This type of support combines the optical and magnetic proprieties of certain materials, making the most of each of their advantages. These materials contain both magnetic proprieties, in the sense that above a certain temperature called the Curie temperature the particles are no longer bound to one another and dispose themselves according to an applied magnetic field (applied by a coil traversed by current for example), and as well as these they also have optical proprieties, in that they can reflect incident light.
Let's now take a look at how recording and reproduction takes place on these support systems.
Recording:
A very powerful laser is beamed towards the disc in order to heat up a particular area on its surface to a temperature that is above the Curie temperature.
Once the material in this area begins to melt, the magnetic particles dispose themselves randomly upon the disc's surface.
A magnetic head containing a coil applies a magnetic field to the area in question which follows that of the signal that will be stored (clearly this is the sampled audio signal, that is, converted in a 0 and 1 sequence).
When the laser moves away from the area, the latter cools off and the magnetic particles remain facing the position induced by the head.
Reproduction:
In this phase the support behaves in every respect like an optical support and therefore is read by using a laser beam as with all normal optical readers.
This system uses just this type of magneto-optic support. The following is a specifics-table supplied by the manufacturers:
Table 19.1. MiniDisc Specifics
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The first thing to take into account when analyzing these specifics, is the compression system called ATRAC (Adoptive Transform Acoustic Coding). This system is based on the frequency masking principle which theorizes the possibility of ignoring information pertaining to frequencies adjacent to other ones with far greater amplitudes. This approach is justified by the fact that, as we have seen, the basic membrane inside the human ear [Inner ear ] is covered in hair follicles and each of its different areas receives certain frequency bands. It may happen that if a frequency with a high amplitude is stimulating a certain area of the membrane, the hair follicles all get used up in receiving that particular frequency that has a greater amplitude. In this sense the adjacent frequency with a smaller amplitude would be masked by the greater one. By and large this system works and the quality of sound produced by the MiniDisc is undoubtedly high for what it costs and the technology it uses, however though, compression is to say the least highly inadvisable in professional practice. So, the MiniDisc and all other systems that use data-compression algorithms (such as the famous MP3) ought to be used in contexts where quality isn't an absolute priority. For example a radio promo can happily be stored in MP3 form or on a MiniDisc seeing that the sound quality of radio-television shows is such as to not highlight the difference between compressed and uncompressed data.
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