What is DVD? Updated from Ultimate DVD #6

DVD UNCOVERED

Here's our A to Z overview of the most important things you may need
to know about choosing and using a DVD system.
Part 2 is here, including our run-down on those crucial Region formats

DVD

Digital Versatile Discs. They may look like compact discs, but they can contain sound and vision, computer programs and exceptionally good audio recordings.

It can hold over two hours of high-quality digital video and up to eight hours on a double-sided, dual-layer disc (see Layers and ‘The Future’) and up to eight tracks of digital audio (which often includes foreign soundtracks on DVD).

As the discs are not touched by anything except laser light (which reads the information on the disc) they should not wear out.

100Hz scanning

A method of reducing the flicker on larger televisions by doubling the number of times the picture on the screen is re-drawn every second. Requires large amount of digital processing and can often introduce unwanted effects in the picture, especially on areas of fast motion.

16:9

Widescreen television ratio (see Anamorphic) [aka 1.85:1]

4:3

Standard television screen ratio (see Anamorphic) [aka 1.33:1]

5.1

Any sound system using 6 separate sound channels: 2 main left and right, 2 rear left and right, a centre speaker, and the optional woofer speaker (hence the .1). It needs to be decoded usually in a Home Cinema amplifier, although more and more DVD players have decoders built in.

Bit-rate Meter

Included on many DVD players to monitor the datarate (up to approximately 10Mbits/s) of the MPEG-2 bitstream which carries pictures, audio and graphics. Of little real use other than to demonstrate that scenes with little movement in them require only a low bit-rate, whereas fast, detailed action requires a much higher bit-rate.

Compression

This is a way of dramatically reducing the amount of data that needs to be recorded on the DVD by using the fact that television pictures do not generally change much from one frame to the next.

Take for example a cow eating grass in a field. The cow will move slightly from one frame to the next, but the background will hardly change...

continued next column

Compression (continued)

So it’s only really necessary to send the initial picture and then send the small changes between this picture and the next.

DVD uses a compression system called MPEG-2 (the same as is used for digital television broadcasting). The first frame of a shot is called an ‘I-Frame’ and is basically a complete picture. To avoid the image deteriorating too much, new I-frames are generally inserted every 15 frames or so, or whenever there is a shot change.

When DVDs are made the compression rate is changed depending on the scenes. The cow scene will have lots of compression (establish the first frame and then record the few changes after that), while the fast action scene will have a low compression rate as everything changes all the time.

Digital

A method of representing constantly changing information such as an audio signal by sampling the signal at short intervals and storing it as a numerical value as a series of on or off signals.

By employing sophisticated error correction, DVD players can generally correct or conceal errors caused by dirt or scratches on the disc surface so that the viewer never sees any problems – a far cry from the dropout problems that plague VHS cassettes!

Dolby Digital

A digitally compressed sound system providing up to six separate full-range sound channels.

Dolby Pro-Logic

A extremely popular successor to Dolby Surround, it had an added centre dialogue channel.

Dolby Surround Sound

A sound system using four speakers, where the front two were stereo and the rear two mono with a limited frequency range.

DTS

Digital Theatre Sound. An alternative to Dolby Digital, but using lower compression to give a supposedly better quality audio, but it does take up more disc space. It is currently primarily used on some Region 1 discs.

Easter Eggs

Hidden surprises on the DVD. They are hidden in all sorts of ways, so happy hunting!

Feature © Visual Imagination Ltd 2000. Not for reproduction

For details of Anamorphic displays – get the full feature
in each issue of Ultimate DVD

The Future

DVD-Audio
Audio DVDs are known as DVD-Audio (makes sense!) and will have better quality sound than compact discs. They are expected to be released early in 2000.

DVD-R
Recordable DVD (DVD-R) may be coming out in 2000, but it probably depends more on what copying protection etc is insisted on by the software providers. It will not work with current, analogue, TV signals.

Fitting more on a disc
Currently the lasers that read all DVDs and CDs use red light, but lasers using blue light are being researched. These will be capable of reading even smaller areas of the discs than the red, and so more information can be packed onto the disc…

Layers

DVD discs can have up to two layers per side. The top layer is semi-transparent, allowing the player’s laser scanning head to focus through it onto the lower layer. By changing the focus position, data can be read from either layer as required. These are the best kind of discs to buy.

Some work by having a layer on each side of the disc, which means that at the end of one layer/side you have to turn the disc over. This is so unpopular that in the States it has almost vanished!

Macrovision

An analogue copy-protection system employed in both sell-through VHS cassettes and DVD players to prevent the programme being copied onto videotape. The Macrovision circuit is built into the DVD player and is turned on by an instruction from the disc.

Not all discs turn the Macrovision on, as this requires a royalty payment to the Macrovision Corporation, which cut into the disc producer’s profit margins. Unfortunately the Macrovision signal sometimes causes unwanted side-effects on some video projectors and a small number of televisions.

MPEG

A way of compressing very high bit-rate digital video to a more manageable size so that it can be stored on discs or transmitted. MPEG-2 is the variant used by digital broadcasting and DVD.

Plasma screens

Flat, thin and wall-mountable television sets. These hi-tech televisions are currently very expensive.

S-Video

Super-Video. This way of outputting a video signal gives one of the best possible quality pictures. It must, however, be connected to a TV and/or amplifier that can accept S-Video signals.

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