DisplayPort 2.1 has a serious issue with UHBR-certified cables — it could explain Nvidia sticking with DP 1.4 on RTX 40-series

Aaron Klotz
·6 min read
Aorus.
Aorus.

One hallmark of DisplayPort 2.1 is its maximum 80Gbps UHBR20 specification, making it the only consumer-friendly display connection standard in the world that supports 4K 240Hz monitors without Display Stream Compression (DSC). But several notable issues with the UHBR 80Gbps standard significantly hinder its adoption among both consumers and manufacturers alike.

The biggest problem is cable length. The maximum cable length for a DP80-certified cable is currently only 1.2 meters (3.9 feet), according to the VESA database, with many of the cables only reaching 1m or 0.8m. (Monitors Unboxed was one of the first to show the implications of this problem, revealing how problematic 1/1.2 meter cables are for most PC setups.)

Most desktop PC users have likely become familiar with cables that are around 2m in length. That's plenty long to reach a PC sitting on the floor while the monitor is on the desk. 1m cables are a different story, and they're among the shortest video cables you can purchase, including DisplayPort and HDMI. Generally, this length can work for laptops and mobile devices, but it's impractical on desktops unless the PC sits on top of the desk. (Yeah, we get it: Desktop has become a bit of a misnomer in the modern PC world.) For typical desktops, a 2m or longer cable connects the monitor (or monitors) to the PC.

This is a serious problem for users who want to take advantage of the UHBR20 spec. Currently, only an extremely limited number of people even have access to graphics cards and monitors that support UHBR20 — AMD's W7900 Pro cards have a single DP80 port (i.e. DisplayPort 2.1 UHBR20), while the others are DP54 (DisplayPort 2.1 UHBR13.5) — and even fewer are likely to have a DP80 capable display. Those who have the hardware side covered will inevitably be using a high-end PC, which means said PC could be sitting on the floor.

Technically, it's not VESA certification limiting DP80 cables to 1.2m and shorter lengths. Rather, the cable manufacturers need to be able to make such cables. If you have a monitor, graphics card, and cable that are all DisplayPort 2.1 rated, your system will likely try to run at the maximum DP80 bandwidth ... but it may not work on longer cables, leading to artifacting or signal loss. We've seen this with long cables even on DP1.4a, and higher speed signaling will experience greater signal degradation.

There's also the problem of branding. DisplayPort 2.1 is divided into several different specifications that are not all equal. DisplayPort 2.1 UHBR20 mode is the only spec that allows for the standard's maximum 80Gbps bandwidth capacity, which has now been labeled DP80. UHBR 13.5 (DP54) and UHBR10 (DP40) support require substantially less bandwidth at 54Gbps and 40Gbps, respectively. But all three standards are still lumped together under the "DisplayPort 2.1" umbrella, and many cables exist that claim DisplayPort 2.1 compatibility without actually being certified for DP40 or DP80 — and no DP54 certified cables exist at present.

This results in DisplayPort 2.1 having widely varying capabilities, depending on which specification is used. In a real-world context, this could mean that some future DisplayPort 2.1 capable 4K 240Hz OLED monitors might only be capable of delivering their maximum resolution and refresh rate with DSC enabled (which can cut bandwidth requirements and thus speed by up to a third), even though they might have the DisplayPort 2.1 standard equipped.

Display Stream Compression (DSC) isn't necessarily a bad solution. There are already DisplayPort 1.4 monitors that support 4K 240Hz via DSC, and many newer high-resolution, high refresh rate monitors support DSC to achieve their maximum resolution and refresh rate. But what if you don't want to use DSC? (Currently, using DSC on Nvidia GPUs disables DSR support.) Answer: Too bad.

Reaching the highest signaling speeds of DisplayPort 2.1 has proven to be very challenging, and is undoubtedly why adoption has been exceptionally slow. DP40 and DP54 modes effectively offered a way for the standard to be released without having to hit higher speeds, but this split in features and specs muddied the waters.

So far, the AMD RDNA 3 RX 7000-series GPUs only support DP54, with the professional W7900 cards adding a single DP80 port. Intel Arc Alchemist GPUs also have DisplayPort 2.1 support, but they're limited to DP40 — which means HDMI 2.1 offers higher bandwidth and resolution support (at up to 48 Gbps). The Nvidia Ada Lovelace RTX 40-series GPUs opted to forego DP2.1 entirely, even on the professional cards, instead sticking with the older DP1.4a spec.

And the display market adoption has been, if anything, worse than the GPU support. The only monitor that currently supports the full DisplayPort 2.1 DP80 spec is the Gigabyte Aorus FO32U2P, a brand-new 4K 240Hz OLED monitor that was only just released. All other concurrent 4K 240Hz and higher resolution displays depend on either HDMI 2.1 (which maxes out at 48Gbps) or less potent DisplayPort variants combined with DSC.

The DisplayPort 2.1 issues will continue to be problematic as adoption goes mainstream. Resolutions and especially refresh rates continue to increase, thanks to continuing advancements in OLED technology. Sooner than later, outgoing standards such as DisplayPort 1.4a won't have enough bandwidth to drive the latest flagship monitor tech even with DSC.

This already happened with Samsung's latest Odyssey Neo G9 dual 4K 240Hz ultrawide, where DisplayPort 1.4a can't run the monitor at its max resolution even on an RTX 4090. (HDMI 2.1 with DSC can get there, however, as 7680x2160 240Hz 'only' requires around 42.5 Gbps of bandwidth with DSC enabled, so the 40-series GPUs should be able to run this monitor over HDMI.)

The good news is that VESA and cable manufacturers are working to increase cable length for DP80 beyond the current 0.8–1.2 meters — just like they did recently with UHBR 13.5 cables through the new DisplayPort 2.1a standard. One approach is to use active cables rather than passive copper cables. That of course would increase the price of the cables, so that a 2m cable might end up costing closer to $50 than the $10 you'd pay for a passive cable. But DP40 already has certified 3m cables, and DP54 should allow 2m cables, providing for something of a middle ground.