The CD revolutionized portable storage for the PC.  The floppy had always been a major obstacle for software vendors.  Windows 95 came on 22 diskettes !!  When CD's came out, they could fit any program in the world.  Enter DVD  -  although it has been slow to take hold, it's only a matter of time.  Graphic capabilities continue to grow, and people are fed up with games that require 2 or more CD's.  Standard DVD's (DVD-5) store roughly 7 times as much data as a CD does.

Despite the huge difference in capacity - the DVD technology is amazingly similar to CD.  Both use stamped pits, that act as reflectors of light.  Both use lasers, which aim their beam down onto the surface, and read the amount of reflected light coming back, as 1's and 0's (bits).  Both have one, long, spiral track (as opposed to the cylinder tracks that hard drives and floppies use).

Read Data Rate - DVD's are rated for both Read and Write Data rates, such as 2x, 4x, etc.  It seems as if DVD's are very slow when you compare their "x" ratings to CD drives.  However, with DVD drives, the "x" relates to the base factor of 1200 kBps (kilo-Bytes per sec), which is 8 times as much as the CD "x" value, where 1x = 150 kBps.  Therefore 2x = 2400 kBps, 4x = 4800 kBps, etc.  Rule of Thumb  .  .  .  a 4x recordable DVD drive burns a DVD in about 15 minutes. 

There are, of course, complex differences in the technology that we won't cover here.  But DVD's are just the normal evolution of enhanced technology - smaller and more compact - just as hard drives are continually able to pack more and more data.  But unlike hard drives, where the 10% to 20% improvements are available on the store shelves each year with bigger and bigger drives  .  .  .  the industry waited for years before coming out with their bigger optical disc, and that disc is the DVD  .  .  .  a 700% improvement !!!

How DVDís have so much more Capacity than CDís


Increase factor
Track Pitch
1600 nanometers
740 nanometers
Minimum Pit Length
(single-layer DVD)
830 nanometers
400 nanometers
Minimum Pit Length
(double-layer DVD)
830 nanometers
440 nanometers

Since the reduced track width (pitch) of DVD allows 2.16 times as many lines to exist in the spiral, and the reduced pit length allows 2.08 times as many pits to be stamped - this gives us an increase factor of 4.5.  But we said earlier that a DVD-5 holds 7 times as much as a CD.  The rest of the increase is due to other factors, such as greatly improved error correction (which requires much less overhead), and improvements in other technologies.

In addition, the following parameters have been slightly enhanced, the biggest change is in the error correction method (CD's use very inefficient error correction and must include massive amounts of data for it) :

*      slightly larger data area (~ 1.02 x CD)

*      more efficient channel bit modulation (~1.06 x CD)

*      more efficient error correction (~1.32 x CD)

*      less sector overhead (~1.06 x CD)

See also www.mpeg.org/MPEG/DVD/General/Gain.html