Re: Why are there no double sided CDs?

As a matter of fact, there ARE double sided CDs!!!  These new "CDs" are
of a new format called the Digital Versatile Disk, or just DVD (they are
also called Digital Video Disks, but I prefer the first name).  This new 
format uses a smaller laser wavelength (635 to 650nm) than the 'old' CD 
(780nm).  This means that the laser beam can be focused down to a 
sharper point to read the pits of the CD.  As a result, the pits
(that store the binary '1's and '0's of the data) can be made a lot 
smaller, giving the DVD a data spiral of over 11km (more than double that of 
the CD).  This single layer, single sided DVD can store about 4.7Gbits.
(Compare this to a CD, which can only store about 0.68Gbits!)

Another feature of the DVD is that the discs are made thinner and are 
composed of two substrates. (Think of it as two CDs fused together.)  A 
lens is then used to focus the read-out laser beam onto one of the two layers.  
(Both of these substrate layers can be read from the same side of the disk.)  
With the two substrate layers, this brings the data storage total to about 
8.5Gbits.

Now, since these DVDs are made much thinner than CDs (0.4 microns per substrate 
layer; as opposed to 1.2 microns, for CDs) they decided to take this whole data 
storage thing one step further and make them double sided! (Just picture two of 
the single layered (or double layered) DVDs placed back to back.)  With double 
siding, the total storage for these disks has now  gone up to about 9.4Gbits 
(single substrate) or 17Gbits (double substrate). (Once again, compare this to 
a CD, which can store about 0.68Gbits......quite an improvement, if you ask me!)

Add all this to the fact that the new DVD players can play back data at
11Mbits/s (as compared to the CD's 1.2 to 4.8 Mbps) and you have one very 
impressive optical storage device.  With these many improvements to the 
CD technology, the DVDs have opened may doors that where not possible 
before.  You will soon start seeing many applications using DVDs in the 
areas of computer multimedia, video discs, and various other high-bandwidth 
applications.

Well, in a nutshell, that's about all I've learned about DVDs.  I have 
listed several places for further reading on this topic below.  I also 
want to point that all of the facts and figures that I spewed out above 
did not come from my little head (although I wish they did  :-)  ); they are 
from the Scientific American article, "Next-Generation Compact Discs", 
written by Alan E. Bell.  I highly recommend that you look up this article.

I hope this helps you.

	Kurt Frost
	kfrost@sympatico.ca

=============================

REFERENCES:

	Bell, Alan E. "Next-Generation Compact Discs." Scientific American,
	  July 1996, 42-46.

FURTHER INFORMATION:

	Gunshor, Robert L., and Arto V. Nurmikko "Blue Laser CD Technology."
	  Scientific American, July 1996, 48-51.
	Toshiba DVD Homepage
	MPEG_DVD Resources on the Web
	DVD