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A PLAY ON DATA

by Tom Cantrell

Start ı Oops ı Once More, With Feeling ı Optical Options ı Have It Your, and Our, Way ı Lesson Learned ı Sources and PDF

OPTICAL OPTIONS

Under the hood, DataPlay demonstrates the marvelous mechanics and integrated electronics that characterize the latest generation of mini-me disks (such as the 1-GB IBM MicroDrive that I write about in "Disko Boogie" coming up in the January issue of the print magazine (Circuit Cellar 138)).

Letıs see where DataPlay fits in the laundry list of optical options. Basically, the differences between the alternative technologies boil down to whether or not, how many times, and how fast data can be written in the field.

Traditional CDs are pre-mastered with actual pits in the shiny layer to modulate the laser reflection, a process you canıt duplicate at home. The first big breakthrough was CD-R, allowing one-time programming in the field. CD-R media has a dye layer that is burned by a recording laser to simulate the pit in a CD. The burned areas absorb the playback lasers light, allowing CD-R disks to be played back by conventional CD drives (see Figure 1).

Figure 1ıCD-R drives mimic conventional CDs by burning spots in a dye layer to create a virtual (non-reflective) pit.

Another approach is Magneto-Optical or MO, the first to deliver multiple write capability. As the name implies, it relies on a combination of magnetic (Curie point for writes) and optical (Kerr effect for reads) properties. A laser heats the media to a high temperature, which allows the magnetic polarity to be changed. Subsequently for playback, a slight shift in the polarization of the reflected light indicates a 1 or 0.

The original MO drives used a constant magnetic field and modulated the laser. Thus, they were like a flash memory chip in that the media had to be erased each time prior to recording. The need for an erase cycle requires separate read and write heads or two passes with a single head.

MiniDisc uses a slight variation on the MO theme by modulating the magnetic field instead of the laser, allowing direct overwrite much like a conventional magnetic disk. Thereıs no need for an erase cycle, so MD can re-record in a single pass.

Most recently, CD-RW has emerged. It replaces the dye layer in CD-R with a special alloy that responds to the heat of a write laser with an amorphous phase change. If heated a lot, when it cools it becomes shiny. Heat it less and it ends up dull. Note that the difference in reflectivity isnıt as dramatic as for CD or CD-R, so CD-RW media may not work in older CD drives. Newer CD drives (and DVD) include automatic gain control (AGC), which compensates for the weaker reflection from CD-RW media.

On a side note, DVD and DVD-R work the same way as CD and CD-R, only with higher density (i.e., smaller pits, and a narrower track). Rewritable DVD formats (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.) incorporate various aspects of CD-RW (phase change) and MO technology, but itıs a complicated story Iıll leave for another day.

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