Well, now that we know what the basic structure of a MiniDisc looks like,
lets examine how the Recordable MD actually earns its name. The
technology that allows one to record onto a disc is known as the
Magnetic Field Modulation Direct Overwrite Process. This is how it
works, seen graphically at left in Diagram 4:
-
From the bottom of the disc, a strong laser beam hits the revolving disc,
bringing the magento optical layer up to Curie temperature (180°C).
At this temperature, the magnetic particles on the MD lose their storing
power, and can thus be re-recorded.
- The overwrite head above the disc adds the recordable binary digits
"0" or "1" to corresponding magnetic North or South poles. As the laser
beam moves away from the recorded area, the temperature falls, and the
signal remains.
And there you have it - the secret in recording onto a disc depends on
the latest magneto optical technology. Now you know how information is
recorded onto an MD, but how do we retrieve it?
Unlike a regular CD, or a Premastered MD, the binary information for
playback is not generated by simple reflection off a pitted surface.
Rather, it depends on the Kerr Effect: In general, a laser beam
aimed at a magnetized layer will be affected by the magnetization,
resulting in a slight rotation. And so playback can be achieved in
the following manner.
The direction of the laser beam's rotation depends upon the polarity
of the location on the disc that has been hit (N or S). A polarization
beam splitter distinguishes this, changing it to distribution ratios
sent through one of two photo detectors. These varying amounts of light
are assigned values of either "0" or "1", and the digital information has
been safely retrieved. To the left we see Diagram 1 again, to remind of
you of just what the surface of the Recordable MD looks like.
Well, that ends this section. Hopefully you understand just how this
exciting new Recordable MD technology works. Return to the Audio index
page to see the Advantages of the MD System, or a product Showcasing it.
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