Physics of MiniDisk Players

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Introduction:

In 1986, designers at Sony began the quest to develop a sound medium that combined the benefits of the compact disc, such as sound quality and track accessibility, with the recordability of the cassette tape, in a player with unprecedented portability. They gave engineers the task of creating what would later be called the Minidisc, a wallet sized unit that could record, play, and store music on cartridge enclosed discs. Engineers integrated several technologies to produce the Minidisc, all utilizing the laws of physics. They found that:

-Laser optics would be the obvious choice for reading data.

-Magneto optic technology would be created to record information.

-To allow recordability, new disc properties had to be defined.

-Compression and acoustic principles would be incorporated to store information on a disc 1/5 the size of the compact disc.

The laser used in the minidisc player serves two purposes. Its primary role is to read digital information in a similar fashion as a CD player. Its secondary, but equally important, function is to supply focused heat to the minidisc itself during the recording process.

LASER HEAT: The minidisc recording system takes advantage of a discovery by Pierre Curie in the 1880's. He found that when iron is heated up to a certain temperature, now called the Curie point, it loses its magnetic properties. Similarly, when the minidisc laser is directed at a tiny point of the disc during recording, it heats that spot up to 180 degrees celsius, and the disc material loses its magnetic properties. A magnetic field can then be "frozen" onto that point once it moves past the laser beam, storing data. For more information on the recording process, visit...

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...o-acoustic principles, which states that the ear is more sensitive to some frequencies of sound than others. Psychoacoustics also explores the relationship between the intensity of sound (its decibel level) and its frequency. As you can see from the equi-loudness curve below, a low frequency, high intensity wave will produce the same loudness as a high frequency, low intensity wave.The masking of sound is another factor considered during the compression process. This occurs when one sound blocks another sound from being heard. For example, on a crisp fall day you would be able to hear leaves rustling in the wind until a marching band stomps by. ATRAC would omit the rustling leaves data that was masked by the band, saving storage space. Once the audible sound data is singled out, it can easily be compressed and stored with an inperceivable change in sound quality.

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