DisplayPort 1.4 MST Hub

Updated on 20-01-2020 in MST Hub
6 on 24-05-2019

I’ve noticed a couple Thunderbolt 3 docks (such as the HP Thunderbolt Dock G2) containing DisplayPort 1.4 MST Hubs. I am wondering why there aren’t any standalone DisplayPort 1.4 MST Hubs yet and will there be any soon?

Currently, for four lane DisplayPort 1.4 support, the HP dock requires a Thunderbolt 3 connection to a computer with a Titan Ridge Thunderbolt 3 controller having Nvidia or AMD graphics supporting DisplayPort 1.4.

The HP dock can instead take input from a USB-C with DisplayPort alt mode port (because the HP dock uses a Titan Ridge Thunderbolt 3 controller), but that only allows two lanes of DisplayPort.

A DisplayPort to USB-C cable does not work with the HP dock. I guess a USB signal is required so four lanes isn’t possible this way.

The only way to get a four lane DisplayPort 1.4 output from a graphics card to the HP dock is to use a GC-TITAN RIDGE. A standalone MST hub would remove this requirement.

One interesting thing a DisplayPort 1.4 MST Hub can do is drive a 5K dual cable DisplayPort 1.2 display (such as the Dell UP2715K) from a single DisplayPort 1.4 signal. The max color depth is reduced from 10 bpc to 8 bpc because DisplayPort 1.4 has only 75% the bandwidth of dual cable DisplayPort 1.2. I have only one such 5K display, so I don’t know if this connection counts as one display or two displays. I do know that an Nvidia graphics card which supports only 4 displays will allow one of the 4 displays to be a 5K dual cable display but I don’t know if two or more of the 4 displays can be 5K dual cable displays.

 
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0 on 28-05-2019

Hi Joevt,

I must say I am very impressed with your knowledge in this field.

Club3D has been working on the next generation MST Hub that supports DP1.4 technology. Since MST hubs in the same sense as USB C Altmode depends purely on the signal from the source device, its hard to define the final specs since DSC 1.2 specifications are not bound to DP 1.4 specs and thus not all DP 1.4 sources are able to support DSC 1.2. Thus the delay in the MST 1.4 introduction….

Stay tuned as I have heard they are close to announcing it this year.

in regards to the DP 1.4 source to the Dell monitor (MST) – I do think that the 8/10 bpc is a limitation of the monitor and not the DP1.4 capacity

if you have any other questions or just want to trade thought please get back to me 🙂

 

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0 on 28-05-2019

I’ve used the DP 1.4 MST hub of the HP Thunderbolt Dock G2 using a PC running Windows 10 and having a GTX 1070 card. I’ve posted the results of my tests in a thread at eGPU.io.

The specs for the Dell UP2715K say it supports 10 bit per component color. The Nvidia control panel lets me choose 10 bpc color when using two separate DisplayPort 1.2 connections but only 8 bpc when using two DisplayPort 1.2 connections from the DisplayPort 1.4 MST hub.

If you do the math, using a 966.5 MHz pixel clock, you can see that 10 bpc exceeds the bandwidth of DisplayPort 1.4 (without DSC) but fits in the bandwidth of dual DisplayPort 1.2.

The “HP THUNDERBOLT DOCK G2 FEATURES AND TROUBLESHOOTING” technical white paper pdf says that “HP Thunderbolt Dock G2 is DSC ready”. It is unclear if this means it is support now or in the future. The next line implies now: “In order to fully support DP 1.4 DSC customers must be using a host platform that fully supports the DP 1.4 DSC”, but there is a later line that implies later: “Triple 4K @ 60 Hz when connected to DP 1.4 systems with DSC (display stream compression) – future”.

I don’t know if there are displays that support DSC (maybe 8K TVs?) or if the AMD or Nvidia graphics support DSC in their drivers.

It would be cool when the graphics card (and drivers) supports DSC, if the MST hub could accept DSC signals and output decompressed SST signals. This could allow 10 bpc dual DisplayPort 1.2 link 5K from a single DisplayPort 1.4 port instead of just 8 bpc. I don’t know if such a mode is possible or maybe it’s the default mode (or it should be) when DSC is supported – in that case the graphics card will choose to send DSC only when the bandwidth limitation of DisplayPort 1.4 requires it. If the user doesn’t want DSC then the user will choose resolutions/refresh rates/color depths that don’t require DSC.

I saw no indication in my tests that any DSC was happening (I could not exceed non-DSC displayPort 1.4 bandwidth). It may be possible that DSC is not support by the drivers, or the MST hub, or that DSC requires the displays to support DSC (all my displays are older than DisplayPort 1.4).

The specs for the GTX 1070 says 7680×[email protected] is supported which means that DSC is support, or that dual cable link DisplayPort 1.4 is supported (as used by the Dell UP3218K 8K display), or both methods are supported.

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2 on 29-08-2019

The Delock 87737 is a stand-alone DisplayPort 1.4 MST hub. I think it uses the same Synaptics chip that the HP Thunderbolt Dock G2 does.

https://www.delock.com/produkt/87737/merkmale.html

But it’s only sold in Europe? I guess one of those stores will ship to North America.

on 29-08-2019

Hi Joevt

Thank you very interesting product indeed.

Unfortunately I do not know where other vendors sell there products.

on 29-08-2019

BizLink has a DP 1.4 MST Hub but I don’t know if it’s something they sell in single units:

https://www.bizlinktech.com/products/detail/1285/DP+1.4+MST+Hub

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0 on 20-01-2020

Looks like Club 3D has finished their DisplayPort 1.4 MST hub, the CSV-7300.

StarTech has also announced one (MST14DP123DP). It looks identical to the Delock-87737.

Does the CSV-7300 use the same Synaptics VMM5330 chip as the StarTech and Delock?

What does the following statement mean? Or what should I take away from knowing that the CSV-7300 has this feature? What’s a slice? Is there a 3 or 5 slice decompression? What’s the difference?
“DSC 1.2 transport and up to 4-slice decompression”

The reason I ask, is because there is something I don’t understand about the supported combinations of resolutions. For example, as far as I know, 8K30 24bpp only requires HBR3 (non DSC). If DSC has a 3:1 compression ratio, then shouldn’t two or three of those be allowed?

The specification sheet has the following asterisk note:
“*Requirement: – Host must support DP1.4 with DSC1.2 feature”

The asterisk seems to apply to all the resolutions but this is unclear because the asterisk is in the middle of every resolution like this “3840×2160*60Hz, 24bpp”. I think you need a separate table for non DSC? But you have the table above that which should be sufficient but it neglects to show the bits per pixel and it’s missing the 8K30 option. The table with heading “DP1.4 Output Port 1, DP1.4 Output Port 2, DP1.4 Output Port 3” and showing all three combinations of connecting one or two displays is redundant if you fixup the first table and just say that displays can be connected to any output port without change in behavior.

The asterisk seems to state that the hub can’t even do a single 8K30 24bpp without DSC – even though HBR3 has enough bandwidth. Does this mean MST adds a lot of bandwidth? I know it adds some, but this seems excessive. Well, I did the calculation, assuming CVT-RB timing, an overhead of only 4.823% will make 8K30 24bpp not work. That seems reasonable. What is the actual overhead? Using CVT-RB2 timing, more than 5.9% overhead would cause 8K30 24bpp to not work.

This amount of overhead doesn’t explain why DSC only allows one 8K30 24bpp instead of two. Maybe the compression ratio is not 3:1? It doesn’t seem to be at least 2.5:1 because otherwise 8K60 24bpp would be supported. 2:1 is not enough because it only allows 4.41% overhead (using CVT-RB timing).

The tables don’t say what kind of timing is used (CVT-RB or CVT-RB2).

Does the MST hub support converting DSC to non DSC? It does not say otherwise so I assume that is true. In other words, DSC can be used on the input to allow output to more 4K60 non DSC displays than could otherwise be supported by non DSC HBR3.

I believe the MST hub can convert between different link rates and link speeds. For example, it can convert HBR3 x4 MST to dual HBR2 x4 SST to drive a dual cable DisplayPort 1.2 display like the Dell UP2715K at 5K resolution (only at 24bpp though) from a single DisplayPort 1.4 port. Or it can convert HBR2 x4 to HBR3 x2 (if you have an old DisplayPort 1.2 source and want to use a new DisplayPort 1.4 two lane adapter to its full capability).

Back to the statement in the specification sheet that says “DSC 1.2 transport and up to 4-slice decompression”. While I don’t know what a slice is, I believe that it means that the hub can transmit DSC from the GPU to a display that supports DSC, and it can decompress DSC from a GPU to a display that does not support DSC, but it cannot compress a DisplayPort signal from a GPU and and send that compression output to a DSC display. Is that correct?

Does HDCP affect the ability to decompress?

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