Lasers
and layers
The
single biggest advantage of a DVD is its vast storage capacity – a
direct consequence of the type of laser DVD drives use. The
laser, with wavelengths of 635 to 650 nanometers, can be
focused on a smaller spot than a typical CD-ROM drive’s
780-nanometer laser. This
means that the microscopic pits used to represent data on
DVD can be smaller than those used on a CD, allowing a disc
of the same size to store more data for every square inch.
Most of the current DVDs on the market are single-sided and
single-layer, but a DVD can use up to four physical surfaces
for storage (double-sided and double-layered).
A
single DVD disc can theoretically hold as much as 30 gigabytes
of data – roughly equivalent to 50 650-megabyte CD-ROMs.
That is enough data space to store eight hours of high-quality,
full-screen digital video with a high-fidelity 5.1-channel
soundtrack and multiple language and subtitle tracks. It
is this factor that explains why the first commercial application
for DVD is in video.
Four
layers, four possibilities
The
typical DVD disc can store in excess of 4 gigabytes of
data, but with newer emerging technologies this number
can easily exceed 30 gigabytes of data on a single disc.
DVD-ROMs can have as much as four different data recording
surfaces - two layers for each side of the disc. This is
a rough description of how DVD discs can store so much
information:
• Single-sided,
single-layer = 4.38+ gigabytes
| top
laminate |
| recording
layer A |
| base |
| bottom
laminate |
• Double-sided,
single-layer = 7.95+ gigabytes
| top
laminate |
| recording
layer A |
| base |
| recording
layer A |
| bottom
laminate |
• Single-sided,
double-layer = 8.75+ gigabytes
| top
laminate |
| recording
layer A |
| recording
layer B |
| base |
| bottom
laminate |
• Double-sided,
double-layer = 15.9+ gigabytes
| top
laminate |
| recording
layer A |
| recording
layer B |
| base |
| recording
layer B |
| recording
layer A |
| bottom
laminate |
DVD
vs. CD