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Como se graba físicamente un CD

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Como se graba físicamente un CD

escrito por Administrator

Saturday, 26 de November de 2005

Hay dos tipos de cd's vírgenes en el mercado, los normales y los regrabables. Pero, ¿como son grabados físicamente por las grabadoras de cd's?
Compacts CD-R (Compact Disc-Read)

Como ya sabes los CD-R sólo se pueden grabar una vez. Basta que haya un mínimo fallo en la grabación para echar a la basura el CD. Cada CD Grabable tiene una capa reflectante y un sustrato orgánico o superficie. Cuando grabamos un CD-R la superficie de éste se funde, literal e irreversiblemente.
Los datos se almacenan de forma que la lectora pueda leer ese código (o formato ya definido), y luego decodificarlo para dar la señal acústica, o bien datos, imágenes, texto, etc.
Para esto la grabadora crea unos pits y unos lands cambiando la reflectividad de la superficie del CD. Los pits son zonas donde el láser quema la superficie con mayor potencia, creando ahí una zona de baja reflectividad. Los lands, son justamente lo contrario, son zonas que mantienen su alta reflectividad inicial, justamente porque la potencia del láser se reduce.
Según el lector detecte una secuencia de pits o lands, tendremos unos datos u otros. Para formar un pit es necesario quemar la superficie a unos 250º C. En ese momento, el policarbonato que tiene la superfie se expande hasta cubrir el espacio que quede libre, siendo suficientes entre 4 y 11 mW para quemar esta superficie, claro que el área quemada en cada pit es ridícula.

Compacts CD-RW (Compact Disc-Read/Write)

En este caso ya no es necesario tirar el CD a la basura en caso de un fallo, ya que como su nombre lo indica, estos CDs se pueden grabar una y otra vez. ¿Pero cómo es posible si se funde la superficie? En realidad, en este caso la superficie no se funde como en los CD-R, sino que el láser hace que la superficie cambie de propiedades.
Esto es posible ya que es una superficie algo "especial". Está formada en esencia por plata, teluro, indio y antimonio. Inicialmente (el disco está sin nada, completamente vacío de datos...) esta superficie tiene una estructura policristalina o de alta reflectividad. Si el software le "dice" a la grabadora que debe simular un pit, entonces lo que hará será aumentar con el láser la temperatura de la superficie hasta los 600 o 700 ºC, con lo que la superficie pasa a tener ahora una estructura no cristalina o de baja reflectividad. (Ingenioso, ¿eh?) Cuando debe aparecer un land, entonces se baja la potencia del láser para dejar intacta la estructura policristalina.
Para borrar el disco se quema la superficie a unos 200 ºC durante un tiempo prolongado (de 20 a 40 minutos) haciendo retornar todo este "mejunge" a su estado cristalino inicial. En teoría deberíamos poder borrar la superficie unas 1000 veces, más o menos, aunque con el uso lo más probable es que se te estropee el CD y tengas que tirarlo antes de poder usarlo tantas veces.

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