I was wondering, given that all CD-ROMs have the same surface area, why do they come in different capacities?
The condensed, simple answer to your question is; the tightness of the spiral, and the size of the pits. When the laser makes the pits and bumps to represent 0’s and 1’s, it does so in an outward spiral, like a vinal record. The cd itself has a track to follow in it’s writing. So if the spiral is slightly tighter, then you get more memory.
As a cool aside, some DVD burners can fit 1.4 gigs on a 700MB cd. It does this by using the fine DVD writing laser to use the normally allotted space for one bit of data, and writing 2 bits there. My dvd writer does this, t’s pretty neat as far as pure data storage. (You can’t play a cd like this on a regular cd rom anymore.)
I see… I thought that the spiral was written by the recording laser at the time of burning. :smack:
There’s also the fact that CD-ROMs can be written in two modes. Normal data CD-ROMs are Mode 1, where additional error-correction information is written, giving 2048 bytes per data block.
Other types of CD, such as VideoCD, write in Mode 2, giving 2352 bytes per block. The lack of error correction information is not as important for these formats. You might see a temporary glitch in the video stream, but the error can be recovered at the next key frame.
The data block occupies the same amount of space on the CD regardless of which mode it is written in.
Hold a blank CDR up with the underside at a shallow angle to a bright point light source. You’ll see a diffraction pattern. (I.e., “purdy colors”.) That’s the spiral.
I need a bigger :smack:
sigh
I´m supposed to pay attention to the visual characteristics of objects (part of the job) I think I can safely mark this in the calendar as my “Be Dumb for a Day” day.
That “pregroove” doesn’t just tell the drive where to write, it also holds data about the manufacturer and construction of the disc, which is encoded by varying the width of the pregroove. Some PC games have copy protection that tries to read that information (which is only there if the game has been copied onto a CD-R), and quits if it succeeds.
I really don’t see how you could feel dumb about this. It’s not exactly something you talk about over dinner. And if you are required to be able to tell how tight a spiral is based on the difference in purdy colors, I don’t envy you your job.
At least you asked and found out the answer.
Well, I´m good at my job (texture creation for 3D computer graphics) but I´m not that good; however the light difraction caused by wavelenght interference should heve hinted me that, well, the surface comes pre-grooved…
People look at me funny when I stare at rust stains, freckles, paint chips and other surfaces like a texture obsessed version of Ray Man. 
I’m nitpicking, but I want to point out that all CD formats contain very substantial amounts of data redundancy, error-correction data, and damage-resistant data encoding and formatting. Something like 20-30% of a CD’s true capacity is devoted to such things. It’s just that CD-ROM formats intended for use with computer data contain even more (as you mention when you say “additional error-correction.”)
The “no error correction” canard sometimes comes up when people discuss audio CDs and the theoretical differences in sound between one CD player and another. That misconception can lead to others. Just a pet peeve of mine.
You are of course absolutely correct. My apologies for oversimplifying my explanation.