CD-ROM Basics
Compact Disk - Read Only Memory.
Originally designed for audio industry.
Designed to deliver a constant data rate - variable RPM.
Implemented with optical storage
Non-changeable storage.
Inexpensive.
Fairly durable.
Very portable.
CD-ROM Technology 1
Single track recorded from the hub out to the rim. 22,188 revolutions.
Constant linear velocity 120 cm/sec. (Constant data delivery)
RPM 530 RPM at hub to 200 RPM at rim.
Bits stored by creating reflective (lands) and not reflective (pits)
zones on the disk.
Pits and lands read by an infra-red LED laser.
Bits are noted by the change between a pit and a land.
Data actually exists on the top side of the cd right under the label.
CD-ROM technology 2
Data written once and permanent.
Commercial disks have physical pits of 1/4 wavelength depth.
Commercial Worm use high powered lasers to physically modify recording
surface.
Home burned disks use a strong laser to burn a spot in a dye coated disk.
Dyes
Cyanine dye - cyan blue.
Original dye for cd-r.
Sensitive to bright or uv light. (www.cd-info.com)
Phthalocyanine dye - faint aqua tinge.
Less sensitive to bright or uv light.
Requires a more accurately tuned laser for writing.
Formazan dye - light green.
Metalized Azo - dark blue.
Reflective layer.
Gold - real gold.
Silver - proprietary (secret).
Quality of recordng affected by choice of media and drive manufacturing.
Also, older 16x speed cd-r drives may not be able to record on cd designed
for the newer 48x writers.
Bit/byte storage
CD-ROMS use a variety of techniques to provide error free data.
Symbol - the smallest unit of storge - 14 bits in size.
Composed of an 8 bit byte
and an additional 6 bits for error correction.
Frame - composed of 42 symbols or 588 bits.
24 symbols are data.
18 symbols are for error correction and control.
Sector - composed of 98 frames.
Classified as Mode 1 or Mode 2.
(1) Mode 1 - used for digital data, contains error correcting code.
(2) Mode 2 - used for audio, additional audio replaced ecc.
Mode details
All sectors contain 2352 bytes.
All sectors contain a 16 byte preamble.
1st 12 bytes used to signal beginning of sector.
Next 3 bytes are sector i2^24 or 16 million sectors.
Last byte contains the Mode type.
Mode 0
Mode 1 - contains 2048 bytes of data and 288 byte ecc field.
Mode 2 - contains 2336 bytes of data and no ecc field.
Modes 0 and 2 for analog.
Only about 28-30% of data on disk is user data.
Interface protocols
The low level protocols of bit/byte definition have been defined.
However, protocols are also required by computers and their operating
systems.
Macs, Wintel, Linux, Unix
Early on, the computer industry insisted on a single standard that
was compatible on different systems.
Known originally as High Sierra, it evolved into ISO 9660.
ISO 9660 has there levels defined.
ISO 9660
Level 1
8 character file name with a 3 character extension.
Only upper case alpha characters, numbers and underscore.
Directories can be eight deep.
Directory names may not use the 3 character extension.
All files must be recorded contiguously (sequentially in single write).
Level 2 - 32 character filenames
Level 3 - Non-contiguous writes (for re-writable disks).
Rock Ridge - Unix specific protocol that supports Unix inode filesystem.
Reference to Blazing Saddles.
CD-ROM Improvements CD-R
Basic design offers cheap durable storage.
Commercial disks already pre-recorded.
Obsolete or incorrect data unrepairable.
To provide recording on a pc, the CD-R was created.
Uses a dye in place of a real pit.
Uses a two level laser that can either read the pit or burn the dye to
create a new "pit".
Contains a second embedded track that provides accurate timing for the
CD-Burner.
Because data is actually just under the top side of the disk
scratching or wearing the top can be more damaging than damaging the
clear underside.
Using a paper label protects data.
Burn CD before applying label. Additional material can affect
rotational speed when recording.
Mark label before placing on disk to prevent physical damage.
Don't use solvent pens directly on disk and make sure paper label dry
before applying to disk.
Some newer cd-r's claim to have writable surfaces.
Ball point pen can indent surface.
Solvent from permanent markers can attack disk.
CD-R technology
Limitations.
Constant linear speed requires data to be delivered at constant
When data cannot be delivered in time, it is called buffer underrun
and trashes the disk.
VTOC - volume table of contents - lists the files and their sectors.
Found at the beginning of the track (1st sector).
Original CD-ROM always looked in 1st sector.
VTOC had to be written first and had to be complete.
Thus the write once read many (Worm) nature of a cd.
CD-R improvements
With level 3 protocol, write once nature somewhat modified.
Level 3 allows the original VTOC to be invalidated and rewritten.
The new VTOC will be stored elsewhere on the disk.
The OS will search for the most current VTOC.
This allows the user to add new files or hide old files (delete).
Storage for deleted files not recoverable.
Rewriting VTOC limited to 13 times and takes at least 1 sector.
CD-RW
CD-Read/Write - allows unlimited(?) re-writing of data.
True deletion of data.
Dye replaced by a reflective layer of antimony and other metals.
This alloy has two states, crystalline and amorphous.
Crystalline is reflective (lands).
Amorphous is non-reflective (pits).
The CD-RW is equipt lasers with three power levels capable of
Reading
Converting to Crystalline
Converting to Amorphous
Because change is an actual physical process
Disk will fatigue and fail with extended changes (100?)
Also, often CD-RW are readable only on CD-RW drives and occasionly
accessable only on the drive it was written.