Optical disk drives use a laser in conjunction with a reflective disk. They come in
many types, are highly reliable and are effectively immune to magnetic fields.
Recordable formats are invariably more expensive than conventional drives.
Compact Disc
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  A Compact Disc – Read Only Memory (CD-ROM) disc can store over 600 M of data
— equal to 1,900 diskettes or over 250,000 typed pages. An audio CD can hold up to
80 minutes of sound whilst a Photo-CD can store 5,000 high quality colour images.
Each disc has a diameter of 4.7 inches and contains a continuous spiral track 5,000 metres long, giving an effective density of 15,875 tracks per inch (tpi). As the head moves towards the inner ‘tracks’ the drive motor accelerates to ensure a constant linear velocity (CLV).
ë In this GUIDE the word disk is spelt with k at the end — but for CDs it ends in c!
CD-ROM Drives
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In a standard drive the disk spins at 200-500 revolutions per minute (rev/min) to give a seek time of 300 ms to 1.5 seconds and a data transfer rate of 150 K per second. Although slow compared to a hard disk there’s a huge amount of information available!
In a 4-times speed drive the access time is shorter and the transfer rate increases to 600 K per second. An 8-times drive, found inside many PowerMacs, is even better! The latest 12-times drives provide a data transfer rate of 1.8 M per second.
ë For fast data transfers buy a 6, 8 or 12-times speed drive.
Connections and Caddies
The drive is connected to your Mac via SCSI using a 50-way Amphenol cable, and often needs an external SCSI terminator. A switch on the drive lets you select its SCSI ID number.
Older drives use a CD caddy to protect each disc from dust and damage. Unfortunately every CD needs to be put in its own caddy — including audio discs. You could transfer discs in and out of caddies (to reduce costs) but that rather defeats the object!
Recordable CDs
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To create your own CDs (of any type) you must use a CD Recorder that accepts blank CD-Recordable (CD-R) discs. These drives often play at 4-times speed but may be limited to 2-times speed during recording.
  Some Plasmon CD-ROM drives can also accept a 5.25 inch 650 M re-writable
Phase-change Disk (PD). Data is recorded with a high-power laser that changes the
crystalline nature of the PD’s surface for up to 10,000 read-write operations. The
PD’s unique format makes it impossible to read on a standard CD-ROM drive.
Data Standards
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CD-ROM drives accept High Sierra (HS), ISO 9660 or Macintosh HFS discs, providing the appropriate files are in the Extensions folder within the System Folder. High Sierra defines the basic CD formatting whilst ISO 9660 is a revised version of the system.
A hybrid disc contains duplicated data for both PC and Mac — and consequently holds less data. This is necessary for high-speed multimedia presentations since the Intel processor in a PC must receive its data bytes in the opposite direction to a Mac!
  To use a Multimedia PC (MPC) disk on a PC requires a suitable drive and computer.
You must use a MPC drive with a buffer or cache of a least 64 K — small buffers of
16 or 32 K aren’t enough. Most 4-times speed drives have a 256 K cache.
Some applications need to use the version numbers attached to filenames on ISO 9660 and High Sierra disks. If you have problems with files you should:-
• Ensure the application is active
• Drag the CD-ROM icon into the Trash
• Hold down the Option key until you’ve fully reinserted the CD
Modes
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A CD-ROM can work in one of two modes, depending on whether or not it uses a
Table of Contents (TOC):-
z Mode 1 (X)
600 M capacity — at a rate of 150 K per second on a standard drive
3 levels of error detection — suitable for important data
TOC not provided
z Mode 2 (A)
681 M capacity — at a rate of 171 K per second on a standard drive
2 levels of error detection — suitable for multimedia audio or video
TOC is used
A CD-ROM/XA or Extended Architecture (XA) disc contains material in both modes to accommodate low or medium quality audio and data for multimedia presentations. They can be played on some CD-ROM drives but you may need extra hardware on a PC to take full advantage of them. They use a TOC in order to locate the audio tracks.
The actual capacity depends on the disc itself — the values shown above are for the equivalent of an 80 minute audio CD. Some CDs are limited to 63 minutes (550 M) or 74 minutes (650 M).
Other Types of CD
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Audio CDs
These can be played on most drives, although a digital audio feed into the computer isn’t always provided — even if it is, you’ll usually get better quality from the output sockets on the drive itself.
If the Audio CD Access extension is installed you can use the AppleCD Audio Player application to control your drive.
CD+G discs also provide animation and text alongside the audio track, as required for Karaoke. The CD+MIDI variety include a MIDI sequence that can be used to control musical instruments in step with the sound recording — special software may be necessary to extract the MIDI data.
Photo-CDs
  These hold up to 100 images at a resolution 16 times better than television — and
they can also be played on many CD-ROM drives.
In a single session disc all the images are transferred to the CD in a single pass — hence there’s a single table of contents (TOC). In a multi-session disc the images are transferred singly (or in groups) at different times — there can be any number of TOCs. Therefore any single session drive (which can only read one TOC) will only read the first TOC and the associated image or images.
ë Install the Apple Photo Access extension to view Photo-CDs on a Mac.
CD-Interactive (CD-I)
CD-I discs are usually used with special player — such discs don’t use a TOC. They can’t usually be played on a CD-ROM drive.
Compatibility
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Not all kinds of CD can be played on every player. A disk that can be played on either an XA drive or CD-I player is called a bridge disc — a Photo-CD disc is an example.
The table below shows compatibility between formats:-
Drive n CD-ROM CD-I Photo CD
Disc
j
Audio ‚àö ‚àö ‚àö
Photo CD Some ‚àö ‚àö
ISO 9660 / HS ‚àö x x
Mac HFS ‚àö x x
CD+G Audio only ‚àö ‚àö
CD+MIDI ‚àö ‚àö ‚àö
CD-I x ‚àö ‚àö
CD-ROM/XA Some x x
Other proprietary formats include:-
Memorex VIS
Commodore CD-TV
Sony Multimedia (Dataman) MM/CD — really a version of XA.
All of these are incompatible with CD-ROM.
Other Optical Formats
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  There are plenty of optical formats in both 3.5 and 5.25 inch sizes. Drives with a
larger capacity can often read smaller disks of the same type but may not be able
to record onto them!
Magneto-Optical
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MO disks are recorded using the combined effects of a laser and magnetic field — replay uses the laser on its own. They’re very durable — a typical disk has a 40-year life at 25 °C, can be overwritten over 10 million times and read over 100 million times. These disks are particularly suitable for use as a universal startup disk for configuring any Mac.
During writing the disk is usually first erased, the erasure is checked, the recording is made and finally the recording is checked. For digital audio applications one or both of these check phases may be omitted.
Disks can be formatted using HDT Primer or Spot On — preferably with all extensions switched off. The default settings for formatting and partitioning should be used. The process can take 20 minutes or more.
Manufacturers suggest that you clean the drive lens monthly and the cartridges every three months — don’t tamper with the disks or drive!
3.5 inch MO
These have a capacity of 128, 230, 540 or 640 M — the 128 M and 230 M versions are specified by the International Standards Organisation (ISO) and are recorded at 512 bytes/sector whilst the largest version uses 2048 bytes/sector.
Data can be written to a 128 M disk at 700 K per second or read at 2 M per second.
MO jukeboxes can be used for large amounts of data.
5.25 inch MO
Standard sizes are 600 M, 650 M, 1.2 G, 1.3 G, 2.3 and 2.6 G — the 600 M and 650 M versions are specified by the ISO. The data capacity is spread between the two sides of the disk — you must turn it over to read data on the other side. As you can see, the disk sizes come in three ‘paired’ sizes — the smaller varieties are recorded at 512 bytes/sector, the larger at 1024 bytes/sector.
Digital Video Disk
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The DVD format was designed as a video playback medium, similar in operation to CD. However, it can also be used to store huge amounts of data in the form of DVD-ROM.
Floptical
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  A 3.5 inch disk with 1,000 tracks per inch, rotating at 600 rev/min to give a
capacity of 20.8 M. Seek time is 135 ms — more than six times slower than a
standard hard disk drive.
These drives can also read HD diskettes but not the DS variety. Flopticals have never been supported by Apple and at the time of writing appear to have been discontinued.
Write Once Read Many
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 A Write Once Read Many (WORM) optical disk is recorded by using a laser to
deform the surface of the disk — once recorded it can’t be modified. They come
in a range of sizes, from 5.25 inch disks storing 110, 600 or 800 M to a 14 inch
version holding up to 3 G. WORMs have been largely superseded by MO disks.
Sony have manufactured an 3.5 inch MO drive that also accepts WORM disks. You can record your own data on any part of these WORM disks, with two notable exceptions:-
O-ROM (Optical ROM)
A factory pressed disk in which pits are used to represent bits
P-ROM (Partial ROM)
A disk which has been partially pressed, also incorporating a writable section
