


COMPACT DISC WRITING MODES


March 13, 1995



You may have seen on the data sheets of CD-R drives terms such as "Track-at-Once",
"Disc-at Once", and "Track Incremental".  So just what does that mean?  Well, to begin
with, CD-R drives do not write data in the way a hard disk or other storage drive writes.
CD-R drives "stream" data onto a disc, usually writing at least a track.  The "Write Strategy"
or "Write Mode" the CD-R drive uses depends on two things; A. Which write modes the CD-R
drive supports, and B. Which write modes the pre-mastering software supports.


TRACK AT ONCE

As the name implies, this write mode will write a track of any format (CD-DA, CD-ROM, ect.)
on the disc.  The size of a track must be at least 300 blocks (4 seconds), which equates to
around 700Kb.  As specified in the Red book, you can record up to 99 tracks on a Compact
Disc.  Before multi-session was a reality, users could only create 1 track on the disc that would
have to contain all the data they wanted to place on the disc.  This can be very wasteful of disc
space; if the user only had 100Mb of data to write, the other 550Mb available on the disc was
wasted.  This type of disc is considered a "Single Session Disc".  This brings us to . . . .

TRACK MULTI-SESSION

This write mode is very similar to Track at Once.  In the Multi-session environment, each 
"session" must contain at least one track.  Again, the size of the track must be at least
300 blocks.  Track Multi-session, as you have probably guessed, this mode allows you 
to incrementally add tracks to a disc.  Not to be confused with Incremental Writing, which
I'll cover later.  There are several important things to remember here.  Firstly, each session
will take up about 13.5Mb of disc space in overhead; what we call Lead-in and Lead-out areas.
So it does not make sense to record small amounts of data (say, less than 50Mb), because each
time you write, you lose that 13.5Mb on the disc.  And keep in mind that you can only write to
the disc 99 times).  A second thing to remember is that CD-ROM readers implement multi-session
reading differently.  Some follow the Kodak Photo-CD method of multi-session (CD-ROM/XA format),
while others follow the CD-ROM Mode 1/Mode 2 format.  Lastly, it is very important not to
mix CD-ROM and CD-ROM/XA on the same disc.  Some premastering software packages will
allow you to do this, but you will have problems reading back the disc.  Chose one format and stick
with it.

While most CD-R drives support this recording mode, some will allow the user to record more
than one track within a session.  This method of recording is very useful because the 13.5Mb of
session overhead can be avoided.  In a session that has more than 1 track, the tracks are separated
by 150 blocks (2 seconds) of pre-gap.  A good example of a multi-track is a CD audio disc.  While
this type of disc is usually single session, there is a Lead-in with Table of  Contents (TOC), some
number of tracks (each song being one track), and the Lead-out area.  The same approach works
for data tracks and can easily be multi-session.






DISC-AT-ONCE

This writing mode is especially useful for creating a master disc that you will send off to a
replicator for mass production.  In Disc-at-Once mode, the whole disc - Lead-in, Data, and Lead-out
areas are written starting from the beginning of the disc to the end of the disc without ever turning
off the recording laser.  With Track-at-Once and Track Multi-session, the data area is written first,
then the Lead-out area, then the Lead-in area; each time turning off the recording laser to jump to
the next area.  Each time the recording laser is turned off and on, link blocks are created on the
disc.  These link blocks "link" tracks with the Lead-in and Lead-out areas.  However, these link
blocks are interpreted as "uncorrectable errors" on most mastering systems at the replication plant.
Writing in Disc-at-Once mode eliminates the link blocks because the recording laser never turns
off.  Disc-at-Once requires the pre-mastering software to send a "cue sheet" to the CD-R drive that
describes the disc layout.  From there, the CD-R drive accepts the data and begins writing the 
Lead-in with the Table of Contents (TOC), the actual data, and the Lead-out in that order without
interruption.  Disc-at-Once creates a single session disc only.


TRACK INCREMENTAL

Another way of saying multi-session.  When you think about it, adding data to a disc
one track at a time is "incrementally" adding data to the disc.  Not to be confused with
"Incremental" or "Packet Writing"


INCREMENTAL (PACKET) WRITING

This is also referred to a just "Incremental Writing".  This is a writing mode that is just a little
ahead of it's time.  The concept behind Incremental Writing is simple: make the CD-R drive
act like a hard disk or other storage device; able to write as little as a file at a time to the CD-R
disc.  While simple in concept, implementation is a whole different story.  ISO9660, the current
file system for CD-ROM, is not really appropriate for a disc written incrementally.  The reason
is the indexing  system (path tables) for ISO9660 depend on knowing about all the files that are
to be recorded on the disc.  Without this knowledge, the ISO9660 file system will have a very
difficult time to retrieve a file.  With current multi-session writing, each time a session is
written, the whole file system must be re-written.  A new file system has emerged called
ECMA 168.  ECMA 168 builds on the ISO9660 file systems and adds the flexibility to add
data to a disc "a file at a time", without having to re-write the file system each time.  The
key drawback is that a incremental disc cannot be read back on today's CD-ROM drives
without a special driver.  In addition, ECMA 168 has yet to be approved and implemented.
Nonetheless, this mode of writing will become popular soon.  



Written by Bob DeMoulin, Product Manager of Ricoh Corporation's Compact Disc
Recordable Products.
