires more than 17 bits for each byte
(due to the added merge and sync bits), EFM+ is slightly more efficient because only slightly more
than 16 bits are generated for each byte encoded.
Note that although no more than ten 0s are allowed in the modulation generated by EFM+, the sync
bits added when physical sectors are written can have up to thirteen 0s, meaning a time period of up
to 14T between 1s written on the disc and pits or lands up to 14T intervals or bit cells in length.
Recordable DVD Standards
The history of recordable DVD drives began as a troubled one. It dates back to April 1997, when the
DVD Forum announced specifications for rewritable and recordable DVD: DVD-RAM, and DVD-R.
Later, it added DVD-RW to the mix. Dissatisfied with these standards, the industry leaders in optical
recording and drives formed their own group called the DVD+RW Alliance and created another
standard—DVD+R and DVD+RW. For several years, drives based on one family of standards could
not freely interchange media with drives using the other family of standards.
Fortunately, all recent drives support both DVD-R/RW and DVD+R/RW media, including dual-layer
(DL) DVD+R media, and most also support DVD-RAM. Thus, by using a modern drive that supports
all of these types of media, you can choose the right media for a particular task. For example, use
DVD-RAM for easy drag-and-drop file backups and DVD-R for creating video DVDs compatible
with older DVD set-top boxes.
Table 11.11
compares the competing recordable DVD standards, and
Table 11.12
breaks down the
compatibilities between the drives and media.
Table 11.11. Recordable DVD Standards
