Can't convert HDMI to DisplayPort - root cause?

Question

I have a DisplayPort based monitor, and a laptop with a DP out. I recently got a second laptop with HDMI out, and went looking for a HDMI -> DP converter. I was very surprised to find that such a thing effectively doesn't seem to exist.

This answer and the Dell page it links to seem to imply that it's just not possible to go HDMI -> DP, without a very expensive converter box (not just an adaptor). However, I can't find anything that explains why this is the case. Is DP designed to be backwards compatible but can't interpret HDMI? Is it a case of HDMI-quality signal not being high enough quality to be convertible? Some other reason?

Note that this question is not looking for an adaptor - I'm looking for the underlying cause of the lack of them.

Solution 1

HDMI is a clock-less video format, DP uses a clock to keep video in sync.

DP adds the clock to the video at the sending device; so a DP-receiving device needs to see a clock on the incoming video, HDMI doesn't. This is why it's very easy with a simple adapter to go from a DP output on a laptop to a HDMI input monitor because the monitor doesn't care about the clock.

When going from HDMI to DP, the DP-receiving device is looking for a clock; HDMI sender does not have one, so this does not work. You need an 'active' adapter in this case and all that adapter does is add a clock to the video format and sends it out via DP. Notice I said "active" adapter--that's the magic word you need to look for when getting an adapter to go from HDMI to DP.

It has absolutely nothing to do with the age of the video format (as another user put it "older standard"). HDMI 2.0 is newer than DP1.2 and the issue would still exist for the very reason I stated above.

Solution 2

HDMI and DisplayPort are completely different from the point of view of the signals that they send down the wires, as well as the connectors that they use. However, a DisplayPort output has the ability to detect when it's connected to a HDMI display, and send HDMI signals down the wire to that display. This only requires a passive adapter or cable with the right connectors on each end.

The reverse isn't true because HDMI is an older standard, and such a capability has never been added to the standard in the meantime. A HDMI output port can't send DisplayPort signals because it doesn't know how to do that, and it's not required to.

Solution 3

dead link for UGREEN.

Hope this CableMatters is still a thing when you search for this converter.

https://www.cablematters.com/pc-825-141-gold-plated-hdmi-to-displayport-converter-supporting-4k-30-hz-in-black.aspx

Solution 4

This works fine for me:

• Raspberry Pi Settings > Resolution > "Default - preferred monitor settings"
• UGREEN HDMI to DP, HDMI to Display Converter, 4Kx2K Resolution, HDMI to DisplayPort Converter with USB Power for HDTV, LCD, monitors etc $19.95
• Displayport 1.2 male to Mini Displayport female adapter (generic) $6.95
• Apple Cinema Display LED 24-Inch

View more solutions

Comments

[Appleoddity]

Your question is somewhat misleading. It is possible to convert hdmi to display port and adapters do exist to do that. I’m not claiming to be an expert here, but DP was designed to be backward compatible with HDMI. They are two different technologies. Just like DVI was designed to easily convert to VGA. Or, how blu-ray players still play DVDs. It is not uncommon at all for newer technology to be backwards compatible with older technology to aid in adoption. Something tells me you know that, so what are you really asking? Of course it is more difficult to “convert” an old tech to a new tech.

[Secret]

@Appleoddity I’m looking for why it only works in one direction. You can convert VGA to DVI (although it may not look good) as well as DVI to VGA. You can play a DVD in a blu-ray player, but you can’t play a blu-ray disc in a DVD only player because the disc isn’t encoded for it. If the answer is “DP controllers can detect they’re talking to an HDMI device and change protocols”, that’s a valid answer. So is “DP is HDMI plus some extra information, so it’s easy to strip but hard to generate that info”. Does that help clarify?

[user3660103]

@Bobson Don't go too far with that reasoning. You can convert DVI-I and DVI-A to VGA with just a simple adapter, and the other way around, but you to convert the DVI-D (that is to say, actual DVI) to VGA, you need active device. You also need an active device to convert VGA to DVI-D. That's because the DVI-A and DVI-I actually have pins for VGA signals within the connectors.

[Secret]

This is most of what I’m looking for. Can you add why a DisplayPort input isn’t also capable of interpreting hdmi?

[Secret]

That is definitely far cheaper than I've seen similar items anywhere else. Interesting.

[Rules]

@daveyost, will it degrade quality during conversion?

[daveyost]

I haven’t used this setup for 9 months, but I don’t remember any problems, and I would’ve said something if it didn’t work perfectly.

[Attie]

"HDMI is a clock-less video format"... what do you mean? The pinout alone disagrees with this statement...

[Attie]

"all that adapter does is add a clock to the video format" - this is incorrect... they are two very different and incompatible standards (electrically and logically). The fact that many DP sources can also operate as HDMI sources (hence the existence of passive adapters / cables) is a red herring.

[mythofechelon]

It seems to be possible now? amazon.co.uk/…