Optical Disks

Various 312, 4.7 and 514 inch optical disk drives are available, all of which employ a laser to read data, which is usually detected by variations in the intensity of light reflected from the disk. This kind of medium is usually highly reliable and can be invaluable for use as a backup. The following kinds of disk may be encountered:-

Most read/write drives can read disks created by different kinds of drives that use the same ‘family’ of disks, although some newer drives can’t write to smaller disks, as originally written by older drives.

  CD and DVD

The most common formats are based on Compact Disc (CD) or Digital Versatile Disc (DVD). Although a DVD drive can read most types of CD, a CD-only drive can’t read DVDs. Both types of disc are highly reliable and are immune to magnetic fields.

The following table provides information about the basic formats:-

FormatTypeMaximum Capacity
CD-AudioRO74 minutes 80 minutes
CD-ROMRO650 MB 700 MB
CD-RWORM650 MB 700 MB
CD-RWRW650 MB 700 MB
DVD-ROMRO4.7 GB •9.4 GB •17 GB
DVD-RWORM3.95 GB 4.7 GB
DVD-RWRW4.4 GB
DVD-RAMRW2.6 GB 5.2 GB 4.7 GB 9.4 GB
DVD+RWRW4.7 GB 9.4 GB
Not all DVD-ROM drives support the larger disc sizes

As you can see, the actual selection of a drive can be a complex business, particularly since the two competing DVD read-write formats aren’t compatible with all types of drive.

  Magneto-Optical (MO) Disks

Magneto-optical disks use a laser and a magnetic field during recording, but only require the laser for playback. Each disk has a 40-year life, can be overwritten 10 million times and can be read 100 million times.

In a standard WMRM drive, areas of the disk are first erased and then the erasure is checked. Next, data is recorded and then verified. For digital audio recording, where speed is vitally important, one or both of the check phases may be omitted.

The more advanced Light Intensity Modulation-Direct Overwrite (LIM-DOW) format uses a special type of dual-layer disk, with the erasure check omitted to improve speed. Fortunately, LIM-DOW drives also accept standard MO disks and LIM-DOW disks can be used in standard MO drives, although without any improvement in performance.

The following common varieties of magneto-optical disk may be encountered:-

Miniature MO

These 312-inch disks come in 128 MB, 230 MB, 540 MB, 640 MB or 1.3 GB sizes, as well as a 640 MB LIM-DOW variety. Typically, data is written to a 128 MB disk at 700 KB/s or read at 2 MB/s, with newer drives running at up to 5.9 MB/s.

The 128 MB and 230 MB versions comply with International Standards Organisation (ISO) formatting of 512 bytes/sector whilst the largest versions employ 2048 bytes/sector.

Standard MO

These 514-inch disks rotate at 3600 rev/min and come in 600 MB, 650 MB, 1.2 GB or 1.3 GB sizes, as well as 2.3 GB, 2.6 GB and 5.2 GB LIM-DOW varieties. The data is spread over both sides of each disk, requiring you to turn it over to read all the information, meaning that a 2.6 GB disk has 1.3 GB on each side. A standard drive provides transfer rates of up to 4.35 MB/s, whilst a LIM-DOW drive can reach as high as 10 MB/s. Typical seek times are around 25 ms or better.

The 600 MB and 650 MB versions meet ISO standards. The 600 MB, 1.2 GB and 2.3 GB versions use 512 bytes/sector but the larger 650 MB, 1.3 GB and 2.6 GB varieties employ 1024 bytes/sector.

Advanced Storage MO (ASMO)

This 4.7-inch disk has a capacity of 6 GB, an ASMO drive also accepting both CDs and DVDs. Initially, the low cost of this format made it competitive with DVD itself, although it now seems to have fallen out of favour.

Professional Disc for Data (Pro Disc)

Recently introduced by Sony, this offers 23.3 GB at rates of 18 Mbit/s on a 4.7-inch disc, effectively replacing earlier 9.1 GB 514-inch MO disks. Unlike older optical drives, which use a red laser, Pro Disc employs a blue laser to create a smaller spot on the disk. Later versions are planned to accommodate 50 GB, increasing to 100 GB with a double-sided disk.

Other Optical Formats

Some non-standard optical formats have been developed by specific manufacturers. Unfortunately, it can be difficult to predict which ventures will survive, as most proprietary formats seem short-lived. Here are some examples:-

Apex

Produced by Pinnacle, this 514 inch disk has a 4.6 GB capacity and is partially compatible with earlier formats.

Floptical

This 312 inch disk has a capacity of 20.8 MB, contains 1,000 tracks per inch and rotates at 600 rev/min, but is no longer available. The seek time is around 135 ms, over six times slower than a typical hard disk drive, which is probably the reason for its demise. Although a Floptical drive can read standard HD floppy disks it doesn’t accept the DS variety.

WORM Optical Disk

These formats use a laser to deform the disk’s surface. A 514 inch disk stores 110, 600 or 800 MB, whilst the 14 inch version holds up to 3 GB. Most WORM formats have been superseded by other MO disks and, more significantly, by CD and DVD.

©Ray White 2004.