What is HD?

HD (high definition) is everywhere these days. Games consoles play HD games, the BBC and Channel 4 have dedicated HD channels, and streaming services like Netflix and Lovefilm play movies in HD. Even phones and tablets are now arriving with HD resolution screens.

Now with the arrival of smart TVs, we’ve never been so spoilt for choice when it comes to warm, glowing banks of HD panels in the high street.

What is HD? The difference between 720p, 1080i and 1080p

But what is HD? HD refers to a level of detail on screen that provides more fluid video and more vibrant colours. HD follows from standard definition - the level of detail in analogue colour TV that most of us grew up with, and never even thought about until someone told us it was just, well, ordinary.

Technically, HD is defined by the number of pixels on the screen (see '720p vs 1080p'), but confusingly there are three different types of ‘HD’ resolution out there, so it’s worth knowing a bit more when shopping around for high definition TV sets and related gear. Just because a TV set or monitor has ‘HD’ slapped on the side, it might not be what you want or need. We take a closer look at 720p, 1080i and 1080p here, and what you need to know.

HD: 720p, 1080i and 1080p - what’s the difference?

720p, 1080i and 1080p are all versions of HD, but they’re all different. It’s important to note that you can’t actually buy any TV sets at 1080i - it only refers to TV and video. So to begin with we’ll just look at 720p and 1080p and the differences between those.

720p vs 1080p

In the analogue TV days, all UK TVs used the PAL broadcast system, which used the standard definition (SD) of 576i. This is 720 pixels wide by 576 pixels tall. The more pixels on the screen, the higher the definition of images will be.

A pixel is the smallest visible element on a display, the ‘dots’ that make up the picture, although back, then, TV pictures were scanned line-by-line, and the width wasn't so important. We'll get onto the 'i' later.

A 720p video (and a 720p monitor) is 1280 (wide) x 720 pixels (tall). That's more than twice the detail of standard definition straight away, and 1080p goes even further, racking up the pixel dimensions to 1920 x 1080 - that's five times more detailed than SD.

In our debut episode of Jacked, Recombu Digital’s video podcast, we took a humorous look at the history of broadcast definitions which might help explain the differences between the two a little better.

An HD TV with a resolution of 720p will only be able to display video at this resolution and no higher.

So if you’re planning on playing HD games on your PlayStation 3 (which supports 1080p) or streaming the highest quality movies from Netflix you might want to avoid getting a 720p TV set.

That’s not to say that a PS3 or Netflix won’t work on a 720p set - you just won’t be able to get the absolute best performance on a 720p TV.

Most 720p TV sets you’ll see in shops these days tend to be toward the cheaper end of the price spectrum and will be marketed as being ‘HD Ready’. This is because 720p is the absolute minimum required to meet this standard.

Most 1080p sets you’ll see will be marketed as being ‘Full HD’ or 'True HD' as it gives you a richer, more well defined viewing experience.

What is HD? The difference between 720p, 1080i and 1080p

OK, but does having 720p and 1080p make any real difference?

Each TV set will have what’s called its native resolution. This basically means that a 720p set is better at displaying 720p HD broadcasts.

Every broadcast or format your TV receives will be displayed in its native resolution. So if your 720p set receives a 1080p signal from a broadcast, Blu-ray player or games machine, it will downscale it to fit on the screen. Similarly, any 576p broadcasts will be upscaled.

All HD channels from the BBC, Channel 4, Sky and Virgin Media for example are broadcast in 1080i. A 720p HD TV would then downscale this resolution to fit while a 1080p TV set would be able to handle it natively. We’ll get on to the difference between 1080i and 1080p in a bit.

The real plus that 1080p TV sets have over 720p sets is when it comes to watching Blu-ray movies. Blu-ray is a native 1080p format, so they look their absolute best on 1080p TV sets. If you’ve got a growing Blu-ray collection then the choice is clear - go for a 1080p set.

In terms of picture quality on a 24-inch or 26-inch screen the difference is negligible unless you’re sat up close. Only on bigger screens (32-inches and above) can you start to appreciate the benefits of 1080p.

What is HD? The difference between 720p, 1080i and 1080p

OK great. So what’s 1080i and how is that different to 1080p?

Right. 1080i and 1080p are very similar. 1080i and 1080p broadcasts both display images at the same pixel count as each other - 1920 x 1080. The difference is in how the images are made up on screens.

The lowercase ‘i’ in 1080i stands for interlaced scan. The lowercase ‘p’ in 1080p stands for progressive scan.

Interlaced scan renders images in vertical lines, breaking down the picture into individual columns and then displaying every other line at a very high rate - at 1/25th of a second. Odd-numbered lines get painted on the screen first, then even numbered lines. While this is incredibly fast and impossible for the human eye to detect it can create a ghostly flickering effect on live TV broadcasts (like sports).

Progressive scan renders images sequentially, all at once. This makes for a much smoother image overall that doesn’t suffer from the ghosting effect. What’s more, flat-panel displays like LCDs, LEDs and plasmas (the most common types of HD TVs) will automatically convert any incoming 1080i signal to 1080p. Good quality TVs (generally the expensive ones from well-known brands) will use clever processing to replace the missing lines, but cheaper TVs won't look as good.

Interlaced scan was introduced for analogue TV both as a form of data compression (only sending half the signal at any one time) and because cathode ray tubes (CRTs) in the past couldn't physically scan screens fast enough for a progressive picture.

What is HD? The difference between 720p, 1080i and 1080p

Should I buy a 720p HD Ready TV or a 1080p Full HD TV?

The advantages of a 720p TV set are that they are undoubtedly cheaper. If you’re not after the full home cinema experience, you're buying a smaller TV, and you’re perhaps not so bothered about gaming online, then a 720p TV set will suit you.

Additionally, if you’re after an HD TV set to stream movies (via Netflix or Lovefilm) to then a 720p TV set might be a good idea if your broadband connection isn’t quite up to handling 1080p streams reliably.

Furthermore, there’s not a great deal of 1080p content that’s available for streaming from these services just yet. Most of the titles we’ve seen available from Lovefilm, Netflix, Blinkbox, Acetrax and the rest are standard definition with a fraction of HD titles available. Of those HD titles, most of those are likely to be 720p as well.

Of course if you’re not bothered about streaming to your TV or quality is what you’re after, then get an 1080p TV set. You’ll appreciate the difference when watching movies on a Blu-ray player and on games machines that support 1080p.

We mentioned the Sony PlayStation 3 earlier, it's worth mentioning that the Xbox 360 is a native 720p device. While it will work on 1080p screens (and some 1080p games are supported) the majority of titles are 720p and so will be upscaled to fit on bigger 1080p screens. You’re not likely to notice a major difference unless you’re up close. But on the flipside, if you own an Xbox 360 and want a TV purely dedicated to gaming then you could always save a bit of cash and get a 720p TV.

The forthcoming Xbox 720 is expected to be a native 1080p device, as is the Nintento Wii U. The Nintendo Wii is the only console of the current generation of machines that isn’t natively an HD device, supporting a measly 480p.