
MULTI- SESSION CD RECORDING:  THE REST OF THE STORY


I am often asked "I have recorded a multi-session disc but my CD-ROM drive can
only read the first session, why?"  This problem is most often a problem of playback
on the CD-ROM drive, not any problem with the CD-R drive.

One of the key reasons CD-ROM has been so successful is that physical disc standards
and logical file format standards were established.  Red Book (audio), Yellow Book (CD-ROM),
Green Book (CD-I), White Book (Video) are examples of physical disc standards.  They
describe how the tracks are organized on the disc.  ISO9660 is the logical file format for
CD-ROM.  It describes how the data is organized within a CD track.  These standards were
around before CD's were recordable.  Enter the Orange Book.  The Orange Book is a physical
disc standard that describes how all the other book formats are recorded on the recordable
compact disc.  

The Orange Book made the provision for recordable compact discs to be recorded in several
increments called "sessions".  Recordable compact discs cannot be erased or re-written, but
they can be appended.  This means that two or more recorded sessions on the disc can be
joined together to create one logical structure that a CD-ROM drive would display to the
user.  If the user looked at the contents of the disc, he (or she) should see all the data that
was recorded in all sessions, displayed as one logical directory/file list; even though the
actual data spans several recording sessions.  That is, assuming, that the sessions were 
appended together at recording time, and the CD-ROM drive and it's device driver are
intelligent and multi-session capable.

What does that mean?  Well, as I said before, the Orange Book controls how a multi-session
CD-ROM is recorded, not how it gets played back.  In the early days of multi-session, Kodak
adopted it for their PhotoCD discs.  PhotoCD discs are Yellow Book CD-ROMs all right, 
but they use a track type called XA (extended architecture).  This type of track is ideal for
PhotoCD, but is not the same as the standard CD-ROM track type we call Mode 1.  So
the CD-ROM drive manufacturers modified their drives to handle PhotoCD multi-session.
More often that not, when a CD-ROM drive manufacturer claims it's drive is multi-session,
what they really mean is "PhotoCD or XA multi-session".  

Back to the Orange Book.  The Orange Book does not specify which track type to use 
for multi-session discs, it's really rather vague.  The OB does say that whatever track
type is used, all subsequent sessions must be of that same type.  So you can see that each
CD-ROM manufacturer has some freedom in determining how their drive will react to a
multi-session CD-ROM.  Some drives, like Toshiba, Mitsumi, and NEC look for a XA 
type disc for multi-session.  If a XA disc is inserted in one of those drives, it will read
multi-session.  If the disc was recorded in Mode 1 multi-session, those drives will only
see the first session.  Other drives, like the Sony CDU-561, CDU55, Plextor 4Plex, and
most of the Panasonic drives will read a multi-session disc regardless of the track type.
And generally, most manufacturers are going to this approach.  With the advent of low
cost CD recorders turning out Mode 1 multi-session discs, they really have to.  Also,
the device driver controlling the CD-ROM drive is important.  The driver can force a
XA multi-session drive to read a Mode 1 multi-session disc.  However, most of the generic
CD-ROM drivers bundled with multi-media kits will not do this.






So what should you do if this happens to you?  First of all, find out what kind of multi-
session your pre-mastering software is creating, XA or Mode 1.  Then find out what type
of multi-session your CD-ROM drive is looking for.  Third, how did you create the multi-
session disc?  Most pre-mastering packages have a "load track" or "append multi-session
CD" feature that allows you to load in previous sessions, add your new data, then record
the new session to disc.  In almost all cases, the data from previous sessions is not re-
recorded in the new session.  The software places "pointers" that tells the CD-ROM
drive where to find the data in previous sessions.  Remember, multi-session CD-ROM
drives read the most recent session first.  If you have appended sessions together, there
will be pointers in that session back to other sessions.  If you didn't append sessions
together, you have a multi-volume disc, where each session is separate.  You will need
a software driver to access each individual session in this case.  The best bet is to select
a CD-ROM drive that will read a multi-session disc no matter what track type was used
to create it, and append your sessions. 

A word to the wise:  If you are preparing a disc to send to someone else, and you're not
sure of the other person's CD-ROM drive, don't send a multi-session disc.  He or she
may run into this problem we have here.  Send a single session disc and you will be
safe!


Written by Bob DeMoulin, Product Manager for Ricoh Corporation's recordable compact
disc products.  For more information on Ricoh Corporation's CD-R products, please call
1-800-955-3453.

When you think CD-R, think RICOH!
