The high-definition (HD) hype has been hard to miss.

Most new television sets bear stickers proclaiming them to be ‘HD ready’, broadcasters such as Sky have spent millions aggressively marketing packages of HD channels and even BBC One is available in HD, so equipped viewers can watch the latest Eastenders in full HD glory.

However, all the jargon surrounding HD can be confusing.

Venture into an electrical store and the assistant is likely to tempt you with phrases like 1080i, 720p, Blu-ray and much more – not to mention sell you a few expensive cables to improve the picture quality.

And that, of course, is just the equipment: there are numerous confusing TV-service packages to navigate, including cable, satellite and traditional over-the-air options such as Freeview and Freeview HD.

In fact, a surprising number of people still don’t know what they need to watch films or live TV in high definition. Some are even convinced that they’re watching HD when they’re not.

But don’t panic – understanding all the terminology attached to HD is quite straightforward, and in this article we’re going to explain all you could ever want or need to know about HD. By the end, you’ll understand the difference between standard- and high-definition pictures and have confidence that you’re watching HD.

Looking for clarity
No-one likes to receive a photograph with details that are hard to make out, watch a TV channel where the faces are fuzzy, or be handed a printout where the text and graphics are blocky and tricky to decipher (from a fax machine, for example).

Issues like these are often a matter of definition, or resolution – the amount of available detail.

However, trying to define definition is complicated and depends on various factors.

Fax machines send and receive images measured by the number of dots per inch (dpi), for example, while a traditional printed photograph’s definition can be traced right down to the film’s grain.

Electron gun
The resolution of television sets, on the other hand, was for a long time defined by the number of picture lines displayed by the screen, from the top to the bottom.

These lines were created by an extremely fast-moving electron gun that fired at phosphors adhering to the screen’s rearside.

On European sets, there were 625 of these lines (though some of these were used for the old Ceefax and Oracle teletext services, rather than the picture – but let’s not digress).

With the digital revolution it has all changed.

The TV screens and computer displays that we all use today have digital screens controlled by sophisticated electronics. The picture is made up of a matrix of tiny dots, called pixels.

Each pixel can have its brightness or colour changed to help make up the whole picture, whether that picture is supplied by the graphics card in a computer, a broadcast TV signal or a DVD or Blu-ray disc.

The digital equivalent of a traditional TV picture is often referred to as ‘standard definition’ and made up of a line of 720 pixels across the screen, and 576 lines, or pixels, from top to bottom; 576 may seem less than the 625 of analogue TV, but because of that old teletext information, the actual picture definition is roughly the same.

American pictures – which used to be 525 lines – have a height of 480 pixels for standard definition, but the same width.

A better standard
So, if that’s standard definition, what’s HD and why does anyone need it?

The second question is fairly simple to answer – with a limited number of dots, the bigger the screen or the closer the viewer is to the picture, the more they are likely to notice the individual pixels.

As TV screens in living rooms become ever larger, standard definition quickly starts to look less detailed. The same is true of a PC screen, incidentally, which is why they have more dots than a TV screen – typically at least 1024 x 768 pixels – because they’re viewed at much closer distances.

It would be nice to think that there’s only one way to describe HD but, unfortunately, that’s not quite true.

There are two distinct picture sizes that fall under the HD banner and, just to make it more complicated, some variations within those sizes.

To start with, the size of HD pictures can be either 1,280 pixels across with 720 lines down (1,280x720), or it can be even better at 1,920x1,080, a size often referred to as ‘full HD’ by TV manufacturers. Often, though, these two standards are shortened to just the number of lines, so you’ll see them referred to as 720 or 1080.

However, these two figures are themselves almost always suffixed with a letter ‘p’ or ‘i’.

These stand for ‘progressive’ or ‘interlaced’ and refer to how the picture is created on the screen.

When a picture is interlaced, first the odd-numbered lines are displayed and then the even-numbered ones.

Interlacing was a requirement of older TV technology, as electron guns could only move so fast: only half the picture was drawn with each sweep, but human persistence of vision helps to build a complete picture.

Some sensitive people may notice flickering caused by interlacing.

Interlacing is no longer a technical requirement of modern screens, but it has persisted because displays have needed to remain compatible with existing broadcasts: standard definition pictures are broadcast interlaced, and this is sometimes referred to as 576i.

The most common standards for HD are 720p, 1080i and 1080p.

In-store salespeople will often explain that the best TVs you can buy are the ones that can display 1080p and to an extent they’re correct.

However, before we explain why it’s important to think about not just the resolution of those HD pictures, but where they are coming from.

In the picture
As explained, regardless of whether they are standard or HD, pictures have a certain size – measured in pixels.

So too does a display, like the 1,024x768 of a computer monitor, or the 1,920x1,080 of a ‘full HD’ flat-screen TV set.

So what happens when they don’t match?

This is actually easy to see. Fire up a PC with a DVD player and often the video will first appear in a window that doesn’t fill the screen.

This is because the DVD is in standard definition (576 lines), which is usually rather less than the display’s resolution. There’s usually an option to play it full screen, but this will make it look a little less sharp.

Much the same thing happens when viewing a standard-definition picture on an HD TV: the picture has to be made bigger to fit.

This process, managed either by the TV or DVD player, is known as ‘upscaling’.

The results can be variable, with some upscaled pictures looking good, but others looking decidedly dodgy and disappointing.

Why is this important? For many people in the UK, much of what they are watching is still standard definition, even if the TV boasts ‘full HD’ on the front. In other words, much of the time your flash HD TV may be displaying nothing more than upscaled standard-definition pictures.

Where does HD come from?
Watch a DVD and it is standard definition.

Watch Freeview, cable or even satellite and most channels are standard definition too.

That may come as something of a shock, but only a small proportion of the channels on these services are broadcast in HD.

There’s no guarantee that programmes broadcast on HD channels were actually shot in HD in the first place, so they might be standard definition too.

 Watch a Blu-ray disc on an HD TV, though, and the picture really is HD – it’s the best quality currently available to home viewers.

The point to understand here is that unless the source of the programme is itself HD, it doesn’t matter what the label on a TV set or PC display says, or how big the picture is; you are watching standard definition.

Remember that upscaling will have been used to make the picture fill the screen and, while technical trickery can make this look a bit better, a standard-definition source can’t magically be turned into HD.

Regardless of TV service, there are both standard-definition and HD versions of some channels – usually depending on the package chosen.

But even when there’s an HD channel available, viewers still need to select it.

BBC One and BBC One HD have different channel numbers, for example: tune in to the standard version, and you’ll be watching standard-definition pictures, regardless of the TV set.

Both Sky and Virgin Media subscribers can pay for packages that include HD channels, though the complexity of these deals is so great that we can’t cover all the options here.

Freesat and Freeview viewers don’t have to pay any extra for HD channels, but neither has as many channels as the pay-TV services.

There’s another element – a receiver that can decode and display HD pictures. Sky subscribers need a Sky HD box, Freeview viewers a Freeview HD one, and so on.

In many cases, new Freeview boxes and TVs have HD decoders built in, but it’s always worth checking before signing a contract or paying for an expensive new bit of kit.

Anyone who’s been with Virgin or Sky for a while may need to upgrade their box to be able to receive HD channels.

Are you really HD ready?
One area that causes no end of confusion is the ‘HD ready’ label that’s stuck to most new TVs, especially when it comes to people who want to watch Freeview without using a set-top box.

The HD-ready label – and its close relative ‘HD-ready 1080p’ – essentially means that a display has at least 720 lines (or pixels) from top to bottom and that it has one or more HDMI connectors, through which it can accept an HD signal to display either 720p or 1080i.

 The 1080p version of the label means the set can display a 1080p image too, while ‘full HD’ usually means the display has a resolution of 1920x1080.

HD ready does NOT mean that the set can receive and decode HD signals.

That’s especially important in the case of a TV with a built-in Freeview tuner – and these days you’ll find that it’s hard to buy one without.

Crazy as it may sound, it’s common for a TV that’s labelled as ‘HD ready’, ‘HD ready 1080p’ or ‘Full HD’ to have a tuner – the bit that picks up the Freeview signal – that is only standard definition.

In fact, any set sold before around Easter 2010 will certainly fall into that category, as the first equipment capable of receiving Freeview’s HD channels didn’t go on sale until then.

In short, the HD ready label means merely that a TV is ready to display HD pictures – not that it can or will when you switch it on. Appalling, but true.

To be sure that HD programmes can be picked up through the aerial, look for the Freeview HD logo.

Without it, it won’t be showing HD pictures unless it’s connected to a separate device (such as a Sky HD box or a Blu-ray player).

Can I watch HD on my PC?
There’s no simple answer to this question, because it depends what you want to watch.

Most computer monitors can display HD pictures – some have a higher resolution than a ‘full HD’ TV set, in fact.

If a monitor has an HDMI connector, it will be possible to plug in a set-top box, or a Blu-ray player, and watch in HD.

If necessary, the picture will be scaled, just as on a TV.

Many newer PCs have a built-in Blu-ray drives, so can play HD discs and it’s also possible to play HD content downloaded legally from the internet.

When it comes to watching TV, though, things are a little more complicated. As a rule of thumb, the PC will need an up-to-date graphics card, as well as either a satellite or Freeview TV tuner card.

For satellite, a DVB-S2 expansion card is needed, and for Freeview HD, a DVB-T2 card.

In both cases, the ‘2’ is vital – DVB-S and DVB-T cards are cheaper, but use older versions of the standards and, in the case of Freeview, will only pick up standard-definition channels; while for satellite, many HD channels are switching to use DVB-S2.

Ultimately, though, watching TV on a PC tends to be a compromise: computer screens are designed to be viewed up close, not from a sofa across the room, and often the supplied software is rarely as simple to use as a real TV.

Simple facts
It might seem like there’s a lot to think about, but it is actually pretty simple.

 If you want HD, then you need three things: a screen with enough pixels for an HD picture; something that supplies HD pictures – like a set-top box, a Blu-ray player, or a built-in Freeview HD tuner – and the HD picture itself, like the HD version of a TV channel, or a Blu-ray disc (rather than a standard DVD).

Unless all three are in place, you’re not watching HD – and don’t let anyone tell you otherwise.