What does DVD mean

A. DVD stands for Digital Versatile Discs.

Q. How does a DVD store data

A. Like a CD, the data on a DVD is encoded in the form of small bumps in the track of the disc.

Q. How do DVDs differ from CDs

A. A DVD is very similar to a CD, but it has a much larger data capacity, holding about seven times more data than a CD does. This huge capacity means that a DVD has enough room to store a full-length film, as well as a lot of other information.

Q. How do DVDs store more data than CDs

A.There are three main reasons for this. DVDs have smaller bumps and tracks and so have a higher density storage capacity. A CD uses an old and inefficient error correction scheme, so repetitive, superfluous information has to be encoded, whereas a DVD stores much more real information. Finally, a DVD can have up to four layers, two on each side.

Q. What are DVD discs made of

A. A DVD is composed of several layers of polycarbonate plastic with microscopic bumps, representing encoded data, arranged as a single, continuous and extremely long spiral track of data. A covering of reflective layers protects the bumps: aluminium is used behind the inner layers, and a semi-reflective gold layer is used for the outer layers. After all of the layers are made, each one is coated with lacquer, squeezed together and treated under infrared light.

Q. How does a DVD player work

A. A DVD player shines a laser beam onto the surface of the disc to read the pattern of bumps. Light reflected back is picked up by a photodetector and microprocessors decode the different bumps light patterns back into data.

Q. How can a DVD have two layers of data, doesn't this confuse the DVD player

A. The large storage capacity of a DVD is due partly by using two layers of data, instead of one, like on a CD. The semi reflective coating used for the outer layer allows the laser to focus through the outer and onto the inner layers.

Q. Are only new films available on DVD

A. No, older ones are being moved onto DVD format. Also, because DVD manufacturing costs are relatively low, often, a film comes out on DVD before it comes out on video.

by Lisa Cardy

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