4K Distribution Might Be Easier Than We Imagine
There is a lot of talk currently within the custom installation industry about how 4K, HDMI 2.0, and HDBaseT won’t cut it for 4K.
Depending on your needs and for most people, that’s simply just wrong. BT Vision and Sky are both working on new 4K set-top boxes, with BT Vision already aiming to get theirs out before the new football season starts. Meanwhile, the Blu-Ray association (BDA) has announced new Ultra HD 4K disc format and players that are due before the end of 2015.
This is all great, but I can hear you thinking, ‘How am I going to get this round my customer’s house?’ Simply put, you can use what is already on the market, just check it supports 3Gbps (300Mhz) transmission speed and, if you want to share Ultra HD Blu-Ray media, HDCP 2.2
What is 4:4:4?
There is lots of talk right now of handling the maximum video format of 4K 60Hz, 4:4:4. Chroma subsampling is what is being referred to here. You don’t really need to worry too much about the mathematics behind it, but it relies on the fact that the human eye is terrible at detecting colour. It is really good, however, at detecting luminance.
For the most part, what we are watching on TV is encoded using Luminance (Y) and then two chroma components: Cb and Cr. The historical reason for this is that when black and white televisions were in use, the transmission comprised of just luminance, which is simply how bright or dark a given pixel was. When engineers started working on colour TV there was a strong desire to produce a backwards compatible system, so adding the chroma data would allow existing TVs to still operate by reading the luminance information.
Chroma subsampling is commonly expressed as a three part ratio (i.e., 4:4:4 or 4:2:0). Let’s call this J:a:b with:
J = Number of sample pixels, commonly 4
a = number of chroma samples in the first row of sample (J) pixels
b = number of changes of chroma samples between the first and second row of sample (J) pixels
When a chroma sample is 4:4:4 there is no subsampling taking place. Simply, for the 4 pixels in the sampling reference, there are 4 chroma samples in the first row and 4 in the second row, and this means every pixel contains the maximum possible amount of data. This is great. However, that’s a lot of data, which is why we need the 6Gbps data links that are in the new HDMI 2.0 specification.
As the specification was being written, it was apparent this was going to be too much data for broadcasters and that they could not transmit this load both to or from the set-top box in the first place. Even though there is a strong desire to keep the uncompressed video format within HDMI, a means to reduce the amount of data being sent was desperately needed.
To handle this engineers turned to the fact that the human eye is pretty bad at detecting position and motion of colour compared with its ability to detect luminance (brightness). By simply reducing the amount of chroma data in the frame you can end up with a high resolution image and with colour data that looks at normal viewing distances exactly the same.
To do this you take the two pixels side by side and these are given the same chroma information—this will half the amount of data you need in the X axis.
This is known as 4:2:2, a format supported by HDMI at 1080p for a very long time. With 4K video it wasn’t enough. Thankfully you can also perform the same trick in the Y-axis as well and share one pixel worth of chroma information for 4 pixels—this is 4:2:0 (4 Sample pixels, 2 Chroma bits in the first row, 0 Chroma bits in the second row).
Unless you’re streaming 4K gaming, however, you don’t need 4:4:4, and to be honest, there is little need to distribute a gaming PC over a residential video system; the latest generation video consoles have just got started, they are not moving to 4K any time soon.
Lots of studies have been carried out and in double blind tests you can’t tell the difference at normal viewing distances. If you don’t believe me, read up on the Blu-ray specification. 4:2:0 is the standard format your Blu-ray movie is encoded in right now; it’s the format that all TV broadcasters use for HD content as well.
Best of all, think of it like this, with 4:2:0, you have 4K luminance with 1080p colour and no one will ever know the difference.
Martin Ellis is managing director at Pulse-Eight Ltd based in Poole, Dorset, UK.