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Text File | 1994-11-23 | 6KB | 108 lines

Compact Disc Formats

Andrew Davidson
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...