The telly is pretty much dead and gone now. Walk into any high street shop that sells televisions and all you’ll see is a vast array of wide-screen flat-panel television sets.

If you’re thinking about buying a new flat-screen TV then you’ve probably heard the advice that says you should look out for the HD-Ready logo that ensures a TV can be used to display high-definition television and films. That’s good advice ­ but it’s not the whole story.

There are actually three different types of HD (high-definition) formats currently being used by the television and film industries, and different TV sets will handle these formats in different ways. It’s therefore worth going into a little more detail so that you know exactly what you’re getting before you splash out hundreds of pounds on a new set.

Format frenzy
High-definition television broadcasts, such as those available from Sky or Virgin Media, tend to use two different formats, known as 720p and 1080i. The third format is called 1080p, and is used by Hollywood film studios when they release their films on Blu-ray or HD DVD discs. We’ll explain what these numbers mean in a second.

The good news is that any television that carries the HD-Ready logo will work with all three of these formats. That’s why it’s so important to look out for that logo, as it ensures that you’ll never miss out on watching any HD films or television programmes because you accidentally bought the wrong type of set.

However, each of these three formats provides a different level of image quality, and some of the more basic HD-Ready television sets may not be able to give you the full image quality made possible by each format. So to understand what’s going on here we need to explain how these different HD formats work.

Line by line
The most basic of the three HD formats is 720p. Less expensive HD-Ready television sets will primarily be designed with this format in mind. The screen on 720p sets is divided into 720 horizontal lines of pixels. In order to create the appearance of a smoothly moving image, those 720 lines are rapidly updated (or ‘refreshed’) dozens of times every second.

The ‘p’ in ‘720p’ means that the image is updated using a technique known as ‘progressive scan’. The TV starts by updating the first horizontal line in the image at the top of the screen, and then progresses down to the bottom of the screen one line at a time. Once that’s done ­ and the whole image has been ‘refreshed’ ­ the TV jumps back to the first line of the image and starts all over again. If you’ve got a 720p television set and you’re watching a television programme on Sky HD that is also broadcast in 720p format then your TV set will be a perfect match for that broadcast, and you’ll see the programme in all its high-definition glory.

But, as we’ve mentioned, high-definition TV broadcasts can also use another format, called 1080i. 1080i programmes are designed to display on a TV set with 1,080 horizontal lines across the screen. However, the ‘i’ in 1080i means that the image is updated using a technique known as ‘interlacing’ rather than the progressive scan technique of the 720p format. This means that instead of updating every line in the image, the TV only updates every other line ­ in effect, it only updates half the image at a time. Once it’s done that it goes back and updates the second half of the image.

It’s important to know that a basic HD-Ready TV set that has a 720-line screen can still handle a TV signal that is broadcast in 1080i. But, in order to do so it will need to shrink the image down from a 1,080-line image to 720 lines, causing a noticeable loss in detail.