MP 7,1 Thunderbolt cable longer than 2meters for XDR display

[chfilm]

Hey,
I'm sorry to open up another thread for this but I would just really like to understand why this is such an issue. Maybe someone with more technical knowledge can elaborate on this.

I'm trying to extend the cable connection between my XDR display and my Mac Pro. First I tried it with a thunderbolt 4 dock by OWC - no luck, only 5k resolution. Then today I got the 5meter TB cable from corning for 430$. Same result, still only 5k max resolution.

I spoke with a senior person at Apple and he also couldn't explain me the reason why it wouldn't work. Can anyone please shed some light on this? What is the difference between Apple's own cable and the Corning one? Is there any way to make this work?

 

[blackadde]

The Corning site is pretty clear:

Thunderbolt™ Optical Cables | OEM Optical Communication Solutions | Corning

Corning’s Thunderbolt™ 3 and Thunderbolt™ 2 optical cables enable high-speed data transfer over longer distances for media, broadcasting, and entertainment applications industries.

www.corning.com

• Supports one 5K display or two 4K displays

I'm not an electrical engineer but I'm pretty sure 6K / 60hz / HDR is right at the very limit of the TB3 spec, which is why they need to use DSC and leaves almost no bandwidth for anything else. It's not a surprise that active / optical cables designed to defeat that spec aren't compatible with extreme edge cases like the XDR.


I found this article, which involves a lot of speculation pre-release, but has some thoughtful analysis near the bottom:

Apple 6k3k LCD, 2019

An Apple KB article was just published today with further specifications. Additionally it has been brought to my attention that Intel's Titan Ridge TB3 controllers actually do support DP 1.4. So that resolves some of the feature discrepancies I'd noted above: Display Stream Compression and HDR support were both added in version 1.4. I still think DSC is a poor choice for such a high-end product, so Apple needs to find a way to send ~36.6Gbps of raw video data (and probably 38.3Gbps, with CVT-R2 timing) through a cable.

The supported Macs section offers some clues. Old macs which can only drive the display at 5k resolution get USB 3.1 gen 1 support on the downstream USB-C ports. And that makes sense: TB3 has enough DP bandwidth to support 5k resolution, DP1.4 provides HDR feature support, and there's 5Gbps spare for USB 3.1. But the new 16" MBP, newest 15" MBP, 2019 iMacs, and of course the Mac Pro, can all drive 6k resolution. The old MBP and iMacs all seem to have Intel's Titan Ridge controllers, and perhaps they use DSC to drive the display. BlackMagic's eGPUs also have Titan Ridge controllers. I still don't see any way, compliant to the standards as I understand them, to drive 6k at 10 bits per color channel through TB3 without DSC. The total bandwidth is (just barely) there, but as far as I can tell is not completely allocable to DisplayPort.

So there are three options. Apple may use DSC to reduce the required bandwidth. I may not adequately understand TB3, and there is some way to get more than ~35Gbps of TB3's bandwidth allocated to DisplayPort. Or Apple has cheated, perhaps by detecting the Pro Display XDR and running the TB3 link or its encapsulated DP links at slightly higher-than-spec speeds.

Also, don't buy a $430 cable to get a sit-stand desk to work. That's crazy. Just mount the MP to your desk or something.

 

[DFP1989]

Why would you spend such an extraordinary amount on a cable without reading the description?

The Pro Display XDR is quite the edge case as noted above.

 

[th0masp]

Why would you spend such an extraordinary amount on a cable without reading the description?

Two weeks return window, no questions asked for online shopping? Pretty sure.

Crazy price indeed. Can't the computer be put on a (literal) pedestal so it sits halfway between the desk's highest and lowest position and the short cable length is sufficient for both settings?

 

[goMac]

Apple is supposed to release a 3m cable, it’s in their PDF guide for the Mac Pro. But a price has never been released and it’s been MIA for a while now.

 

[joevt]

I'm not an electrical engineer but I'm pretty sure 6K / 60hz / HDR is right at the very limit of the TB3 spec, which is why they need to use DSC and leaves almost no bandwidth for anything else. It's not a surprise that active / optical cables designed to defeat that spec aren't compatible with extreme edge cases like the XDR.

I found this article, which involves a lot of speculation pre-release, but has some thoughtful analysis near the bottom:

Apple 6k3k LCD, 2019

Yes, 6K / 60hz / HDR is right at the very limit of the TB3 spec - it uses two DisplayPort HBR3 connections. Normally, two HBR3 connections (51.84 Gbps total) would exceed the maximum bandwidth of Thunderbolt 3 (40 Gbps), but 6K 60Hz HDR only requires 38.94 Gbps (assuming 649 MHz 30 bpp per 3008x3384 tile as specified by the EDID) and Thunderbolt doesn't send the DisplayPort stuffing symbols that would be used to fill the HBR3 bandwidth. Apple is going through some hoops to setup this dual HBR3 connection - so maybe it can't work with a dock or an optical cable between the Mac and the display? Maybe it works only with Apple's drivers (need to check Linux and Windows - Boot Camp and not Boot Camp).

But if your GPU supports DSC, then the XDR will accept a single HBR2 connection (17.28 Gbps) to do 6K 60Hz HDR, leaving plenty of bandwidth for USB 3.0. This connection mode should work with optical cables and docks.

 

[chfilm]

Yes, 6K / 60hz / HDR is right at the very limit of the TB3 spec - it uses two DisplayPort HBR3 connections. Normally, two HBR3 connections (51.84 Gbps total) would exceed the maximum bandwidth of Thunderbolt 3 (40 Gbps), but 6K 60Hz HDR only requires 38.94 Gbps (assuming 649 MHz 30 bpp per 3008x3384 tile as specified by the EDID) and Thunderbolt doesn't send the DisplayPort stuffing symbols that would be used to fill the HBR3 bandwidth. Apple is going through some hoops to setup this dual HBR3 connection - so maybe it can't work with a dock or an optical cable between the Mac and the display? Maybe it works only with Apple's drivers (need to check Linux and Windows - Boot Camp and not Boot Camp).

But if your GPU supports DSC, then the XDR will accept a single HBR2 connection (17.28 Gbps) to do 6K 60Hz HDR, leaving plenty of bandwidth for USB 3.0. This connection mode should work with optical cables and docks.

Yea thanks for this good explanation. Would be interesting if that works for anyone with the W5700, but I have two Vegas, so no luck
I’m sending the cable back and will find a way to mount the Mac to the desk I guess. Just couldn’t find a good mount yet that supports the weight.

 

[timerickson]

Another poster in another thread was successfully using an optical cable with the W5700X and the PD XDR, but it suddenly stopped working after a firmware or macOS update, I can't recall which. I will need to dig for that thread.

If someone wants to buy me an optical cable I can test it

 

[schweigert85]

Just wanted to say I'm using an XDR through the OWC TB3 dock - and getting full 6k res on the display, as well as no issue with additional USB devices on the doc