DVD Player Advanced Guide

The principles
A DVD is actually a group of thin plastic layers glued together to create a disc that is 1.2mm in depth. One of the layers contains a series of bumps (often referred to as pits, although it is just a question of which side of the disc you view them from) that can be read by a laser in the DVD player. The bumps are arranged in a single, continuous spiral and the DVD player spins the disc to pass the bumps under the disc-reading mechanism.

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.

How it works
The bumps of data on a DVD are microscopically small, measuring just 320 nanometers wide (a nanometer is one billionth of a metre). This is about half the size of the bumps on a CD.

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.

Glossary
Dolby Digital – A multi-channel sound format that delivers audio to a home cinema speaker system

DTS – A rival sound format to Dolby Digital that does the same job. Opinion is divided on which is the superior format

DVD – Digital Versatile Disc. Can be DVD Video or DVD-Audio discs

DVI – Digital Visual Interface, a digital video-only input or output

HDMI – High Definition Multimedia Input – also known as the ‘digital Scart’, it carries video and audio signals in digital form

High-definition – A superior delivery format for TV that presents more lines of detail, resulting in a much sharper, more detailed image. Due to launch in the UK on Sky in 2006.

Progressive scan – A method by which image detail on a TV picture is enhanced by showing both halves of a single frame at the same time, as opposed to showing only half a frame at a time

RGB – A video signal format that offers better picture quality than a standard video connection. Requires RGB-capable Scart inputs and outputs

Scart – An audio-visual cable that carries pictures and sound from a DVD player or VCR to a TV