Differences between magnetic en optical recording

Magnetic discs and tape


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.


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


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.


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 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.


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.