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Widescreen TV 5. 'The HDTV Challenge' by Richard G. Elen


A 42-inch flat-screen plasma television

A 42" flat-screen plasma television suitable for high-definition broadcasts

Most of the discussion so far has centred on standard-definition TV - and particularly in the UK, as it is more complex here. But we also need to consider HDTV, as over time it will become increasingly important. Although any kind of digital TV is capable of delivering better pictures (because the transmission is less susceptible to interference, multipath and other effects from which analogue TV suffers) assuming broadcasters allocate enough bandwidth to them, (which is not always the case), and while HDTV is digital, not all digital TV is high definition. We have already see this with the UK system, which is standard definition, digital TV. So let's look at what we mean by 'high definition' - the meaning has evidently changed since the BBC's 240/405-line Television Service was introduced as the 'world's first high-definition service' in 1936!

The 'resolution' of a TV image depends to a large extent on the number of (horizontal) lines displayed on the screen - this is technically known as the vertical resolution. For a change, let's use some US specifications: although the BBC was one of the first to transmit high-definition television experimentally, today there are more HDTV sources (around 40 channels at the time of writing) in the US than in the UK (where there is currently just one).

A standard NTSC television image consists of 525 lines, of which 480 are visible. By comparison, a regular DVD has a resolution equivalent to about 450 lines, and standard VHS, 240 lines. A regular TV image is also interlaced, which means that each frame of video consists of two fields of 240 lines each, and a complete frame (all 480 visible lines) is displayed every 30th of a second. Thus standard NTSC is known as 525i or 480i. The PAL equivalent can be described as 625i or 576i - the latter if we want to refer to visible lines).

Digital video can also employ a different method of scanning the screen, known as progressive. Progressive scan means that instead of being interlaced, the entire 480 lines are displayed at once, and this occurs in the same time as a regular field, ie in the US, in a 60th of a second.

This means that twice as much information is delivered in the same time, and the resulting image has less flicker and movement is smoother. A progressive scan system with the same number of visible lines as NTSC would be called '480p', and in PAL, '576p'. Progressive scan is the standard method used by computer monitors. Incidentally, to give a computer monitor equivalent to these screen resolutions for comparison, NTSC is 854 x 480 and PAL, 1024 x 576 pixels.

Today, the most common HDTV formats in the US are 720p (720 visible lines, progressive scan - 1280 x 720 pixels) and 1080i (1080 visible lines, interlaced - 1920 x 1080 pixels). These will also be the formats used for HDTV in Europe.

1080i requires the smaller bandwidth of the two and because it has more lines, the image is clearer and sharper than regular television, particularly when the images is static or only moving slightly: it has a higher 'spatial resolution'.

720p displays entire images more frequently, making it better for displaying graphics and fast-moving images without blurring: it has higher 'temporal resolution'.

HDTV sets have to handle both these formats, and in addition, something in your home entertainment system should be able to 'scale' (convert) other common formats including SD to these specifications.

However it should be noted that 1080i was originally designed for conventional (CRT-based) TV sets. For any kind of matrix display, such as a plasma flat-screen, DLP projector or LCD panel, a progressive input is best. These displays may handle an interlaced input, but they will de-interlace it internally, and the results may be disappointing.

Also bear in mind that you will get the best results on-screen if you minimise the amount of processing you are performing on the image. Don't carry out multiple standards, progressive/interlaced, or scaling conversions unnecessarily - you'll see the degradation this causes, especially on a big screen.

It should be noted that HDTV specifies only a 16:9 aspect ratio - there is no 4:3 mode. As a result an HD display - or the HD inputs on a multi-standard display - may only accept 16:9 signals. What happens if you input a 4:3 source will vary between manufacturers, but it is quite common for the input to be stretched, automatically and unchangeably, to full screen width. You do not want this. As a result, if you are using a single display interface such as HDMI, which may be the only interface available for HDTV (as Sky have announced in the UK), you should scale your video sources externally so that the display is always fed with a 16:9 input. This means, for example, that your scaler should 'pillarbox' a 4:3 source into a 16:9 frame.

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The HDTV Challenge

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