How DVD Players Work

The key to DVD technology is the increased data-storage capacity, and this is enhanced even further by the use of double-sided and double-layered discs. This is one of the factors that allows the technology to far outstrip the old Video CD format, as well as VHS tape.

The data track is also more tightly compressed on a DVD, with just 740 nanometers between each groove. Again, this is roughly half the distant between the grooves on a CD. Extra storage is also made available by using more of the disc surface, and the error-correction needed for DVD is not as wasteful of space as it is for CD.

This only goes part of the way to explaining the excellent picture quality of DVD. The problem has also been addressed from the other angle - the amount of data that needs to be stored in the first place.

Video-compression technology made a huge advance with the introduction of MPEG-2 encoding. Formulated by the modestly titled Moving Picture Experts Group, this compressed a video signal into a fraction of the space normally needed, without introducing picture degradation.

The compressed video signal is read by the laser pickup in a DVD player and decoded, resulting in a video signal that can be delivered to a display. Again this area sees an improvement on CD technology - the laser used in a DVD player has a shorter wavelength, enabling it to read the smaller, more tightly packed bumps on a DVD.

The laser light bounces off a reflective layer behind the data layer and back to an optical pickup. This can distinguish between light that has bounced back from a bump, or from a 'land' (the area between bumps). This simple process allows the DVD player to construct a stream of data, forming bits, bytes, and eventually gigabytes of information.

Equally critical is the tracking mechanism, which moves the laser over the spinning disc. This needs to be able to move nanometers at a time, and because the rate at which data passes underneath the laser pickup has to be constant, the speed at which the disc spins has to gradually slow down as the pickup moves from the centre of the disc to the outside.

The data retrieved is in digital format and in order to be understood by your display device, it needs to be converted. This is the job of the video and audio DACs (Digital to Analogue Converters) in your player, although decks are now appearing with digital video outputs to eliminate the need for this potentially degrading process.