As mentioned previously, a CD-ROM or
DVD-ROM drive uses a laser to read data, continually firing it at the reflective
surface of the disk. An optical sensor synchronized with the laser records any
direct reflection returned from the disk as a digital '1' and the lack of a
reflection as a '0.' The bumps ('pits') and lands that guide the reflection of
the laser are formed into a continuous spiral track over the surface of the CD.
The
laser needs to pass over this track at a constant speed in order to reliably
read data from it, so the motor of the CD drive varies the speed
that it spins the CD in order to accommodate reading from the inside tracks to
the outside edge of the CD at a consistent rate.
DVDs work in a similar manner, with the addition of the ability
to vary the strength of the reading laser in order to read the multiple layers
of data that may be present on a DVD disk.
CD standards for storing data.
As the current CD format was
gradually established, a series of technology guidelines, called 'books' were
put forward to guide how CDs handle the storage of data and guarantee
compatibility. A brief synopsis of the ones important to this article
follows:
Red Book - 1980 - the audio CD
standard. Allows for 74 minutes of digital audio on a single CD, and up to 99
tracks.
Yellow Book - 1983 - An
extension of the Redbook standard to cover the use of CDs as a data storage
medium (CD-ROM).
Orange Book - 1988 - An
extension of the Yellow Book standard to allow writeable CDs. Essentially
created the CD-R as we know it. Later revised to allow multiple 'sessions' per
disk each with its own table of contents, meaning that the entire disk did not
have to be written at one time. This is known as multi-session writing.
Like any other method of mass
storage, writeable CDs need a file-system to arrange the data that is written to
them. Given the relatively rigid nature of writing to CD as opposed to a
hard-disk drive, where any section can be written to or written over at will,
data CDs have no need for a constantly updated catalog of the contents of the
disk. Rather, they need a simple table of contents to guide the reading device.
CD writing, and the
'ISO-9660' format.
The most
common data CD file system is the ISO 9660 format.
Drafted by the International Organization of Standards in 1988, and modified and added to many times
since then, it is the accepted standard for storing data
on CD.
If you are burning files
to a CD-R disk, you are using the ISO 9660
file system.
ISO 9660 CDs can be read by any
modern operating system (though the files on the CD may not be useable), thus
MP3 files stored on a CD burned in an iMAC could be read by a Windows system,
even though the two computers' operating systems normally cannot read
each-other's files.
In simple terms, an ISO 9660
formatted CD contains a list of the contents of that particular CD, including
the number of sessions, directories, subdirectories and files, their names and
where to physically locate them on the disk. This data is read first by the
drive once the disk is inserted, and is subsequently kept in memory, reducing
the time needed to find data on the disk. Unlike other file systems intended for
use on magnetic disks, ISO 9660 provides no way to delete or add to a file once
it has been written to the disk, reflecting the inflexible nature of CD-R/RW
media.
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