4K UHD at 144 Hz without compromise will be delayed

Screens with 4K resolution and running 144 Hz are now a reality and even more models are on the way. But none of them can run an image signal in 4K and 144 Hz without resorting to compromises such as reduced colors. If you also use HDR without color reduction, today 98 Hz is the highest possible frequency for 4K screens.

The limitation is due to the fact that today’s Displayport is not enough. There is simply not enough bandwidth to transmit the required image signal.

So no – the compromise does not depend on the screens. The compromise does not depend on the graphics cards either. Because not even an Nvidia Geforce RTX 2080 Ti can conjure and show an image beyond how the Displayport standard works. And it may take a long time before we have a new standard.

If not, HDMI 2.1 will emerge as an alternative in 2019. However, the practical solution to this problem may take several years. It has historically also taken several years from the time the specification was released until the technology made a breakthrough on a broad front.

The problem is that Displayport has a basic maximum bandwidth that is too low to run the screens fully in 4K resolution and with high frequencies. HBR3, High Bit Rate 3, which is the highest transfer speed over Displayport, is enough for a maximum of 32.4 gigabits. That speed came with the Displayport 1.3 specification, which was completed in September 2014.

4K UHD: The capacity of HBR3 (32.4 Gbps) in Displayport 1.3 / 1.4

A speed of 32.4 gigabits corresponds to a pixel clock for the image signal of about 1,080 MHz, which is comparable to the 600 MHz (18 Gbps) used in HDMI 2.0 and the 300 MHz (9 Gbps) which is still the practical limit of Intel’s integrated graphics circuits. Displayport has long had a much higher capacity compared to HDMI, which also has a certain ironic harmonic.

This is because this superior bandwidth to HDMI has never been used by monitors before. But now that the bandwidth is really required in the picture signal, Displayport is still not enough – and can also be trumped by HDMI 2.1.

Until the first generation of 4K / 144 Hz screens now appeared in 2018, such high bandwidth over Displayport has never been used since 2014. Most models on the market require around 18 to 20 gigabits per second.

The de facto limit has long been at the 600 MHz / 18 Gbps level, ie the HDMI 2.0 specification. The exceptions have been a few G-Sync monitors that have stepped slightly above the 600 MHz limit, for example for 2,560 × 1,440 pixels at 165 Hz and 3,440 × 1,440 pixels at 120 Hz.

HDMI 2.1 is emerging as a possible solution

In November 2017, the HDMI 2.1 specification was nailed down after several years of work. Already at the end of 2016, it was clear that 48 Gbps is a desired bandwidth because it is enough for 4K resolution in 120 Hz with 10-bit HDR (or 8K / 30 Hz with 10-bit HDR) It rhymes more with what is required for digital video. This corresponds to a pixel clock of 1,600 MHz.

4K UHD: Upper capacity for HDMI 2.1 (48 Gbps)

This upper bandwidth in HDMI 2.1 is much more useful even for computer graphics as we can really reach 144 Hz and include HDR. And reach 180 Hz if we skip HDR.

What then is the crux? HDMI 2.1 is still a paper specification. Products that use the higher bandwidth do not yet exist. Even though 2019 is expected to be the year where HDMI 2.1 functions sneak into products like TVs, it is still a very open question about how high they reach in bandwidth. The need to run 4K / 144 Hz on the TV side is not at all in demand in the same way as for computer screens.

DSC uses destructive compression

VESA has a conceivable – or perhaps unthinkable – solution in the form of DSC, Display Stream Compression, which provides up to 3: 1 destructive compression at a level that is “visually acceptable”. DSC is available with the Displayport 1.4 standard and is in any case a possible solution because the bandwidth requirement is more than halved compared to the original bandwidth. A reduction which is largely based on the discarding of image information.

What speaks against DSC is that it is primarily presented as a solution for desktop graphics, photography and moving images, especially when video in demanding high resolution like 8K – and with low frequencies such as 24 and 30 Hz. There is no information on how it works with fast games 144 Hz. The technology for DSC is in the graphics cards, similar to Nvidia’s RTX series. However, the function is not yet available in any screen and it is therefore a completely open question whether this is now a possible solution for game screens.

Thunderbolt 3 runs Displayport 1.4

According to the specification, the Thunderbolt 3 can transmit up to 40 Gbps, which of course is more than the 32 Gbps that come with HBR3 in Displayport. A margin for 4K UHD in 144 Hz without reduced colors. Worth remembering is that Thunderbolt is a general transfer technology. When Thunderbolt is used for image signal, a “muxed” Displayport signal is sent, ie a signal that at best follows the HBR3 speed of 32 Gbps we have already found is not enough. So no, Thunderbolt 3 does not solve anything here.

What happens now?

On paper, HDMI 2.1 is thus the only technology that at the time of writing (October 2018) can transmit a signal with sufficient bandwidth for 4K UHD in 144 Hz with full color resolution and also with HDR. So on paper. For the increased bandwidth in the HDMI 2.1 specification has not yet been realized in products and it is unclear whether the technology in 2019 will even reach as far as 48 Gbps.

For Displayport, there has long been talk from VESA’s side of a change and a possible version 1.5, but nothing has yet appeared there. A possible opportunity is during the CES trade fair in January 2019.

When a newer and better specification is available, the technology must also find its way into both screens and graphics cards. Manufacturers must become familiar with the new and circuit manufacturers must return to the drawing board again and develop new circuits. Circuits that have to find their way into new screens and of course also new graphics cards. A process that can therefore take several years. Before this happens, it is the limits described above that apply until further notice.