Technology
At first glance, a DVD
disc can easily be mistaken for a CD: both are plastic discs
120mm in diameter and 1.2mm thick and both rely on lasers to
read data stored in pits in a spiral track. And whilst it can
be said that the similarities end there, it's also true that
DVD's seven-fold increase in data capacity over the
CD has been largely achieved by tightening up the
tolerances throughout the predecessor system.
Firstly, the tracks are placed closer
together, thereby allowing more tracks per disc. The DVD track
pitch (the distance between each) is reduced to 0.74 micron, less than half of CD's 1.6
micron. The pits, in which the data is stored, are also a lot
smaller, thus allowing more pits per track. The minimum pit
length of a single layer DVD is 0.4 micron as compared to
0.834 micron for a CD. With the number of pits having a direct bearing on
capacity levels, DVD's reduced track pitch and pit size alone
give DVD-ROM discs four times the storage capacity of CDs.
The packing of as many pits as possible onto a disc is,
however, the simple part and DVD's real technological
breakthrough was with its laser. Smaller pits mean that the
laser has to produce a smaller spot, and DVD achieves this by
reducing the laser's wavelength from the 780nm (nanometers) infrared light of a
standard CD, to 635nm or 650nm red light.
Secondly, the DVD specification allows information to be
scanned from more than one layer of a DVD simply by changing
the focus of the read laser. Instead of using an opaque
reflective layer, it's possible to use a translucent layer
with an opaque reflective layer behind carrying more data.
This doesn't quite double the capacity because the second
layer can't be quite as dense as the single layer, but it does
enable a single disc to deliver 8.5GB of data without having
to be removed from the drive and turned over. An interesting
feature of DVD is that the discs' second data layer can be
read from the inside of the disc out, as well as from the
outside in. In standard-density CDs, the information is always
stored first near the hub of the disc. The same will be true
for single- and dual-layer DVD, but the second layer of each
disc can contain data recorded "backwards", or in a reverse
spiral track. With this feature, it takes only an instant to
refocus a lens from one reflective layer to another. On the
other hand, a single-layer CD that stores all data in a single
spiral track takes longer to relocate the optical pickup to
another location or file on the same surface.
Thirdly, DVD allows for allows for double-sided discs. To
facilitate the focusing of the laser on the smaller pits,
manufacturers used a thinner plastic substrate than that used
by a CD-ROM, thereby reducing the depth of the layer of
plastic the laser has to travel through to reach the pits.
This reduction resulted in discs that were 0.6mm thick - half
the thickness of a CD-ROM. However, since these thinner discs
were too thin to remain flat and withstand handling,
manufacturers bonded two discs back-to-back - resulting in
discs that are 1.2mm thick. This bonding effectively doubles
the potential storage capacity of a disc. Note that
single-sided discs still have two substrates, even though one
isn't capable of holding data.
Finally, DVD has made the structure of
the data put on the disc more efficient. When CD was developed
in the late 1970s, it was necessary to build in some
heavy-duty and relatively crude error correction systems to
guarantee the discs would play. When bits are being used for
error detection they are not being used to carry useful data,
so DVD's more efficient and effective error correction code
(ECC) leaves more room for real data.
| DVD Disc
Format |
Type of DVD
Unit |
| DVD
Player |
DVD-R(G) |
DVD-R(A) |
DVD-RAM |
DVD-RW |
DVD+RW |
| R |
W |
R |
W |
R |
W |
R |
W |
R |
W |
R |
W |
| DVD-ROM |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
No |
| DVD-R(G) |
Yes |
No |
Yes |
Yes |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
No |
| DVD-R(A) |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
No |
Yes |
No |
Yes |
No |
| DVD-RAM |
No |
No |
No |
No |
No |
No |
Yes |
Yes |
No |
No |
No |
No |
| DVD-RW |
Yes |
No |
Yes |
Yes |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
No |
| DVD+RW |
Yes |
No |
Yes |
Yes |
Yes |
No |
No |
No |
Yes |
No |
Yes |
Yes |
| CD-R |
No |
No |
No |
No |
No |
No |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
| CD-RW |
No |
No |
No |
No |
No |
No |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
Until late-2001 DVD-RAM drives were capable of writing to
their own media only. At that time drives began to appear that
could also write to DVD-R for General media. Whilst this
increased their appeal, their inability to write to CD-R and
CD-RW media still left them at a disadvantage compared to the
rival DVD-RW and DVD+RW formats.
The first DVD+RW drives had no capability to record to
write-once DVD media. However, this was addressed in
second-generation units which appeared in the spring of 2002
and were able to write to the newly-developed DVD+R media in
addition to rewritable DVD+RW
discs.
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