Is it possible for HDMI <--> DVI-D adapter to make the output picture unstable?

Question

I mean it's not a converter, it's not supposed to make changes in video signal.

My chain is:

Tablet PC -> USB Type-C hub -> HDMI output -> HDMI cable -> HDMI <--> DVI-D adapter -> Monitor

So I am looking for what is making picture unstable in this chain.

The problem with picture is like something is wrong with progressive scan. There are horizontal lines constantly running vertically. Sometimes certain areas of the picture moves left and right very fast. I was able to capture on photo one of this artifacts.

Buying another hub is expensive.

Buying another HDMI <--> DVI-D adapter is most likely useless.

This is adapter that I use.

• My hub is a Vention 6-in-1 USB-C Docking Station (Model: TGNHB).
• My Tablet PC is Dell Latitude 7210 2-in-1.
• My monitor is Acer V276HLCbmdpx.

Answer 1

Tablet PC -> USB Type-C hub -> HDMI output -> HDMI cable -> HDMI <--> DVI-D adapter -> Monitor.

Let me correct this.

Tablet PC -> USB Type-C hub -> DP-to-HDMI converter -> HDMI output -> HDMI cable -> HDMI <--> DVI-D adapter -> Monitor.

The attempt at a USB-C alt mode for HDMI died before it really got started. Specifically:

HDMI LA said that it doesn't know of a single adapter that has ever been produced. Similarly, at the USB Implementers Forum (USB-IF), people who are familiar with the certification process have yet to see a true USB-C to HDMI adapter.

Effectively, all USB-C to HDMI cables, hubs, etc., will be DP from the host fed through an additional converter chip to get a HDMI signal.

You are correct in that HDMI pre-2.0 and DVI share a signal format, and can be adapted by "dumb" pin mapping without requiring extra conversion.

The nature of the artifacts you are seeing do suggest something going wrong during signal conversion. A failure at the passive pin mapping of HDMI=>DVI is unlikely to degrade image quality so much as just not working at all.

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All the discussion above assumes we're dealing with DisplayPort Alt Mode, which sends an actual DisplayPort signal straight from the host (GPU) over USB-C, then out a DP port or through a converter to a HDMI port.

However, there is another potential path:

Tablet PC -> DisplayLink driver -> USB Type-C hub -> DisplayLink chip -> HDMI output -> HDMI cable -> HDMI <--> DVI-D adapter -> Monitor.

DisplayLink, no relation to DisplayPort despite the name, is a proprietary protocol used to send video over USB. We're not talking USB cables with a display protocol running directly over alt mode; we're talking actual USB 2.0 or 3.0. This relies on a software driver that effectively performs a 'screen capture' and encodes it to video, then those packets are sent over USB to a DisplayLink chip that decodes it and renders it for output via a video port.

DisplayLink exists to handle the cases where the host hardware doesn't support direct video output. It's also varying degrees of bad to terrible, and it's not uncommon to see video compression artifacts that almost look like an analog VGA signal gone bad, not to mention strange disconnects and general unreliability. Or, if you're lucky, it might work okay.

I'd suggest checking for two things:

1. Does your host actually support DP alt mode? Check the spec sheet.
2. Is your hub/dock using DisplayLink? Again, check the specs. You may also be able to find a "DisplayLink" USB device attached to your computer, see device manager under "Display Adapters" or "USB Display Adapters".

If your host doesn't actually support DP alt mode, you're pretty much stuck. You can either have DisplayLink or nothing.

If your host does support DP alt mode but your hub is using DisplayLink, you should replace your hub with one that uses alt mode.

If both support DP alt mode then you'll need to look elsewhere for the issue.

Answer 2

Many things are possible, but to solve your problem you need to isolate each component:

• Connect this monitor to a different computer.
• Connect this cable to a different computer and monitor.
• Connect a different monitor to this computer and cable.
• Try a different USB-C dock/hub connected to this computer.

As cables and adapters are the cheapest and easiest things to swap, that should usually be done first. New, you can probably borrow another monitor from someone else and try that connected to this same computer and cable. Connecting this monitor to a different computer should also be relatively easy, but it is best if that computer has a DVI output you can connect to, so you're checking the same port on the monitor.

This is the normal process of diagnosis to be followed in cases like this. Doing these steps has a high chance of indicating quite clearly which component, or combination of components, is having this problem.

As an aside, a monitor that only supports DVI and does not have HDMI is going to be quite old, and this age increases the odds this component will be the problem.

Answer 3

I kinda wonder if this "double conversion" is supposed to work at all. I think it highly depends on whether the DP standard actually has any backward compatibility for DVI "mode". (Unfortunately I have no concrete knowledge on that.)

The story is a bit tricky here, because the way HDMI be backward compatible with DVI is a bit similar to the idea of DP++, that is, there is a way for a HDMI source to detect whether the sink is a DVI sink when it is behind a (passive) adapter. In other words, by design HDMI sources is supposed to (be able to) have a "DVI mode".

To name a proof / evidence of that, because limited color range / black level is a thing in consumer electronics (and media like DVD / Blu-ray and most of the video codecs, I think), while at the same time it was never a thing in PC / DVI (until DisplayPort), the CEA mandates in the HDMI standard logic / heuristic such as determining the color range to output based on whether the current mode is a "CEA mode" (such as 1080p@60 or 2160p@60, but not 1440p@60). AFAIR, the standard even specifically states that such logic / heuristic should not be adopted when the sink is detected to be a DVI sink.

When it comes to DisplayPort, its standard is pretty much a cheap copycat of HDMI with respects like the mentioned one, because VESA naively thinks that making it highly compatible / similar with HDMI would got DP into the consumer electronics world, while obviously the CEA members (like "AV brands") wouldn't give a damn about it because of money et al.

With the two parts of the story combined, you get a tricky result:

1. DisplayPort to HDMI active conversion solutions like those in USB-C hubs / dockings could easily work properly for HDMI sinks

2. For it to further work properly for DVI sinks, essentially you need the DP side to have support for "DVI mode" (even when the conversion is "active"), and the HDMI side (of the "solution") to "detect and notify"

The problem is, because of DP++, other than the "CEA heart" of HDMI, it didn't really need to care about "DVI compatibility" in its standard, because DP++ is essentially a "switch", once the source side detects that the sink is a TMDS sink, it just hand over the job to chip or whatever that is responsible for HDMI support (like literally two ports attached to different links combined into one port physically). In other words, DP wasn't (well, at least "didn't need to be") really designed with "mode switch" kind of backward compatibility, but literally just tries to be as similar as possible to HDMI, without the "mode switch" kind of backward compatibility in that for DVI.

But I could be totally wrong though. After all, there were DisplayPort to DVI active adapter, I think. No idea if they still requires DP++ or not though. Nevertheless, even if the DP standard was designed with DVI compatibility in mind, it doesn't necessarily mean that the solution in your hub was standard conforming enough / made with that in mind, because as you can see, many people thinks that the source is not aware of the fact that the sink is a DVI sink when that's the case. (Vendors could have "missed" it like some DVD/Bluray player vendors "missed" support for closed captions that can be found on some DVDs.)

P.S. In other words, I do wonder if DP sources would be aware of whether connected sinks are HDMI, or DVI, sinks when there is (active) conversion, which doesn't require / leverage DP++ AFAIK.