Philips Interactive Media of America
This note is an attempt to answer the oft-asked questions about CD formats like "What is the difference between CD-DA, CD-ROM, CD-ROM/XA, and CD-I?" and "How is Photo CD compatible with all of these?" and "What is the difference between single-session and multi-session discs?"
In the beginning, there was CD-DA (Compact Disc-Digital Audio), or standard music CDs. CD-DA begat CD-ROM when people realized that you could store a whole bunch of computer data on a 12cm optical disc (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 "disk." 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 disc 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 discs. 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 disc. They teamed up with Philips to develop the standard for Photo CD discs. At this point, a new problem enters the picture, if you'll pardon the expression. All of the disc formats mentioned so far are read-only; there is no way for anyone but the producer of one of these discs to store his/her own content on the disc - that is, to write to it. But there already existed a technology called WORM (Write Once Read Many). This is an optical disc 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 disc and read forever. (Depending on your definition of forever, of course.)
I should say that CD-ROM, CD-ROM/XA, and CD-I discs 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 (or replicants, if you are a "Blade Runner" fan) are pressed from the master. This process is much cheaper than burning for quantities above a few dozen or so. Generally, disc pressing plants can handle all of these formats as the underlying technology is the same; the only difference is in the data and disc format.
The reason that WORM technology was critical for Photo CD is obvious - the content of these discs is not determined by the manufacturer or publisher. For Photo CD, each disc will be different - a roll or few rolls of film per disc from a customer.
Kodak and Philips wanted Photo CD discs 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 disc, or one with multi-platform compatibility, led to the development of the "CD-I Bridge" disc format. A Bridge disc 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 discs calls for supporting certain kinds of sectors identified by "form" and "mode" bits, which is what you usually hear instead of the "Bridge" disc label.) That almost completes the picture, except for the concept of sessions.
Although a WORM disc can only be written to once, it is not necessary to write, or burn, the entire disc all at once. You can burn the disc 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 disc; once a spot on the disc is burned, it can not be re-burned. Each burn operation is referred to as a "session," and a drive or disc that supports this multiple burning operation is called "multisession."
Originally, I believe that all WORMs were single session only. That is, you could not go back and add data to a WORM disc 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 disc as additional rolls of film were processed. So the extension of WORM technology to multisession was developed and adopted for the Bridge disc 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 disc, 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 discs have only a single session on them. (Generally, being mastered means a single session, although it is possible to master a multisession disc. I don't know of any software that currently supports this, however.)
Well, I hope that this answers some questions about CD formats. Sorry it is so long, but it is not a simple story. Perhaps I should have formatted this as a multisession answer...
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