What's the difference between CDs and CD-Rs?

[ytoledano]

What's the difference between the CDs that you get when you buy a game/software and the CD-Rs you burn at home using a standard burner? How do they make the ones you buy?

Roses are <font color=red>red</font color=red>, violets are <font color=blue>blue</font color=blue>, post something stupid and I won't reply to you!

 

[phsstpok]

CD have "pits" and "lands" burned into the reflective material. The pits are 1/4 wavelength deep (I think it's 1/4). This makes the reflected light 180 degrees out of phase from the light relected from the lands. It's the out of phase transistion that make CD work so well. It causes high contrast reflections (light and dark).

Like CDs, CDRs have a reflective material but unlike CD's they have a light sensitive dye over the reflective material to control light and dark areas instead of pits and lands.

Fire the writing laser into the dye and it turns the dye dark (maybe it turns it light - I'm not sure which). Anyway it reverses the dye's state.

The trouble is, on reading a CDR, the dye absorbs some of the laser light so you don't get as good a reflection from the light colored areas as you do with CDs. Also, the dark area's don't stop all the reflected light. The result is you have lower contrast.

This can and does cause problems between some CDR discs with various dye types and various readers. Low contrast means a CDR is harder to read than a pressed CD.

<b>56K, slow and steady does not win the race on internet!</b><P ID="edit"><FONT SIZE=-1><EM>Edited by phsstpok on 09/07/03 02:30 PM.</EM></FONT></P>

 

[ytoledano]

When you say that CDs are pressed, you mean like coins? Boy, that's like Xgazillion speed.

Roses are <font color=red>red</font color=red>, violets are <font color=blue>blue</font color=blue>, post something stupid and I won't reply to you!

 

[phsstpok]

Production involves stamping, chemical coating and some photoreactive process which I don't really understand.

Here's a pretty good description.

<A HREF="http://entertainment.howstuffworks.com/framed.htm?parent=cd.htm&url=http://www.ee.washington.edu/conselec/W94/edward/edward.htm" target="_new">http://entertainment.howstuffworks.com/framed.htm?parent=cd.htm&url=http://www.ee.washington.edu/conselec/W94/edward/edward.htm</A>

<b>56K, slow and steady does not win the race on internet!</b>

 

[ytoledano]

Well, that's a bit too much info than I can handle but I understand in general (I wasn't gonna start my own CD factory at home).
Thanks!

Roses are <font color=red>red</font color=red>, violets are <font color=blue>blue</font color=blue>, post something stupid and I won't reply to you!

 

[jackscotty]

Typically the way that CD is made (in lieu of a CD-R as previously explained) is that the pits are created on what's called a "glass master". For the most part this is a platter that is made of glass and about the same diameter of a CD. A machine using a special type of precision drill puts the pits into the glass. Then then an impression of the glass master is imprinted into the thin reflective material which gets attached to the plastic disk.