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Beginners Guides: Optical Drives & Recording Formats

Beginners Guides: Optical Drives & Recording Formats - PCSTATS
Abstract: Everything from the differing optical media recording standards, read/record speeds to selecting the best recordable DVD format is covered.
Filed under: Beginners Guides Published:  Author: 
External Mfg. Website: PCstats Dec 08 2003   Mike Dowler  
Home > Reviews > Beginners Guides > PCstats

Reading CDs and DVDs

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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Contents of Article: PCstats
 Pg 1.  Beginners Guides: Optical Drives & Recording Formats
 Pg 2.  How CD-Rs store data
 Pg 3.  — Reading CDs and DVDs
 Pg 4.  UDF/Packet Writing
 Pg 5.  Understanding CD-R/RW media
 Pg 6.  Other DVD Recordable Standards
 Pg 7.  DVD-Dual and DVD-Multi formats

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