Using Compact Discs (CD, CD-R, CD-RW)

August 2008

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Using Compact Discs

CADCAM Software for All Woven Textiles

CD-R (Compact Disc Recordable)
CD-R is a "write-once" medium. This means that once you have written data to a CD-R the space occupied by that data is used up and cannot be used for any other data. You can erase data from a CD-R but all that happens is that the data is "hidden" and will not be seen on the CD-R but the space originally occupied by the data will be lost.

This makes CD-R suitable mainly for archiving or distribution purposes where you do not intend to change the data on the CD-R once it has been written.

Generally you will create a CD-R by starting a "session" during which data is written to the CD-R. When you come to eject the CD-R you can choose to leave the session open (which will allow you to add data to the CD-R later) or close the session will prevent any further data being added. A CD-R, which has been left "open", can only be read using a CD-R drive. A CD-R which has been closed can generally be read by any CD drive.

If you intend to use a CD-R for distribution of data or wish it to be compatible with all CD drives then you should create it using the "Disc at Once" option in your CD creation software (as opposed to "Track at Once" which is more suitable for music CDs) and ensure that the "session" is closed when you eject the CD-R. Always test a CD-R after it has been created by attempting to read it in a standard CD-ROM drive as opposed to the drive that was used to create the CD-R.

CD-RW (Compact Disc Rewritable)
CD-RW is a rewriteable medium which means that data can be overwritten on the CD-RW in the same way as a on a hard disk or a floppy diskette. Before you can do this you must "format" the CD-RW, which prepares it for writing. This process uses some of the disc capacity for directory information and as a result the actual data capacity of CD-RW is somewhat lower than the quoted 650 or 700 Mbyte specifications.

A CD-RW that has been formatted and is used for read/write access can only be read by a CD-RW drive. You cannot read such a CD-RW in a normal CD-ROM drive. This limits the usefulness of CD-RW for data distribution since it can only be reliably distributed to other computers that also have CD-RW drives.

The advantage of CD-RW is that it can be used as a removable storage medium - simply place the CD-RW in the drive, write your data, then remove the CD-RW. To change the data on the CD-RW just repeat the same process.

You should be aware that CD-RW has limited lifespan - they can typically be rewritten around 1000 times. This means that a CD-RW should not be seen as an alternative to a magnetic hard disk, which does not have this limitation.

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