4K, Ultra HD, 4K Ultra HD – call it what you want, the 'four times HD' TV technology is now here. The big TV brands will are delivering their third-generation 4K TVs in 2016, the first 4K TV channel is now live, you can stream 4K from Amazon, Netflix and YouTube, and even 4K Blu-ray has made it on sale.
We've seen many Ultra HD TVs pass through our test rooms from all the major brands, and some that are best avoided, and there's no question 4K video can look seriously impressive. We've even seen 4K OLED TVs from LG and Panasonic, delivering what is (arguably) the ultimate TV performance.
So, what's 4K TV all about? On this page we'll run through the basics of ultra high-definition video, and get you up-to-date with the latest 4K news and available content.
MORE: Best 4K TVs
MORE: 4K content guide
What is 4K Ultra HD?
Officially, 4K resolution is 4096 x 2160 pixels. However, in order to shoehorn this higher resolution video on to a normal 16:9 picture format, it has been altered to 3840 x 2160 – still four times the total number of pixels on a Full HD 1080p screen (1920 x 1080).
In order to take full advantage of 4K Ultra HD you will, of course, need a compatible TV, a source and content that packs those all-important extra pixels.
And that's where it gets slightly messy. While we've been reviewing 4K TVs since 2012, it's only really the last year or so that we've had any content to get our teeth into. So if you've resisted 4K until now, you may have made the right decision - but now might be a good time to take the plunge and future-proof your viewing.
What about HDR?
‘HDR’ stands for High Dynamic Range, and it is the next big thing in TV world. The term originates in photography, and refers to a technique that heightens a picture’s dynamic range – the contrast between the brightest whites and the darkest blacks. The theory is that the higher the dynamic range, the closer a picture gets to real life. HDR for televisions is basically the same idea.
An HDR TV is all about contrast and colour performance, and a TV must be able to deliver a certain level of brightness and black level to support HDR. The most readily available spec comes courtesy of the Ultra HD Premium badge, which requires that a TV delivers 4K resolution (3840 x 2160), 10-bit colour depth, BT.2020 colour space representation and support for HDR, for the 'ultimate 4K performance'.
How can you watch 4K video?
To watch 4K video you need a 4K screen, a 4K source and 4K content. The good news is your options have increased exponentially in the last couple of years.
Then you need a source. This can now be as simple as an app - Amazon and Netflix both offer streaming 4K content.
Want to play discs? You of course need a 4K Blu-ray player. The first one we've reviewed is the Panasonic DMP-UB900.
Last but not least, the content - which has finally become a reality (albeit only a pre-order one, in the case of Ultra HD Blu-ray).
How can you stream 4K video?
Netflix 4K streaming arrived in 2014, with House of Cards: Season 2 making history as the first 4K streaming content from the service, and Breaking Bad in 4K following soon after. More content, mainly original, is being added all the time, so it's worth keeping an eye out for the latest releases.
To watch Netflix 4K you will need a 4K TV with the HEVC codec that Netflix is using in order to play the content. Sony and Netflix struck a deal to ensure Sony TVs get the best out of Netflix, but we've since seen compatible HEVC TVs from LG, Samsung and Panasonic. You'll need a Sony Android TV to watch HDR content on Netflix.
Amazon's 4K content comprises mainly TV shows, and these are available to stream for free through the Prime Instant Video service. Other content, a mixture of films and TV shows, is available to stream at a cost.
There's an outside chance that Sony will produce a firmware update to enable 4K video on the PlayStation 4. That would allow far more people to access Ultra HD video, but don't hold your breath.
How can you watch 4K on YouTube?
YouTube also has a selection of 4K videos, but unless you're watching on a 4K TV, you'll need to make sure you have a compatible 4K monitor. And for those who have the required hardware, there's even an 8K video.
YouTube uses the VP9 codec developed by Google, and it's been running in the Chrome browswer since 2013. As with HEVC, you'll need to make sure your TV or monitor support VP9 before trying to stream.
More after the break
What about 4K media players?
If you are in the US, there's the Sony FMP-X5 media player, which can bring 4K streaming content to legacy Sony TVs.
The FMP-X1 server comes pre-loaded with ten 4K films, has a 2TB hard drive, HDMI and USB connections and connects to your network via an Ethernet connection - and it sounds like you'll need that wired connection for accessing Sony's 4K download service.
Officially called Sony Video Unlimited 4K, the download service is live in the US and promises to be the world's first network service to give access to a constantly updated library of 4K films and TV shows.
Philips has also announced its UHD 880 4K media player, a little black box for sending 4K content to Philips TV.
NVIDIA has released the Shield Android games console that's also capable of streaming 4K as it supports all the required codecs. It's now available to buy in the US, but a UK release date is "coming soon".
Read our complete 4K content guide
Can you watch 4K broadcasts on TV?
The road to 4K broadcasts has been a long one but we do now have our first 4K TV channel, BT Sport Ultra HD. While the channel isn't broadcast over airwaves, instead relying on an Internet connection, it's definitely a step in the right direction.
Sky has also pledged its commitment to 4K broadcasts and is planning to add 4K content to its newly-launched Sky Q service this summer. Most likely with a launch date in line with the 2016/17 Premier League football season.
The BBC had said it planned to broadcast 4K as standard by 2016, with the Rio de Janeiro Olympics seen as the key event in the calendar. A BBC survey claimed 23% of viewers could benefit from 4K TV, based in part on the average TV screen size and viewing distance. We shall see...
The BBC and Sky are also part of the UHD-Forum, which has been formed to promote 4K TV. Led by the Digital TV Group, the forum aims to avoid the confusion that emerged when HD TV and HD-Ready TVs first came on sale.
As ever, it seems that Japan is way ahead of us...
Ultra HD Blu-ray discs
After plenty of rumours, Ultra HD Blu-ray is now official.
The first 4K Blu-ray discs are also now on sale, with the number of titles now hitting double figures and including releases such as Batman V Superman - Dawn of Justice, Life of Pi and Fantastic Four.
Ron Martin, Blu-ray Disc Association board member and Panasonic Hollywood Labs VP, revealed some more details to What Hi-Fi?, including why discs would offer better than 4K Netflix pictures. And you know what? We might just agree with him.
How much does a 4K TV cost?
The 4K TV market has now reached a level of maturity that makes room for ultra-cheap Chinese manufacturers alongside more expensive big-name brand sets featuring all the trimmings.
A 43in 4K TV can be bought for as little as £300, but if you still want to break the bank you can - the 2016 LG 4K OLED TV range starts at £3000 for a 55in and goes right up to £25,000 for the flagship 75in.
To make the most of Ultra HD, we wouldn't recommend using anything too small, but a much wider choice in both screen size and price can only be a good thing for the technology.
See all our 4K TV reviews
Any sign of an Apple 4K TV?
The Apple TV has been talked about for longer than many of us can even remember (well, a few years at least) and after rumours of a TV with AirPlay and Siri, or even an OLED Apple TV, the situation is still no clearer. All we have for now is a new generation Apple TV box... with no 4K support.
On the one hand, you have billionaire businessman and influential Apple investor, Carl Icahn, who seems convinced Apple will soon enter the Ultra HD TV market (and car market, for what it's worth). But maybe he would say that.
The flipside is a recent source, quoted by the Wall Street Journal, that claimed Apple did indeed have a team dedicated to working on an Apple TV, but that team has now been disbanded.
We have to say, our money would be on the latter.
4K TV verdict
If you previously doubted whether 4K would ever make the breakthrough into the mainstream - and we wouldn't have blamed you - it does now look like the resolution is here to stay. As such, it should be a real consideration for when you buy your next TV.
The good news? If you're spending a few quid on a TV, it will most likely be a 4K set whether you like it or not. You just have to add the content - which has finally started to arrive.
We're now happy to recommend a number of Ultra HD 4K TVs, all of which deliver the necessary picture performance for SD, HD and 4K content without breaking the bank.
Streaming 4K video has arrived (albeit with limited content), 4K broadcast plans are well and truly underway, and Ultra HD Blu-ray is now a reality.
Sold? In that case, you might want to check our round-up of the best 4K TVs you can buy...