CD ROMs, 650, 700 & 790Mbs?

[Ale]

I was wondering, given that all CD-ROMs have the same surface area, why do they come in different capacities?

[Garfield226]

All the CD-R data you could ever need.

There are 21-minute (80mm/3-inch), 74-minute, 80-minute, 90-minute, and 99-minute CD-Rs. These translate into data storage capacities of 184MB, 650MB, 700MB, 790MB, and 870MB respectively (see below for exact figures). See section (7-14) for more about 80mm CD-Rs, and sections (3-8-1) and (3-8-2) for notes on 80-, 90-, and 99-minute blanks. There used to be 63-minute CD-Rs, but these have largely vanished.

Typical 74-minute CD-Rs are advertised as holding 650MB, 680MB, or even 700MB of data. The reality is that they're all about the same size, and while you may get as much as an extra minute or two depending on the exact construction, you're not usually going to get an extra 30MB out of a disc labeled as 74-minute media. See section (3-8-3) for information on writing beyond a disc's stated capacity.

[The_Llama]

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.)

[Ale]

I see... I thought that the spiral was written by the recording laser at the time of burning.

[acaos]

Originally Posted by Ale

I was wondering, given that all CD-ROMs have the same surface area, why do they come in different capacities?

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.

[ftg]

Originally Posted by Ale

I see... I thought that the spiral was written by the recording laser at the time of burning.

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.

[Ale]

Originally Posted by ftg

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

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.

[Mr2001]

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.

[The_Llama]

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.

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.

[Ale]

Originally Posted by The_Llama

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.

[Lagged2Death]

Originally Posted by acaos

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.

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.

[acaos]

Originally Posted by Lagged2Death

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.