It took a while to gain momentum, but 4K Ultra HD Blu-rays are now a firm fixture of almost all major film releases.
The first 4K TVs went on sale back in 2012, but it wasn't until 2016 that the first 4K Blu-ray players from Samsung and Panasonic went on sale - alongside, thankfully, an increasing number of 4K Blu-ray discs.
So what is Ultra HD Blu-ray, how can you get it and how much does it all cost? Allow us to provide some answers...
What is the Ultra HD Blu-ray specification?
The new format supports a resolution of 3840 x 2160, higher frame rates up to 60fps (frames per second), and high dynamic range (HDR).
HDR is currently delivered in two main formats: HDR10 or Dolby Vision HDR.
That majority of 4K discs come with the standard HDR10. What sets Dolby Vision apart from the HDR10 is the inclusion of dynamic metadata, which is a proprietary HDR technology that adapts its image frame-by-frame and, theoretically, renders each shot at its best.
The first Dolby Vision Blu-ray discs, Universal's Despicable Me 1 and 2, and Sony's Resident Evil: Vendetta, have just been released this June. They may not be the most exciting discs for Dolby Vision's debut, but it's a start.
4K Blu-rays are encoded using the High Efficiency Video Coding (HEVC) standard (also known as H.265), and use 10-bit colour depth and can cover the full spectrum of the Rec.2020 colour space. The discs themselves can be produced in three sizes: 50GB with support for an 82Mbit/s data rate, 66GB with 108Mbit/s, and 100GB with 128Mbit/s.
MORE: What is HDR TV?
MORE: Best HDR TVs 2017
What about Ultra HD Blu-ray audio?
The UHD Blu-ray specification also includes object-based immersive soundtracks, such as Dolby Atmos and DTS:X. Some Blu-ray discs already support these formats, so it's not unique to UHD Blu-ray, but the next-gen audio soundtracks are more prevalent on Ultra HD Blu-ray discs.
You'll find DTS:X on Despicable Me 1 and 2, as well as the new Harry Potter 4K releases.
It's worth noting that you won't find Atmos and DTS:X soundtracks on the same disc; it's one or the other.
MORE: What is Dolby Atmos?
MORE: What is DTS:X?
What else does Ultra HD Blu-ray offer?
The new discs also support an optional digital bridge feature, which will allow you to copy Ultra HD Blu-ray content to an external hard disk drive and to portable devices, such as smartphones and tablets.
The spec also mandates that all Ultra HD Blu-ray players can play legacy Blu-ray discs. Sadly, 3D isn't featured in the Ultra HD Blu-ray spec, but with LG and Samsung phasing out 3D support in their TVs, and with none of Sony's 2017 TVs featuring 3D, it's perhaps understandable.
More after the break
What Ultra HD Blu-ray players are on sale?
Oppo's first Ultra HD Blu-ray player, the UDP-203, shot straight to the top of our list for its excellent picture and sound. At £650, it's not cheap, but with a great picture and support for high-resolution audio it's the preferred choice for audiophiles.
Oppo has also announed the considerably high-end UDP-205, which promises even better audio performance alongside its 4K talents for £1400.
For those that don't quite want to spend that much, there are more affordable models, such as the £450 Sony UDP-X800 - a five-star player that's up there with the best at the money.
Panasonic's first 4K player, the Award-winning DMP-UB900, remains one of our favourites thanks to a hefty price drop from £600 to around £400 now.
On the games console front, Microsoft's Xbox One S also features a UHD Blu-ray disc drive. It was the first console to be compatible with 4K Blu-rays, and at around £300, is one of the cheapest ways to get a taste of the format. Though be warned, it's not quite up to scratch with rival standalone players.
The recently announced Xbox One X (previously codenamed Project Scorpio) also incorporates a UHD Blu-ray disc player with native 4K, HDR and Dolby Atmos support. It’s pricier at £450, and has a more powerful processor to handle 4K gaming, but whether that has an impact on picture quality remains to be seen.
What Ultra HD Blu-ray discs are on sale?
Ultra HD Blu-ray discs are now on sale worldwide and in the UK, with over 100 titles from 20th Century Fox, Sony Pictures and Warner Bros.
Disney has finally joined the ranks, too, with Guardians of the Galaxy Vol. 2 set to be its first 4K disc release - it's out in the US at the end of August.
4K discs currently on sale include Assassin's Creed, Deadpool, Exodus: Gods and Kings, Fantastic Beasts and Where to Find Them, Ice Age: Collision Course, Independence Day, Independence Day Resurgence, John Wick 1 and 2, Kingsman: The Secret Service, Life of Pi, Logan, The Lego Batman Movie, Mad Max: Fury Road, The Martian, Passengers, Planet Earth II, The Revenant, Sicario, X-Men: Apocalypse, X-Men: Days of Future Past, and X-Men: First Class.
Will Ultra HD Blu-ray players play existing Blu-rays?
Yes, Ultra HD 4K Blu-ray players are fully backwards compatible with existing Blu-rays. Ultra HD Blu-ray discs won't play on old players and you can't upgrade old decks, so you will need a new 4K Blu-ray player.
When it comes to regional restrictions, the good news is that 4K Blu-ray discs have just one region code: worldwide. Essentially, they’re region-free. They can be played on any 4K player, in any part of the world.
However, your new 4K Blu-ray player will still have to observe the regional restrictions for DVDs and standard Blu-ray that continue to exist.
Most 4K Blu-ray discs come bundled with a standard Blu-ray version of the same film, and so far, from the discs we have in our testing rooms, these seem to be region-free as well.
Search for the 'ABC' logo or read the fine print on the box, or look it up online to be sure - especially if you're thinking of importing the latest discs from outside your region.
However, it very much depends on the studio, who owns the rights and the distribution of individual films as to whether a Blu-ray has region restrictions or not. We've noticed that the major film studios (20th Century Fox, Sony and Warner Bros) are releasing their Blu-rays as region-free, but smaller studios are more likely to have restrictions.
What TV do you need for Ultra HD Blu-ray?
An Ultra HD Blu-ray player will work with pretty much any TV, but unless it's a 4K Ultra HD TV, you won't see the disc's full-fat 4K resolution on screen. If your TV doesn't have the required HEVC codec, the player will downscale the disc's content accordingly.
Any 4K TV should deliver a 4K picture, but only TVs with the latest spec will be able to take full advantage of what Ultra HD Blu-rays can offer. This includes HDR, which is increasingly seen as one of the key factors for getting the best possible picture.
While 4K and HDR (or Dolby Vision) aren't necessarily linked technologies, if you're looking for a TV with one, it should ideally have the other in order to get the best picture performance.
Look also for a TV bearing the Ultra HD Premium logo - this means it meets certain 4K HDR standards, including support for standard HDR10. However, it doesn't mean the set necessarily supports Dolby Vision HDR.
MORE: Best TVs 2017
Do you need a new AV receiver for Ultra HD Blu-ray?
Yes and no. If your system currently uses an AV receiver to do the HDMI switching, and carry the audio and video over HDMI around your system, then this won't work with 4K Blu-ray unless your AV receiver is 4K-compatible.
The good news is that most home cinema amplifers released in the last three years have been 4K compatible, such as 2016's Award-winning Denon AVR-X2300W, for example. Look for HDCP 2.2 and HDMI 2.0a in the specifications for a clear indication of 4K compatibility.
If your AV receiver isn't 4K-compatible, then you will need to run a separate HDMI cable from your Ultra HD Blu-ray player to your screen for 4K video, as well as one from the player to your amplifier for audio - this is why some 4K Blu-ray players have two HDMI outputs.
MORE: Best AV receivers 2017
Is Ultra HD Blu-ray worth it?
The industry certainly thinks so, but then of course they're eager to see consumers shell out on new equipment and discs.
Victor Matsuda, chairman of BDA promotions committee, said: "For years, Blu-ray Disc has set the standard for high-definition picture and audio quality in the home. Ultra HD Blu-ray will do the same for UHD home entertainment."
Ron Martin, VP of Panasonic's Hollywood Lab and a crucial player in the development of Ultra HD Blu-ray, told What Hi-Fi? that 4K discs would beat 4K streaming: "[A disc format] is the only repeatable, reliable way to do it. Streaming has certain advantages but many disadvantages."
The good news? We agree.
We've compared how 4K streaming stacks up against 4K Blu-rays and standard full HD Blu-rays - and it's no surprise that 4K Blu-ray beats them all.
And we're not the only ones. Sales of Ultra HD kit is on the rise, with 4K being the main sought-out feature for customers hunting for a new TV.
4K Ultra HD Blu-ray may have taken a while to get going, but it's quickly set the standard for the ultimate home entertainment performance.4K Ultra HD TV reviews