MP 1,1-5,1 A 4k mystery: Why am I able to output 4k30 over DVI to HDMI via GeForce 760 on a cMP 3,1?

[Dt990]

i was making a video about running macOS Sonoma on a Mac Pro 2008. I had it connected to an CamLink 4k to capture the video for things OS installation via a DVI to HDMI cable, the exact model is this, an Amazon Basics cable. The HDMI end is on the Camlink, and the DVI end on the GeForce 760.

I then discovered I was able to switch to 4k30 in Sonoma, something that shouldn't be feasible. The cable is DVI-D (but not DVI-DL which is a split bit stream hence why you need active converters for DVI-DL). This means it has it should only support 3.96 Gbps bandwidth. HDMI 1.4 which maxes out at 4k@30hz is 10.2 Gbps.

How is this possible?

My theories are:

• The GPU can force TMDS bit stream that's either within the tolerance of 3.96 Gbps by using compression (I didn't think DSC was a thing for old an old GPU like the 760) and/or lower subsampling like 4:2:0
• And/or DVI-D cable can actually carry more than 2x the data and the GPU ignores the DVI port's limitation and just responds to the HDMI port's negotiation.

Here's the video cued to the segment where I show in the macOS System Reporter that it's outputting 4k.

Anyone have any input here? I only have a surface understanding of DVI/HDMI/DisplayPort bit streams and sadly my BenQ monitor for whatever reason doesn't display super geeky info like bandwidth, bitstream type, subsampling, compression etc so I can't glean any more info than what I have, unless there's an app that can analyze my Camlink 4k. I have both macOS and Windows on my 2019 if a utility exists.

 

[h9826790]

That's a DVI-D dual link cable.

 

[Dt990]

That's a DVI-D dual link cable.

The pinouts are but DVi-D DL is a goofball standard. In the case of going HDMI to DVI-D DL you need an active convertor to drive displays like the Apple Cinema 30". Besides, the max resolution of Dual link is 2560 x 1600 @ 60 Hz from all the technical docs I've seen, unless HDMI supports dual data pathways on the receiving side.

 

[h9826790]

The cable won't really limit the max resolution to 2560x1600. It's the graphic card's limitation. Cable only limit the bandwidth.

2560x1600 60Hz bandwidth is more than enough for 3840x2160 30Hz.

And if I understand correctly, you connect the DVI port to the graphic card, which means the DVI is the input side of the cable. You are doing DVI to HDMI, not HDMI to DVI.

Anyway, I don't think 4K 30Hz need active convertor.

 

[TzunamiOSX]

If you have Dual Link DVI, and a good DUAL LINK DVI to HDMI cable, 3840x2160 at 30 Hz is the maximum. (The Amazon Basic cable is bidirectional? 🧐)

If you want 4K@60 Hz you need DisplayPort, and the right DP -> HDMI cable with an active chip (I have tested 3 DP -> HDMI cable to found a working one.

Edit: you connect it wrong 😂

 

[h9826790]

If you have Dual Link DVI, and a good DUAL LINK DVI to HDMI cable, 3840x2160 at 30 Hz is the maximum. (The Amazon Basic cable is bidirectional? 🧐)

If you want 4K@60 Hz you need DisplayPort, and the right DP -> HDMI cable with an active chip (I have tested 3 DP -> HDMI cable to found a working one.

Edit: you connect it wrong 😂
View attachment 2297642

I think because there is no converter inside the cable, which means, it is bi-directional.