This is intended as brief introduction to the technology of CD-ROMs.
What is a CD?
Firstly, CD is an ancronym for Compact Disc.
There are four main types of Compact Disc formats:
- CD Audio
- CD-ROM (Compact Disc-Read Only Memory)
- CD-I (Compact Disc-Interactive)
- CD-ROM/XA (CD-ROM Extended Architecture)
The data is written in CLV (Constant Linear Velocity) format as opposed to CAV (Constant Angular Velocity)
What is a CD-ROM?
- About 654.74 MB or 74.5 minutes of audio
- One spiral groove with a few billion "pits" and "lands"
- Written in CLV format
- Can be gold (a CD-Recordable disc) or silver (commercially replicated)
Differences between CD-Recordable Media and Commercial CDs
- CD-R media uses gold as a reflective surface, while commercial CDs use aluminium
- CD-R media have a layer of optically active dye (usually green in color) that react with the recording laser to form a pit
- CD-R media have a laser write the information on them while commercial CD are stamped from a master (like ponograph records)
- A CD-R disc is about 10 times as expensive as a commercial CD disc
The above image is a scanning electron micrograph of a commercial CD-ROM stamper. Molten polycarbonate is injected onto this die, resulting in the formation of "pits" and "lands" within the CD-ROM.
The above image is a scanning electron micrograph of a single pit of an audio CD.
The above image is a scanning electron micrograph of a row of pits "burned" into a recordable CD-ROM. Notice the roughness around the edges of the pits when compared with a commercially stamped CD.
What is needed to read a CD-ROM?
- A CD-ROM reader
- The appropriate software drivers for your computer system
Physical Standards
- Red Book - CD Audio
- Yellow Book - CD-ROM, CD-ROM/XA
- Green Book - Compact Disc Interactive
- Orange Book - Recordable Compact Disc Standard for both M-O and W-O. Includes Multisession Discs e.g. Photo CD
Data Standards
- ISO 9660
- Rock Ridge - Named after the town in Blazing Saddles
- HFS (Macintosh)
- "Hybrid" ISO 9660/HFS
Red Book - CD Audio
- Defined by Philips and Sony in 1980 and published in a red binder, hence Red Book.
- Standard needed so a CD made by any manufacturer can be read by any CD player.
- Physical Block Structure, 75 blocks are read per second, each block divided into 98 24 byte frames.
- Frames are encoded using EFM (Eight to Fourteen Modulation), CIRC (Cross Interleaved Reed Solomon Code) codes are inserted for error detection and correction.
Yellow Book Standard
- Sony and Philips found that error rates on audio CDs were low, decided to use them for computer data.
- 1983 Yellow Standard for CD-ROM announced as an extension of the CD Audio standard.
- An extra layer of error correction was added.
Has 2 modes, mode 2 has less error correction and more data
"Mixed mode" disc has both audio and CD-ROM block structure on different tracks.
Green Book Standard
- Announced in 1986 to help with synchronization of audio and data tracks.
- CD-I (Compact Disc-Interactive) built on the CD-ROM mode 2 block structure and interleaves audio and data.
Also has 2 modes, mode 1 with additional EDC/ECC codes, mode 2 without.
Orange Book Standard
Announced in 1990 to address the new recordable optical media and provide specifications for incremental writes.
Divided into 2 parts, Compact Disc-Magneto Optical (CD-MO) and Compact Disc-Write Once (CD-WO)
Covers Multi-session discs, such as the Kodak Photo CD.
The ISO 9660 Format
The ISO 9660 data interchange format, adopted in 1987, is widely used, and is an accepted computer industry standard. CDs mastered in this format can be read by UNIX, Macintosh and DOS machines.
The IS0 9660 level 1 places limits on the directory and file structure that can be placed on the CD-ROM. They are:
- File names are limited to 8 characters plus a 3 character extension, 0-9, A-Z, _ only allowable characters.
- Directory names cannot have extensions.
- Maximum eight level deep hierarchy
Other Data Formats
- Rock Ridge - Uses the System Use Area and Application Use Area in the ISO 9660 directory record to store Unix file specific info, such as UID and GID.
- HFS (Hierarchical File System) - Macintosh specific CD-ROMs
- Hybrid ISO/HFS - HFS volume information written to sectors 0 -15 on the CD, ISO volume info written to sector 16. Mac loads it as a HFS volume, other platforms as an ISO 9660 volume. Uses the System Use area in the ISO 9660 directory record to store Mac file info.
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