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emco: english >Know-how >Computer and Internet >DVD >
What is DVD?
DVD, or Digital Versatile Disc has been making an increasing impact in home viewing format for the last two years and, in my opinion, will eventually become popular enough to see off the old VCR format for good, in the same way that CD technology has largely replaced the audio cassette. The following notes trace the technological history behind the DVD.
In basic terms, a DVD, like a Compact Disc, stores (audio-visual) information digitally and can be replayed using a DVD Player. Sound and picture quality are therefore digitally perfect (although disks are very susceptible to dust and scratch interference and must be treated with care). They will not easily deteriorate over time, thus offering a viewing format far superior to video tape.
Predecessors of DVD
8mm and Super 8mm
Moving images were captured on celluloid film, using a cine camera, and after processing could be viewed by light projection onto a screen.
Reel to Reel videotape
An analogue magnetic format with two separate tape spools of either pre-recorded or home-recorded magnetic tape.
Video cassette
An analogue magnetic format, whereby reel to reel is miniaturised within one case. Light and sound waves are recorded magnetically and are viewed on a television screen. There are some constraints on the use of this medium in terms of reproducing and exhibiting data:
a) The quality of the medium used to store information is often compromised because of the variable standard of coating on the tape which can result in background noise (hiss) which then has to be separated from the signal.
b) The strength of the original signal may not always be reproduced accurately, resulting in dull or saturated pictures and compromised sound quality.
12" laser discs
These were developed in the late 70’s by Philips. They were based on CD technology whereby information is reproduced digitally through binary coding.
Basically the process, for this and other digital formats, involves
analogue source>digitisation > information encoded onto a disc.
Playback occurs when digital info is read and then converted back to analogue format.
DVDs, compression and storage
The quality of a digitally encoded image (resolution) is dependent on the amount of storage space that can be allocated to each frame. The amount of information needed to digitally represent even a single frame is immense, so it is compressed in a variety of ways:
a) Substitution = symbols are used to replace common code patterns
b) Mathematical = formulaic rather than substitution
c) Lossy = a mathematical formula decides on which is the important information and ignores the rest, then compresses the essential info. MPEG is one example of this. A knowledge of context is required. Video is often compressed by just recording the difference between frames, rather than recording all the information within each frame.
Storage capacity
A CD can store 650 mb of info A DVD can store between 4.5 and 18Gb. A DVD stores info in a number of formats which is read by a laser focused at one or two depths (layers) on one or both sides of the disc. This provides greater storage capacity resulting in longer play times.
Different DVD formats
a) Single sided, single layer
b) Double sided, single layer
c) Single sided, double layer
d) Double sided, double layer
The capacity progresses from 4.5gb in a) to 18gb in d)
DVD players must be able to play all forms. Early DVD discs were double / single which meant the disc had to be turned over halfway through a film. Now they are often single / double which results in a few seconds freeze as the laser refocuses. Double / double has the same quality as broadcast TV but it is still not possible to reproduce film quality at home.
Advantages of DVD
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The quality of reproduction is superior to VHS and SVHS video.
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- UK TV = resolution of 625 lines per screen
- VHS = 200
- SVHS = 400
- DVD = 480 plus
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Frames are reproduced digitally which allows a perfect freeze frame and slow motion forward and back
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The capability to encode multiple sound tracks allow producers to incorporate multi language versions
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Sound is reproduced digitally so it is possible to recreate the sound system of the cinema e.g. Dolby pro logic
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Additional features are possible, such as director / actor commentary (great for analysis!)
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DVD offers extras such as trailers, the ‘making of’, outtakes, deleted scenes and alternate endings
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Perfect still frame means that producers can include the screenplay page by page and allow for accurate scene selection
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Increasingly, viewers will be offered a choice of different camera angles (driven by developments in porn technology) which will increase personalisation of viewing.
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Author: Mandy Esseen. © Mandy Esseen 2002. With kind permission of MediaEd.
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