DVD | Functions

• 1. A one-sided digital video disc (DVD) is made up of four main layers. First there's a thick polycarbonate plastic that provides a foundation for the other layers. Next, a much thinner layer of opaque, reflective material is laid on the base. Then come a thin layer of transparent film, and finally a surface layer of clear, protective plastic. Data, including video, audio, text, or programs, is represented, as with a CD-ROM, by a combination of flat areas (lands) and indentations (pits) on two of the surfaces—the transparent film and the shiny opaque layer

• The DVD pits, however, are much smaller, which is part of the reason the DVD holds as much as 8.5MB of data, the capacity of 13 compact discs.

• 2. Like a CD-ROM drive, a DVD drive uses a laser to read the lands and pits. But the DVD laser uses light that has a shorter wavelength, which makes the laser beam narrow enough to accurately read the smaller pits and lands on the DVD surfaces.
• 3. By changing the amount of current flowing through a magnetic coil surrounding the laser beam, the DVD read head focuses the beam so that it's concentrated on only the surface of the transparent film.
• 4. Where the laser beam hits a pit, the indentation scatters the light in all directions.
  
• 5. But when the beam of light hits a flat area, it is reflected back to the read head, where a prism deflects the light to a device that converts the bursts of light energy into bursts of electricity. The computer interprets those electrical pulses as code and data.
• 6. The layer of transparent film accounts for only half the data DVD can contain.
• By adjusting the amount of current in the coils surrounding the laser, the read head can change the focal length of the laser beam so that it passes through the transparent layer with little distortion.
• The beam then strikes the opaque layer and reads the pits and lands on it the same as it does with the transparent layer.
• 7. The capacity of a single-sided DVD is doubled when the same layers of opaque and transparent materials are applied to the other side of the disc. But because current DVD drives have only one read head, you must remove the doubled-side DVD and flip it over to read data on the second side.