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Comment by owenwil | Hacker News Reader

The technology to do 5K @ 120hz just doesn't exist yet. 5K @ 60Hz is already maxing out Thunderbolt 4.

DisplayPort 1.2 (or newer) and HDMI 2.1 both support 5K at 120Hz when using DSC.

What’s DSC?

Display Stream Compression, a lossy compression algorithm optimized for the specific use case of device-display transfers (i.e. relatively low compression ratio == very high quality that's claimed to imperceptible even if it can't run lossless, not needing to work on full frames for speed/sub-frame latency, ...)

I didn't know either. DSC is Display Stream Compression, and has been part of the spec since 1.4. According to Wikipedia [0],

> DSC is a compression algorithm that reduces the size of the data stream by up to a 3:1 ratio.... Using DSC with HBR3 transmission rates, DisplayPort 1.4 can support 8K UHD (7680 × 4320) at 60 Hz or 4K UHD (3840 × 2160) at 120 Hz

0: https://en.wikipedia.org/wiki/DisplayPort#1.4

DisplayPort 1.2 cannot even do 5k at 60Hz at 10 bits per channel. You might be thinking about DisplayPort 2.0, which is not yet widely supported in video cards or monitors, and even then it needs the (essentially non-existent) UHBR 20 data rate to get to 120Hz.

HDMI 2.1 does support 5K@120Hz@10bpc but requires DSC as you say, which is not very commonly found either.

(Note that unlike what a sibling comment states, DSC is lossy. Some argue that the less is not noticeable to the human eye, but not everyone agrees)

> HDMI 2.1 does support 5K@120Hz@10bpc but requires DSC as you say, which is not very commonly found either.

Not very commonly found on what? Nvidia & AMD have been shipping DSC for ~2 generations now (so almost anything within the last 4 years has it)

It's not commonly found on monitors. DSC is included starting with DisplayPort 1.4, and a good portion of monitors even today still ship with DP 1.2

Nvidia and AMD have not been shipping DSC with HDMI simply because HDMI included DSC starting with HDMI 2.1, which only the current generation of graphic cards and very few monitors support.

To shed some light:

DisplayPort 2.0 supports up to 77.37 Gbit/s of bandwidth. Wikipedia's DisplayPort article has excellent tables about bandwidths of display formats: HDR 5K@120hz (57 Gbit/s) is totally possible with DP 2.0.

One could use DisplayPort 2.0 alt-mode to carry that signal over a USB-C cable. With the main disadvantage that the USB ports on the monitor would drop to USB 2.0 speeds [1]. And alt-mode 2.0 is only available on the most recent hosts with USB4 ports.

[1] Native USB4 tunneling only supports DisplayPort 1.4a (for now). That's a huge flaw imo, cause falling back to alt-mode takes over the cable and blocks USB3/PCIe tunneling from working. In fact, that flaw is even present on this monitor:

> When connected to iPad Pro 12.9-inch (3rd and 4th generation), iPad Pro 11-inch (1st and 2nd generation), or iPad Air (5th generation), Studio Display USB-C ports deliver USB 2 data transfer speeds.

Those models don't support TB3 or USB4 on their USB-C port, so they have to use alt-mode with either compression or without HDR.

And yet somehow the technology to do 8k @ 120hz does exist... ( https://www.youtube.com/watch?v=JDUnSsx62j8 )

Does that video show the connection used to display that.

If not, I'm guessing that example case is either using a lossy data compression or multiple cables running to multiple displays.

Lossy Data Compression is used on the Pro Display XDR and Apple doesn't seem to lose any sleep over it. DSC is quite good.

There's also the fact that nearly all content has compression as part of its path to display anyway. Video & images are obvious examples, but also GPUs almost always render to a lossy compressed framebuffer as well. It just saves way too much memory bandwidth not to do.

So as long as 'transcoding' from GPU framebuffer compression to DSC is itself a mostly lossless process, there's realistically not even a quality hit from it anyway

Samsung has 5120x1440 @ 240hz today. That is literally half of a 5K panel at double the speed. It amazes me how many people I've seen say this exact same thing about how 5k120hz is impossible.

You’d say if they can drive 3 displays at 60hz they should be able to drive one at 120hz

Maybe, but I don't see anyone going the way of dual-link ports ever again. It was clumsy enough the first time.

Displays using MST to achieve higher resolutions than a single link allowed worked out more or less fine. The problem there is MacOS doesn't support MST.

Funny, I am typing this on a Mac connected to an MST Dell 5k display....MacOS supported this setup since at least 2015, when I bought my display.

Aha! What MacOS doesn't support is MST daisy chaining for some reason, only MST to a single display to overcome bandwidth limitations.

That's a common misunderstanding. DisplayPort MST allows multiple display streams to be multiplexed onto a single DP link. So you are still bound by the bandwidth of a DP connection (25.92 Gbit/s for HBR3).

The 5K Ultrafine (which I presume you are talking about) is connected via two separate DP links which are tunneled over a single cable using TB3.

That sounds like nitpicking, but is a crucial difference. MST provides daisy chaining of displays, but can not help you exceed bandwidth limits. Whereas the two DP links are completely independent and theoretically allow you to double the max bandwidth. (well 2xHBR3 exceeds the max TB3 bandwidth, so the 5K uses 2xHBR2)

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