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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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1 Answers1

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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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