Hardware Reference
In-Depth Information
is interpreted as the difference between pits and lands, even when read back on a
regular CD-ROM reader or even on an audio CD player.
No new kind of CD could hold up its head with pride without a colored book,
so CD-R has the
Orange Book
, published in 1989. This document defines CD-R
and also a new format,
CD-ROM XA
, which allows CD-Rs to be written incre-
mentally, a few sectors today, a few tomorrow, and a few next month. A group of
consecutive sectors written at once is called a
CD-ROM track
.
One of the first uses of CD-R was for the Kodak PhotoCD. In this system the
customer brings a roll of exposed film and his old PhotoCD to the photo processor
and gets back the same PhotoCD with the new pictures added after the old ones.
The new batch, which is created by scanning in the negatives, is written onto the
PhotoCD as a separate CD-ROM track. Incremental writing is needed because the
CD-R blanks are too expensive to provide a new one for every film roll.
However, incremental writing creates a new problem. Prior to the Orange
Book, all CD-ROMs had a single
VTOC
(
Volume Table of Contents
) at the start.
That scheme does not work with incremental (i.e., multitrack) writes. The Orange
Book's solution is to give each CD-ROM track its own VTOC. The files listed in
the VTOC can include some or all of the files from previous tracks. After the CD-
R is inserted into the drive, the operating system searches through all the CD-ROM
tracks to locate the most recent VTOC, which gives the current status of the disk.
By including some, but not all, of the files from previous tracks in the current
VTOC, it is possible to give the illusion that files have been deleted. Tracks can be
grouped into
sessions
, leading to
multisession
CD-ROMs. Standard audio CD
players cannot handle multisession CDs since they expect a single VTOC at the
start.
CD-R makes it possible for individuals and companies to easily copy CD-
ROMs (and audio CDs), possibly in violation of the publisher's copyright. Several
schemes have been devised to make such piracy harder and to make it difficult to
read a CD-ROM using anything other than the publisher's software. One of them
involves recording all the file lengths on the CD-ROM as multigigabyte, thwarting
any attempts to copy the files to hard disk using standard copying software. The
true lengths are embedded in the publisher's software or hidden (possibly en-
crypted) on the CD-ROM in an unexpected place. Another scheme uses intention-
ally incorrect ECCs in selected sectors, in the expectation that CD copying soft-
ware will ''fix'' the errors. The application software checks the ECCs itself, refus-
ing to work if they are ''correct.'' Nonstandard gaps between the tracks and other
physical ''defects'' are also possibilities.
Although people are used to other write-once media such as paper and photo-
graphic film, there is a demand for a rewritable CD-ROM. One technology now
available is
CD-RW
(
CD-ReWritable
), which uses the same size media as CD-R.
