Digital Audio Media - Magneto-optic media

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This type of support makes the most of the optical and magnetic proprieties of certain materials. Above a certain temperature called the Curie temperature the particles are no longer bound to one another and arrange themselves according to an applied magnetic field (applied by a coil through which a current flows). 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 onto the disc in order to heat up a particular area on its surface to a level 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 applies a magnetic field to the area in question which follows the signal that will be stored (clearly this is the sampled audio signal, namely, converted in a 0 and 1 sequence).

  • When the laser moves away from the area, the affected surface cools off and the magnetic particles remain in 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 is the case with all normal optical readers.

19.4.1. The MiniDisc

This system uses just this type of magneto-optical support. The following is a specifics-table supplied by the manufacturers:

Table 19.1. MiniDisc Specifics 

Channels2 (Left, Right)
Frequency response5 Hz - 20 KHz
Dynamic105 dB
Sampling Frequency44.1 KHz
Data compression systemATRAC
Storage capacity130 Mb
Error correction systemCIRC
Length74 minutes

The first thing to take into account when analyzing these specifics, is the compression system called ATRAC (Adaptive Transform Acoustic Coding). This system is based on the frequency masking principle which states the possibility of ignoring information related to frequencies adjacent to others with far greater amplitudes. This approach is supported by the fact that, as we have seen, the basilar membrane inside the human ear [Inner ear ] is covered in hair follicles and each of its different areas is sensitive to specific frequency bands. It may happen that if a frequency with a high amplitude is stimulating a certain area of the membrane, all the hair follicles are busy receiving that particular high-amplitude frequency. The latter would end up masking an adjacent frequency with a smaller amplitude. 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 format or on a MiniDisc: in radio-television contexts the sound quality is quite low and the difference between compressed and uncompressed data can be hard to spot.