UHD Blu-ray and the rest
When we wrote our Sony VPL-VW260ES true 4K projector review last week, it kind of caught our attention that the whole UHD Blu-ray lingo is a bit of a mine field. And then there are other important parameters that seem to come along with it like colour gamut, colour bit depths, colour subsampling, HDCP, HDMI and HDR. Huh…
We found an excellent article on the topic from Acoustic Frontiers, albeit a bit dated. It got us thinking it would be good to write something comprehensive on the topic of UHD Blu-ray. It gets to the nitty-gritty of UHD Blu-ray specs, but we try to explain it in a way that it makes sense for the average Joe. Obviously, there has to be some level of compromise in technological accuracy, so please excuse us if it is at times only an approximation of ‘the truth’.
So, let’s get cracking. What’s the deal with UHD Blu-ray and the stuff that seems to come with it?
What are the available UHD Sources?
At this point these are the options:
- UHD Blu-ray players (and some game consoles), such as Panasonic DMP-UB900, Cambridge Audio CXUHD, and UHD Blu-ray players from Sony, LG and Samsung, playing UHD Blu-ray discs
- UHD hardware devices such as the those built into your TV, Roku 4K player, Amazon FireTV2, etc. offering UHD streaming services like Vudu, Netflix, Amazon, Youtube, etc.
For this blog, we’ll limit ourselves to the first option. When it comes to streaming devices, things often get a bit unclear, as the service providers often prefer to keep the technical info close-lidded.
So, what is the UHD Blu-ray specification?
The Ultra HD Blu Ray spec is as follows:
- a resolution of 3840 x 2160 (or above)
- frame rate of up to 60fps (frames per second)
- HDR
also
- 4:2:0 colour sub-sampling or higher
- Use of 10-bit colour depth or higher
- Support for wide colour gamut (REC.2020)
- HDCP2.2
- Coded with High Efficiency Video Coding (HEVC, aka H.265)
- Support for object-based sound tracks like Dolby Atmos or DTS:X
Some of these specifications are optional/suggestions, rather than a strict requirement. Some of it is future dreaming or a work in progress.
It’s also noteworthy that 3D isn’t part of the UHD Blu-ray specs.
We hear you think: this is very nice of you to outline what you want or need. But you promised something comprehensive, while in the meantime, you are speaking just as much mumbo jumbo as all the other articles out there. So, let us have a look at all the acronyms that are being thrown around.
4K resolution.
This one you are probably familiar with by now. 4K is 4x the resolution of HD, typically 3840×2160, although some Sony projectors use the slightly larger resolution similar to their cinema projectors (4096×2160). Noteworthy is that 4K is about resolution; nothing else.
High dynamic range (HDR).
While 4K promises ‘more’ pixels, HDR gives you ‘better’ pixels. HDR expands the range of both contrast and colour significantly. Images have greater contrast between light and dark, giving them more “depth”. Most midrange and high-end TVs these days have HDR, and HDR content is also no longer the scarcity it was even a year or two ago. Both on streaming services like Netflix and Ultra HD Blu-ray disc more and more content is becoming available.There are a few major standards for HDR, including HDR10, Dolby Vision (not relevant for projectors) and HLG (or Hybrid log gamma, for broadcasting).
Wide colour gamut (WCG).
Simply put, Wide colour gamut stands for more colours and more lifelike colours. Wide colour gamut, or WCG, is often lumped in with HDR. While they’re often found together, they’re not necessarily linked. Where HDR is an increase in the ‘dynamic range’ of the picture (with brighter highlights being the most noticeable), WCG is an increase in colour: “Redder” reds, “greener” greens, “bluer” blues etc. as well as more shades of a colour. Both HDR and WCG are what makes the images these days so much more exciting, but in a different way.
Previously, Blu-rays were mastered and distributed in the REC.709 color gamut. REC.2020 is the aim for the future. You will notice specs like 80% REC.2020, which basically means it’s 80% there. You want find 100% REC.2020 just yet. You also may find references to P3 which what commercial cinemas can do.
Source: Spectracal
Colour depth
Colour depth (also called bit depth) refers to the number of bits used to define the colour of each pixel and it is expressed as ‘bits per pixel’ (bps). It basically tells you how precisely a certain colour can be expressed. 8 bits, which was used for old Blu-rays means there are 256 levels of blue, red and green, or roughly 16.8 million combinations in total. Stepping it up to 10 bits as per the UHD Blu-ray specs, multiplies the levels of colour by four. That’s 1,024 available levels of green, red, and blue, or a whopping one billion total combinations.
Frame rate
When playing video, “fps” refers to frames per second, also known as frame rate. Simply explained, each frame is a still image; displaying frames in quick succession creates the illusion of motion or animation. The more frames per second, the smoother the motion appears.
The optimal frame rate for 4K HDR is 60fps, but that is still quite a novelty. You will see this displayed as 4K/60 (4K resolution at 60fps), but you’ll also see 4K/24 (4K resolution at 24fps), which is still the most common frame rate for movies.
It’s important to note that there is a difference between frame rate and refresh rate, or the amount of time the screen refreshes (expressed in Hertz). We won’t go in the detail of it here, but for a TV, this can be 60Hz, 120Hz or even higher. It is an important tool when dealing with motion blur and subsequently it is a spec that gamers will look at.
When a TV has a refresh rate of 120Hz while playing a UHD Blu-ray with a 60fps frame rate, it converts the incoming signal to 120 by adding frames (repeated frames, black frames, etc). I know… all the tricks that are being played on our eyes!
As for projectors, the most common refresh rate is anywhere from 60 Hz to 120Hz and beyond, but while you will often find refresh rates in the TV specs, it’s not such a hotly debated topic when it comes to projectors. Projectors employ technologies they call names like ‘Motionflow’, ‘Clear Motion Drive’, ‘Motion Enhance’, ‘Frame Interpolation’, etc. to tackle motion blur issues. They are basically the same technology as what is used in TVs.
The thing to remember with refresh rates is that more is only better up to a point, as the video tends to look quite artificial when overdone.
Chroma subsampling
When you want to send video data over the internet or store it on a disc, it is very important to do this as efficient as possible. The data that is stored in a pixel contains two types of info: how bright it is (luma) and its color (chroma). Because our eyes are more sensitive to brightness info, it is the color info that is being skimped on.
In other words, video created with chroma subsampling has all the brightness info, but not all the color info. How is that handled? Pixels share color data with their neighbors, which you will see displayed as a ratio. The ratio represents a block of 8 pixels. The first number of the ratio represents the top row of 4 pixels, the second number tells you how many of these pixels in the top row contain color info, and the last number will tell you how many pixels in the bottom row contain color info.
Most commonly, you will see these:
4:4:4 (no subsampling);
Source: Crutchfield
4:2:2 (subsampling; two top row pixels share info with the two other pixels in the top row without info and two bottom row pixels share info with two bottom row pixels)
Source: Crutchfield
4:2:0 (subsampling, two pixels in the top row share info with the two other pixels in the top row as well as all the 4 pixels in the bottom row)
Source: Crutchfield
At this stage, while 4:4:4 is used for content mastering, the 4:2:0 subsampling format is used for UHD Blu-ray. It’s important to note that visually, there is no advantage in using 4:4:4 for consumer video content: your eyes can’t tell the difference when you are looking at all these 4K pixels. If anything, the benefit of chroma subsampling far outweighs the increased costs of distribution, it is said. Ah, well…
HDMI 2.1
HDMI (High Definition Multimedia Interface) is the main audio/video connection for TVs, Blu-ray players, game consoles, video streamers, sound bars, computers and more. It carries both video and audio in one cable, allows digital encryption, and generally deliversvideo and audio quality without (more or less) any degradation.
Today’s devices mostly use HDMI version 2.0, but recently more and more devices are supporting version 2.1. HDMI2.1 is quite forward thinking: it offers improved bandwidth from 18Gbps (HDMI 2.0) to 48Gbps (HDMI 2.1; with one Gbps standing for one gigabit per second) and can carry resolutions up to 10K, frame rates up to 120fps.
Mmmm… that’s explaining acronyms with more acronyms.
Think of an HDMI cable as a pipeline. The step up to 4K or even 8K means more data that needs to go through your pipeline. In other words, you need a bigger pipe. If your frame rates also increase, you need an even bigger pipe. HDMI 2.0 is a big enough pipeline for what we are using today. But HDMI 2.1 basically allows for the growth that is thought to happen in the future, but it’s not really needed just yet.
HDCP 2.2
HDCP stands for High-bandwidth Digital Content Protection, a copy prevention technology that’s been used on HDMI connections for over ten years.
HDCP 2.2 is designed to prevent illegal copying of 4K Ultra HD content. Every link in your video chain must support HDCP 2.2 — your TV, video source, and any component the video signal passes through. If one does not, you won’t see a 4K picture. This is why HDMI 2.0 or above and HDCP 2.2 are often mentioned in one breath.
High Efficiency Video Coding
High Efficiency Video Coding (HEVC), is a video compression standard. The great thing about HEVC is the efficiency, allowing for much better image quality for the same bit rate. HEVC supports 8K UHD and REC.2020.
And then this…
While 4K UHD offers an amazing picture and sound that provides the best viewing experience available today, you might have noticed some movies look better than others on your big screen. It turns out that some movies while filmed/shot in 4K (or even 8K!) had all of the editing and post-production work done at the same level of the old Blu-ray standard (2K/1080p). Occasionally, studios use the same 2K transfer as the previous release and they to pass them off as 4K when in reality most special effects shots and sometimes the entire movie only has the quality of 2K/1080p.
That said, it’s important to note that age has nothing to do with potential quality. Classics shot on film such as Lawrence of Arabia, Ghostbusters, and Independence Day can have the original negative scanned at up to 6K resolution! We like to refer to realorfake4K.com as a guide to whether a movie has a final master of 2K or is the real deal.
Source: Projector Central, Cnet, What Hifi, Trusted Reviews, Lifewire, Acoustic Frontiers, Crutchfield