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Reoc A6 DVD - FAQ - What is DVD |
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| What is DVD? |
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DVD (Digital Versatile
Disc), launched in 1997, has quickly become an
established medium for home viewing. It represents the
biggest leap forward in home entertainment technology since
the advent of the compact disc back in the early
eighties. Indeed they may look similar to audio cd's, rely on similar physical technology, 12cm's with a shiny side etc, but there's where the similarity ends. CD's are recorded digitally in a stream from the centre outwards in tracks with 'pits' (or bumps, depending on which side you look at) and 'lands' (flat 'unburnt' areas) which are read by a laser. DVD uses a similar technology but the gap between the spiral tracks is much smaller and 'pits' and 'lands' are also much smaller. Error correction data on DVD players is much more efficient than CD audio so more actual 'content' can be placed in any given area, and with the possibility of adding a second layer on each side you begin to see how far this technology has moved since the humble Audio CD. |
CD |
DVD |
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On a single sided, single layer DVD there is enough room for 133(ish) minutes of high quality audio/visual content, More than enough room for a feature film and a few goodies like extra soundtracks, directors comments and anything else the producers decide to put on the DVD, the limits are virtually boundless, only limited to the designers imagination. Below is a table comparing DVD directly with CD aspect by aspect.
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A Note About MPEG2 Moving images on a video disc consume an enormous amount of information. As incredible as the DVD's capacity is, it would only hold 5 to 10 minutes of a movie if there was no compression involved. Enter MPEG-2, a data compression technology. Developed by the Motion Picture Experts Group. MPEG-2 is the system that is now used to encode satellite television signals.M Put simply, MPEG-2 condenses video by looking for repetitive image signals such as the background of a static shot in a movie. Rather than continuously repeating this information, it uses it only once until the scene or action in that area changes. The result is the capability to encode up to 8 hours of digital video on a double-sided, dual-layer DVD. The magic of DVD starts with several innovations that were developed by Panasonic. First, a unique red laser with a short wavelength is used to read the ultra small pits on a DVD disc. Second, to ensure compatibility with existing CDs and CD-ROMs, Panasonic developed a hologram lens which splits the laser beam so that it can focus on DVD data at a depth of .6mm, and on CD information which rests 1.2mm deep. 
Gold Layer A dual-layer DVD disc is constructed of two thin substrates that are joined by a unique Panasonic bonding system. The first is semi-transparent and gold in colour. The laser reads the information on this layer first. This layer is recorded from the centre of the disc outwards. UV Adhesive The layers of a DVD disc are bonded by a Panasonic process that utilizes a photo-polymer resin. It starts out amber in colour, but the ultraviolet light curing process turns it clear so the laser can read through it. Silver Layer When the laser reaches the end of the first layer, it increases in power slightly and begins to read the second layer, which is silver in colour. This layer is recorded from the outside towards the middle. The switch in layers is (almost) seamless and allows you to watch movies without flipping a disc over. DVD's are surely going to become the widest used data storage medium for video, audio and data because of its physical size, capacity and durability. Although comparitively expensive to create your own at the moment, this is bound to get cheaper in the years to come. |
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