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Devices outputting a video signal through a DisplayPort connector may optionally support DisplayPort Dual-Mode (or DisplayPort++). This allows the DisplayPort cable to be used to carry an HDMI (TMDS) signal. From what I gather, support for this is widespread, even in cases where device manufacturers opt not to use the "DP++" logo.

FWIW, by "active", I mean a cable containing circuitry to convert an LVDS signal (DisplayPort) to a TMDS signal (HDMI, DVI). By contrast, a passive cable would simple rearrange the DisplayPort signal wires to fit an HDMI connector, requiring the source device to detect the HDMI device and output an HDMI signal accordingly.

There are inexpensive, passive DisplayPort-to-HDMI cables that rely on support for DisplayPort Dual-Mode in the video-outputting device. By contrast, the display just sees a regular HDMI signal on an HDMI port.

USB Type-C includes DisplayPort Alternate Mode. This allows the USB cable to be used to carry a DisplayPort signal. In a USB-IF presentation, it is stated that DisplayPort Alternate Mode does not support DisplayPort Dual-Mode:

"USB Type-C will NOT support DisplayPort Dual Mode (DP++)"
Page 23, VESA DisplayPort Alternate Mode on USB Type-C Technical Overview, Jim Choate, USB-IF

USB Type-C includes HDMI Alternate Mode. This allows the USB cable to be used to carry an HDMI signal. From what I gather, this is unsupported by almost (if not all) existing devices on the market.

Edit: everybodywiki.com lists the 2018 HP Envy 13 x360 as supporting HDMI Alternate Mode.

Edit: cablechick.com.au says:

HDMI Alternate Mode is a specification without an implementation (as far as we can tell). Our research turned up no consumer hardware that supports HDMI Alt-Mode by name. Instead, HDMI compatibility over USB-C is generally offered via DisplayPort or Thunderbolt 3 Alt-Modes using adapters.

Combined, this seems to suggest that both scenarios in which a passive USB Type-C-to-HDMI cable could be used – HDMI Alternate Mode and DisplayPort Alternate Mode carrying DisplayPort Dual-Mode HDMI – are, in fact, impossible.

Does this mean that every USB Type-C-to-HDMI cable on the market is active?

Giacomo1968
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Blieque
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  • “Does this mean that every USB Type-C-to-HDMI cable on the market is active?” Please wait… My head is still spinning from this post. My advice? You need to focus it more and perhaps focus on one brand and model. The reality is the market is utterly flooded with all levels of quality of USB-C to HDMI cables and adapters. Some from well know brands, many of seemingly no-name brands. So honestly… Who knows! But do you mean that all of these USB Type-C-to-HDMI cables are basically USB devices that have a video card of some kind in them to output HDMI? – Giacomo1968 Dec 28 '21 at 04:57
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    I wonder why the downvotes. It's actually valid question, albeit it's sort of hardware recommendation-ish. I think you should rephrase them into two questions: 1. Can a Type-C host / source port support both HDMI Alt-mode and DP Alt-mode 2. Do HDMI Alt-mode products actually exist on the market – Tom Yan Dec 28 '21 at 05:22
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    @Giacomo1968 For the record, many (if not all) USB-C to HDMI products on the market actually expects DP Alt-mode on the host and convert the DP signal to HDMI/TMDS signal with a chip (and that's the meaning of "active"). – Tom Yan Dec 28 '21 at 05:25
  • “Does this mean that every USB Type-C-to-HDMI cable on the market is active?” - No – Ramhound Dec 28 '21 at 05:32
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    @Giacomo1968 By "active", I mean a cable containing circuitry to convert an LVDS signal (DisplayPort) to a TMDS signal (HDMI, DVI). By contrast, a passive cable would simple rearrange the DisplayPort signal wires to fit an HDMI connector, requiring the source device to detect the HDMI device and output an HDMI signal accordingly. – Blieque Dec 28 '21 at 05:35
  • @Ramhound Do you know of an explicitly passive USB Type-C-to-HDMI cable? As far as I can determine, almost no video devices would support it. – Blieque Dec 28 '21 at 05:39
  • @Blieque - Your last comment is confusing, you mentioned HDMI to DisplayPort, but your asking me about HDMI alternate mode. Which conversion is your question about? You cannot convert DP to HDMI without an active cable (or it might be the other way, honestly, to tired to look up which way requires an active cable) since this question is so darn confusing – Ramhound Dec 28 '21 at 05:51
  • @Ramhound Sending video from an HDMI device to a DisplayPort display requires an active cable (unless the display can decode LVDS over HDMI – I don't think any displays do this). Sending video from a DisplayPort device to an HDMI display *can* be done with a passive cable, provided the source device supports DisplayPort Dual-Mode (unrelated to USB Type-C "Alternate Modes"). This feature allows the source device to detect that it is connected to an HDMI display via passive cable and subsequently switch to TMDS signalling that the display will understand. Sorry – the standards are complicated. – Blieque Dec 28 '21 at 06:28
  • @Blieque - Yes; I know that hence the reason I brought it up. But your question isn't about HDMI to DP cables. – Ramhound Dec 28 '21 at 19:28
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    Does this answer your question? [What's up with HDMI and DisplayPort over USB C?](https://superuser.com/questions/1192638/whats-up-with-hdmi-and-displayport-over-usb-c) – chx Dec 29 '21 at 02:28

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I think that boiling your question down what you are asking is essentially whether USB Alt Mode DisplayPort is capable of outputting a native HDMI signal, and I think you are right in saying it cannot.

USB C can indeed support various Alt Modes:

  • DisplayPort Alternate Mode
  • Mobile High-Definition Link (MHL) Alternate Mode
  • Thunderbolt Alternate Mode (TB includes DisplayPort)
  • HDMI Alternate Mode

And there is some implications that these modes are exclusive. You can have a Thunderbolt port (where Thunderbolt does also support DisplayPort), but a DisplayPort does not necessarily support HDMI. Most dedicated graphics cards support changing a DisplayPort to output HDMI, but that is a special Mode of the display output.

In the case of USB though supporting DisplayPort does not automatically mean supporting HDMI, so to get HDMI from a DisplayPort or Thunderbolt USB C port you would need an active DisplayPort to HDMI adaptor.

Not all USB C HDMI adapters need to be “active,” especially if the actual USB port supports HDMI output, but if the port only supports DisplayPort then active conversion may well be required.

Yes, this is a minefield of “I need yet another adapter” problems.

Giacomo1968
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Mokubai
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    Approximately nothing supports HDMI altmode. Every hub/cable/etc that you'll find that has USB C on one end and HDMI on the other is converting from DisplayPort to HDMI. – Captain Segfault Dec 30 '21 at 10:07