High definition (HD) is a high-resolution form of digital television. Many more lines and pixels make up the picture, giving a clearer, more-detailed and crisper look than standard definition.

HDTV sets

To get the benefit of high definition pictures, you need an HDTV set to view them on.

HDTV sets come in formats of 720p, 1080i and 1080p. The number indicates how many horizontal lines are in the picture - so a 1080i has 1080 horizontal lines. By comparison, standard definition (SD) screens have only 576 horizontal lines.

High definition screens also use many more pixels to make up the picture. A 1080i TV can show around 5 times as many pixels as a similar-sized SD screen.

But what really counts is the letter. This tells you the type of technology used.

  • "i" stands for interlaced scan. This draws the picture across the screen in two passes, alternating between odd and even lines. Only half the picture is displayed at any one time - but it happens so quickly that the human eye sees one continuous picture. CRT TVs use this method.
  • "p" stands for progressive scan. This delivers the picture to the screen in one pass, meaning the whole picture is on screen at all times. Some 1080p models have effectively twice the picture detail of a 1080i, because the all of 1080p's picture is there at any one time.

A "p" screen produces a smoother picture with less flicker, so it's usually considered better than an "i" -  especially for fast moving action such as sports. Even though it's got fewer lines, a 720p can give you better sports viewing than a 1080i.

Name Resolution
1080p 1920 x 1080
1080i 1920 x 1080
720p 1280 x 720
SDTV 576 lines

Note: 1366 x 768 screens are slightly enhanced 720p models.

Screen size
The size of the screen is important too: a big screen with the same resolution as a smaller one will have bigger pixels. Why does this matter? Because you don't want to be aware of the pixels. They make the picture look "grainy".

As a general rule, you shouldn't see pixels on a 42 inch, 720p provided you're sitting just over 3 metres away. Having a 1080i or 1080p screen of the same size would allow you to sit around 1.6 metres from the screen. But if you normally sit around 3 metres back anyway, the higher resolution will have no real benefit.

HDTV Broadcasts

Picture quality doesn't just depend on the type of screen you have - source and connections are just as important. If you feed a low-quality analogue broadcast into your HDTV, you'll get a low-quality analogue image. For the best picture quality you need an HDTV signal transmitted directly to an HD screen.

Here's a rundown of HD and SD sources:

  • Broadcast TV (including much of Sky's satellite service) is usually SD. The motion blur you see in fast-moving action will usually be from the TV signal, not from your TV set.
  • Freeview terrestrial shows some content in HD but a greater amount in SD. The satellite service doesn't show HD yet.
  • MySkyHDi shows a lot of sport and movies in HD, but most other channels are in SD.
  • DVDs are SD but you can make them look better by connecting with component video or HDMI cables, rather than the more common (red, white and yellow) composite cables. (See Connections for more information.)
  • Blu-ray movies and discs are full HD (1080p) resolution.
  • Game consoles are mostly HD. The PlayStation 3 supports full HD (1080p) and the Xbox 360 supports up to 720p and 1080i. The Nintendo Wii is SD.
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