Hardware Reference
In-Depth Information
However, instead of cyanine or pthalocyanine dye, CD-RW uses an alloy of silver,
indium, antimony, and tellurium for the recording layer. This alloy has two stable
states: crystalline and amorphous, with different reflectivities.
CD-RW drives use lasers with three different powers. At high power, the laser
melts the alloy, converting it from the high-reflectivity crystalline state to the low-
reflectivity amorphous state to represent a pit. At medium power, the alloy melts
and reforms in its natural crystalline state to become a land again. At low power,
the state of the material is sensed (for reading), but no phase transition occurs.
The reason CD-RW has not replaced CD-R is that the CD-RW blanks are more
expensive than the CD-R blanks. Also, for applications consisting of backing up
hard disks, the fact that once written, a CD-R cannot be accidentally erased is a
feature, not a bug.
The basic CD/CD-ROM format has been around since 1980. By the
mid-1990s optical media technology had improved dramatically, so higher-capaci-
ty video disks were becoming economically feasible. At the same time Hollywood
was looking for a way to replace analog video tapes with an optical disk technolo-
gy that had higher quality, was cheaper to manufacture, lasted longer, took up less
shelf space in video stores, and did not have to be rewound. It was looking as if
the wheel of progress for optical disks was about to turn once again.
This combination of technology and demand by three immensely rich and
powerful industries has led to
DVD
, originally an acronym for
Digital Video Disk
,
but now officially
Digital Versatile Disk
. DVDs use the same general design as
CDs, with 120-mm injection-molded polycarbonate disks containing pits and lands
that are illuminated by a laser diode and read by a photodetector. What is new is
the use of
1. Smaller pits (0.4 microns versus 0.8 microns for CDs).
2. A tighter spiral (0.74 microns between tracks versus 1.6 microns for
CDs).
3. A red laser (at 0.65 microns versus 0.78 microns for CDs).
Together, these improvements raise the capacity sevenfold, to 4.7 GB. A 1x DVD
drive operates at 1.4 MB/sec (versus 150 KB/sec for CDs). Unfortunately, the
switch to the red lasers used in supermarkets means that DVD players require a
second laser to read existing CDs and CD-ROMs, which adds a little to the com-
plexity and cost.
Is 4.7 GB enough? Maybe. Using MPEG-2 compression (standardized in IS
13346), a 4.7-GB DVD disk can hold 133 minutes of full-screen, full-motion video
at high resolution (720
480), as well as soundtracks in up to eight languages and
subtitles in 32 more. About 92 percent of all the movies Hollywood has ever made
×
