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Comment by eigen | Hacker News Reader

not a lot of details in this but I'm curious how the AC-coupled USB-C interface works with the DC-coupled HDMI. typically there would be a DP-to-HDMI convertor in the cable plug and just use DP alt-mode but this specifically says:

"This allows native HDMI features and capabilities to be utilized in a source device and to be directly connected to any HDMI display. Other Alt Mode solutions do not enable HDMI-specific features and also require converters and connector adapters to connect to HDMI-enabled displays. A simple HDMI to USB Type-C cable can be used to connect USB Type-C devices to native HDMI displays."

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Maybe an opto-isolator on each signal line at the HDMI end? HDMI inputs are generally on devices that have their own power source, so it's not like they need the power coming over the cable; and they're also generally on devices that have the room for active components like an opto-isolator inside them.

I'd suspect "simple" is doing a lot of work here.

Simple as in “not requiring protocol level conversion”.

There is a similar situation with DP++ to HDMI adapters; they need to only shift voltage levels (3.3V for DP vs. 5V for HDMI) since the DP++ port can output “HDMI on DP wires”, but actual DP to HDMI conversion involves actually translating things at the pixel level.

> native HDMI features

Please tell me that this is not marketing speak for HDCP! What other "native HDMI" features are there that DisplayPort does not already support?

Non-PCM audio (i.e. Dolby Atmos/DTS-X) and HDR10+/Dolby Vision do not appear to be supported by DisplayPort.

(Nor ARC/eARC, but I think those would be of limited utility for the USB-C alt mode.)

Maybe HDMI-CEC? But at least some versions of DisplayPort also support that, including DP alt mode. Some USB-C to HDMI (via DP alt mode) adapters even advertise that capability.

DisplayPort supports HDCP, it's not HDMI-only.

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