The Linux CD-ROM HOWTO
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# 2. CD-ROM Technology

| "CD-ROM is read-only memory, and audio compact disc system is available as package-media of digital data for those purpose. For playing audio CD, please insert Head-phone jack." |
--from a CD-ROM instruction manual |

Don't Panic! The world of CD-ROM technology is not as confusing as your instruction manual.

CD-ROM stands for Compact Disc Read-Only Memory, a mass storage medium utilizing an optical laser to read microscopic pits on the aluminized layer of a polycarbonate disc. The same format is used for audio Compact Discs. Because of its high storage capacity, reliability, and low cost, CD-ROM has become an increasingly popular storage media.

The storage capacity of a CD-ROM disc is approximately 650 megabytes, equivalent to over 500 high density 3.5" floppy disks or roughly 250,000 typed pages.

First generation drives (known as single speed), provided a transfer rate of approximately 150 kilobytes per second. Hardware manufacturers then introduced double speed (300 kB/sec), quad speed (600 kB/sec), and higher. Current drives operate at up to 40 times speed, although the maximum rate is only achievable over certain portions of the disc surface.

Most CD-ROM drives use either the Small Computer Systems Interface (SCSI), ATAPI enhanced IDE interface, or a vendor proprietary interface. They also typically support playing audio CDs via an external headphone jack or line level output. Most drives also allow reading the frames of data from audio CDs in digital form.

CD-ROMs are usually formatted with an ISO-9660 (formerly called High Sierra) file system. This format restricts filenames to the MS-DOS style (8+3 characters). The Rock Ridge Extensions use undefined fields in the ISO-9660 standard to support longer filenames and additional Unix style information (e.g. file ownership, symbolic links, etc.). Microsoft has defined a proprietary ISO file system extension called Joliet which supports long filenames using the 16-bit UNICODE character encoding.

PhotoCD is a standard developed by Kodak for storing photographic images as digital data on a CD-ROM. With appropriate software, you can view the images on a computer, manipulate them, or send them to a printer. Information can be added to a PhotoCD at a later date; this is known as multi-session capability.

CD recordable (CD-R) drives allow writing onto a special "gold" CD which can then be read by any CD-ROM drive. Data can only be written once, although using multi-session new data can be appended to a disc.

CD-RW (rewritable) drives can be erased and rewritten with new data. They use special discs which can be read by most recent CD-ROM drives (but not older ones or most audio CD players).

DVD-ROM expands the storage of a CD to as much as 17 gigabytes. They are commonly used as a medium for distributing full length motion pictures encoded using the MPEG-2 format. The MPEG video decoding is performed using specialized decoder software and/or hardware. DVD-RAM is a writable version of DVD.

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