[LAU] CDRDAO speed, CRC errors, blah

[david]

Arnold Krille wrote:
> On Sunday 25 April 2010 01:34:12 Fons Adriaensen wrote:
>> Writeable CDs don't have pits, either the color or the
>> reflectivity of the surface is modified by heating it.
>> Both require the laser power to be controlled in function
>> of write speed.
> 
> Afaik writeable CDs _do_ use pits. They are created when the laser heats up 
> the colour-bubbles so they expand and raise the reflective layer.

 From Wikipedia (http://en.wikipedia.org/wiki/CD-R#Writing_methods):

"A CD recorder writes data to a CD-R disc by pulsing its laser to heat 
areas of the organic dye layer. The writing process does not produce 
indentations (pits); instead, the heat permanently changes the optical 
properties of the dye, changing the reflectivity of those areas. Using a 
low laser power, so as not to further alter the dye, the disc is read 
back in the same way as a CD-ROM. However, the reflected light is 
modulated not by pits, but by the alternating regions of heated and 
unaltered dye."

Another (older, 2003) article I found said that there are 3 different 
ways of marking the track (without mentioning what the 3 different ways 
  WERE), and the marks are called "pits" whether or not there's a pit 
there, an area of changed dye, or a "bubble".

Non-writable optical discs use different technology. From 
http://electronics.howstuffworks.com/cd1.htm:

"A CD is a fairly simple piece of plastic, about four one-hundredths 
(4/100) of an inch (1.2 mm) thick. Most of a CD consists of an 
injection-molded piece of clear polycarbonate plastic. During 
manufacturing, this plastic is impressed with microscopic bumps arranged 
as a single, continuous, extremely long spiral track of data. We'll 
return to the bumps in a moment. Once the clear piece of polycarbonate 
is formed, a thin, reflective aluminum layer is sputtered onto the disc, 
covering the bumps. Then a thin acrylic layer is sprayed over the 
aluminum to protect it. The label is then printed onto the acrylic."

And continuing on to page 3:
http://electronics.howstuffworks.com/cd3.htm

"The elongated bumps that make up the track are each 0.5 microns wide, a 
minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a 
billionth of a meter.) ... You will often read about "pits" on a CD 
instead of bumps. They appear as pits on the aluminum side, but on the 
side the laser reads from, they are bumps."

So, since non-writable CD media appeared first and actually had 
pits/bumps, I guess it was logical to apply the same terminology to the 
newfangled CD-R and CD-RW media even when they don't have pits.

I always thought lasers actually burned holes in the dye layer, so the 
scanning laser's beam would be reflected back. But, then, I always 
thought an interesting laser printing technology would be to use a laser 
that actually burnt dots on the paper (no need for toner cartridges or 
passing the printed page through a hot press to melt the plastic onto 
the paper).

> Disclaimer: I've been found wrong in the past...

But neither of us have ever been found wrong in the future... ;-)

-- 
David
gnome at hawaii.rr.com
authenticity, honesty, community