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What is an MD?
How MiniDisc works
MiniDisc (MD) comes in two
types: premastered (prerecorded)
and recordable (blank).
Premastered MDs, recorded at
music studios, can be played back
almost endlessly. However, they
can’t be recorded on or over like
cassette tapes. To record, you must
use a “recordable MD”.
Premastered MDs
Premastered MDs are recorded
and played like regular CDs. A
laser beam focuses on the pits in
the surface of the MD and reflects
the information back to the lens in
the recorder. The recorder then
decodes the signals and plays
them back as music.
Recordable MDs
Recordable MDs, which use
magneto-optical (MO)
technology, can be recorded again
and again. The laser inside the
recorder applies heat to the MD,
demagnetizing the magnetic layer
of the MD. The recorder then
applies a magnetic field to the
layer. This magnetic field
corresponds exactly to the audio
signals generated by the connected
source. (The north and south
polarities equate to digital “1” and
“0”.) The demagnetized MD
adopts the polarity of the magnetic
field, resulting in a recorded MD.
How the MiniDisc got so small
The 2.5-inch MiniDisc, encased in
a plastic cartridge that looks like a
3.5-inch diskette (see illustration
above), uses a new digital audio
compression technology called
ATRAC (Adaptive TRansform
Acoustic Coding). To store more
sound in less space, ATRAC
extracts and encodes only those
frequency components actually
audible to the human ear.