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Next: CD Standards Up: Storage Media Previous: Optical Storage

CD Storage

There a now various formats of CD:

In the beginning, there was CD-DA (Compact Disc-Digital Audio), or standard music CDs. CD-DA moved onto CD-ROM when people realized that you could store a whole bunch of computer data on a 12cm optical disk (650mb). CD-ROM drives are simply another kind of digital storage media for computers, albeit read-only. They are peripherals just like hard disks and floppy drives. (Incidentally, the convention is that when referring to magnetic media, it is spelled disc. Optical media like CDs, LaserDisc, and all the other formats I'm about to explain are spelled disc.)

CD-I (Compact Disc-Interactive) came next. This is a consumer electronics format that uses the optical disk in combination with a computer to provide a home entertainment system that delivers music, graphics, text, animation, and video in the living room. Unlike a CD-ROM drive, a CD-I player is a standalone system that requires no external computer. It plugs directly into a TV and stereo system and comes with a remote control to allow the user to interact with software programs sold on disks. It looks and feels much like a CD player except that you get images as well as music out of it and you can actively control what happens. In fact, it is a CD-DA player and all of your standard music CDs will play on a CD-I player; there is just no video in that case.

Next came CD-ROM/XA (eXtended Architecture). Now we go back to computer peripherals - a CD-ROM drive but with some of the compressed audio capabilities found in a CD-I player (called ADPCM). This allows interleaving of audio and other data so that an XA drive can play audio and display pictures (or other things) simultaneously. There is special hardware in an XA drive controller to handle the audio playback. This format came from a desire to inject some of the features of CD-I back into the professional market.

Now, along comes the idea from Kodak for Photo CD - digital pictures on compact disk. They teamed up with Philips to develop the standard for Photo CD disks. At this point, a new problem enters the picture, if you'll pardon the expression. All of the disk formats mentioned so far are read-only; there is no way for anyone but the producer of one of these disks to store his/her own content on the disk - that is, to write to it. But there already existed a technology called WORM (Write Once Read Many). This is an optical disk that can be written to, but exactly once. You can burn data on it, but once burned the data can not be erased, although it can then be used like a CD-ROM disk and read forever. (Depending on your definition of forever, of course.)

CD-ROM, CD-ROM/XA, and CD-I disks are normally mastered, as opposed to burned. That means that one master copy is made and then hundreds, or thousands, or millions (if you're lucky enough to need that many) of replicates are pressed from the master. This process is much cheaper than burning for quantities above a few dozen or so. Generally, disk pressing plants can handle all of these formats as the underlying technology is the same; the only difference is in the data and disk format.

The reason that WORM technology was critical for Photo CD is obvious - the content of these disks is not determined by the manufacturer or publisher. For Photo CD, each disk will be different - a roll or few rolls of film per disk from a customer.

Kodak and Philips wanted Photo CD disks to be playable on both computer peripherals for desktop publishing uses AND on a consumer device for home viewing. For the former, CD-ROM/XA was chosen as a carrier and for the latter CD-I, which was already designed as a consumer electronics device, and dedicated Photo CD players. This desire for a hybrid disk, or one with multi-platform compatibility, led to the development of the CD-I Bridge disk format. A Bridge disk is one that is readable on both a CD-I player and a CD-ROM/XA drive.

This Bridge format is the reason there is so much confusion about CD-ROM drives for Photo CD. A drive that supports Photo CD must be a CD-ROM/XA drive that is also Bridge-compatible. (The technical description of Bridge disks calls for supporting certain kinds of sectors identified by form and mode bits, which is what you usually hear instead of the Bridge disk label.) That almost completes the picture, except for the concept of sessions.

Although a WORM disk can only be written to once, it is not necessary to write, or burn, the entire disk all at once. You can burn the disk initially with, say, a few hundred megabytes of data, and then go back later and burn some more data onto it. Of course, each burn must be to a virgin part of the disk; once a spot on the disk is burned, it can not be re-burned. Each burn operation is referred to as a session, and a drive or disk that supports this multiple burning operation is called multisession.

Originally, all WORMs were single session only. That is, you could not go back and add data to a WORM disk once it was burned, even if it was not full. For Photo CD, they wanted the consumer to be able to add more pictures to an existing disk as additional rolls of film were processed. So the extension of WORM technology to multisession was developed and adopted for the Bridge disk format. This required hardware changes to CD-ROM/XA drives and that is why there are a fair number of single session XA drives on the market and multisession ones appearing more and more.

A single session drive can read a multisession disk, but it can only read the contents of the first session that was burned. Incidentally, all Philips CD-I players are multisession, although all current CD-I disks have only a single session on them.

The capacity of a CD-ROM is 620-700 Mbs depending on CD material, Drives that read and write the CD-ROMS. 650 Mb (74 Mins) is a typical write once CD-ROM size.

next up previous
Next: CD Standards Up: Storage Media Previous: Optical Storage
Dave Marshall