Differences between magnetic en optical recording
Magnetic discs and tape
Writing
A computer hard disk is a magnetic disc, just like a floppy and a (cassette)
tape. In all these cases there is a carrier on which a very thin iron-like
layer has been applied. This one is written onto using a magnetic field. This
magnetizes the magnetic
material on the disc or tape, just like paper clips that can be
magnetized by rubbing them with a strong magnet. The magnetic field is reversed
in polarity (N-S or S-N) according to a certain code and in this way information
can be written. The smaller and more concentrated the writing magnetic field is
made, the more bits and bytes can be stored on the disc.
Reading
At play back this magnetisation is read again. This can be done using the same
electro-magnet coil (read-write head) that is used for writing, but as well
(and even better) using a Magneto Resistive Head. This is a special type of resistor of
which the resistance changes under the influence of a magnetic field. An MRH can
be made much smaller than an electro magnetic coil for reading.
Optical discs
Writing
Optical discs, like CD, CD-ROM, DVD and Blue-Ray don't work magnetic, but by
optics. They are written (CD-R, CD-RW, DVD-RW, DVD+RW, Blue-Ray) by pulsing the
laser for short periods to high power, so creating heat. In RW-materials the
layer melts locally. According to a code you heat up for a shorter or a
longer time. Because the disc is rotating in the mean time, you melt dashes of
different lengths.
Reading
You can read them back because the once molten parts have a darker colour than
the unmolten. This is also true after they cooled down, because the structure
is different (amorph) from the original material (cristaline). A spot of light
of low intensity reads back the differences in reflection.
Erasing
Erasing can be done at half power. The dark dashes are getting pale again as
the layer recristalises in the medium heat.
Read Only
CD, CD-ROM and DVD are read only. The dark dashes are moulded in the surface of
the Poly Carbonate discs in the form of pits. The whole surface is then
evapourated with an aluminium layer. That is the reason why these disc behave
like a mirror. The deepened pits give a lower reflectivity due to interference
of light waves. So the information is stored in a mechanical way in the disc and
gives them a durability of about one century.
Magneto-Optic
To make things complicated, there is also something like Magneto-Optic. On these
discs there is a layer that can be magnetised and of which the reflection
(using all types of trics e.g. polarisation filters) is somewhat dependant on
a magnetic field. The information is written in the form of
magnetisation. This can be done when the layer is hot. Very locally (1
micron2) it is heated by a laser on high power. During cooling
the magnetic field of a coil, that is supported in the direct neighborhood, is
"frozen" into the layer. This coil is rather uncritical, because the layer is
only sensitive at the position of the hot spot.
Reading and application
Reading can be done with the same laser spot, now at low power. Magneto-Optic
(MO) is used by certain exchangeable computer-discs and by Mini Disc. Making
these discs is a rather expensive process.
Advantage is that the layer, while
writing, is heated up much less than in RW discs. MO can be overwritten more than
a million times, RW just (a few) thousand times.