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...

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Copyright 1993 Philips Interactive Media.
All rights reserved.

