CHAPTER 13

Compact Disc and Drive Formats Chapter 13
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writing laser, and the disc is closed. A disc is considered closed when the last (or only) lead-in is fully
written and the next usable address on the disc is not recorded in that lead-in. In that case, the CD
recorder is incapable of writing any further data on the disc. Note that it is not necessary to close a
disc to read it in a normal CD-ROM drive, although if you were submitting a disc to a CD duplicating
company for replication, most require that it be closed.
Track-at-Once
Multisession discs can be recorded in either Track-at-Once (TAO) or Packet writing mode. In Track-at-
Once recording, each track can be individually written (laser turned on and off) within a session, until
the session is closed. Closing a session is the act of writing the lead-out for that session, which means
no more tracks can be added to that session. If the disc is closed at the same time, no further sessions
can be added either.
The tracks recorded in TAO mode are divided by gaps of normally 2 seconds. Each track written has
150 sectors of overhead for run-in, run-out, pre-gap, and linking. A CD-R/RW drive can read the
tracks even if the session is not closed, but to read them in a CD-DA or CD-ROM drive, the session
must be closed. If you intend to write more sessions to the disc, you can close the session and not
close the disc. At that point, you could start another session of recording to add more tracks to the
disc. The main thing to remember is that each session must be closed (lead-out written) before
another session can be written or before a normal CD-DA or CD-ROM drive can read the tracks in
the session.
Packet Writing
Packetwriting is a method whereby multiple writes are allowed within a track, thus reducing the overhead
and wasted space on a disc. Each packet uses 4 sectors for run-in, 2 for run-out, and 1 for linking.
Packets can be of fixed or variable length, but most drives and packet-writing software uses a
fixed length because it is much easier and more efficient to deal with file systems that way.
With packet writing, you normally use the Universal Disk Format (UDF) file system, which enables
the CD to be treated essentially like a big floppy drive. That is, you can literally drag and drop files to
it, use the copy command to copy files onto the disc, and so on. The packet-writing software and UDF
file system manage everything. If the disc you are using for packet writing is a CD-R, every time a file
is overwritten or deleted, the file seems to dissapear, but you don’t get the space back on the disc.
Instead the file system simply forgets about the file. If the disc is a CD-RW, the space is indeed
reclaimed and the disc won’t be full until you literally have more than the limit of active files stored
there.
Unfortunately, Windows versions up through Windows 2000 don’t support packet writing or the UDF
file system directly, so drivers must be loaded. Fortunately, though, these typically are included with
CD-R/RW drives. One of the most popular packet-writing programs is DirectCD from Roxio (Adaptec).
When you remove a packet-written disc from the drive, the packet-writing software first asks whether
you want the files to be visible to normal CD-ROM drives. If you do then the session must be closed.
Even if the session is closed, you can still write more to the disc later, but there is an overhead of
wasted space every time you close a session. If you are going to read the disc in a CD-R/RW drive, you
don’t have to close the session because they will be capable of reading the files even if the session
isn’t closed.
PhotoCD
First announced back in 1990 but not available until 1992, PhotoCD is a standard for CD-R discs and
drives to store photos. You simply drop off a roll of film at a participating Kodak developer, and they
digitize and store the photos on a specially formatted CD-R disc called a PhotoCD, which you can