Over the past year we have seen an increasingly number of vendors, associations, and consortiums lay their support behind the USB Type-C port and take advantage of its alternate mode capabilities. Via alternate modes, a portion of the pins can be reallocated to carry signals other than USB SuperSpeed data, allowing a single port to be a USB port, data port, a display port, and more. This week the HDMI consortium has become the latest group to make use of the alt mode functionality, announcing the HDMI alt mode specification.

Like the other specifications using the alt mode feature, the development of an HDMI alt mode is intended to give laptops and other devices the option of outputting HDMI video over a Type-C port, rather than requiring a separate HDMI port. This saves space and can simplify the process of hooking up an HDMI display, all the more important as devices continue to get thinner and USB-C ports become more common. The addition of an HDMI alt mode means that device manufacturers, when devices are suitably configured, can now offer a direct HDMI connection over USB-C from their devices using a simple cable. Previously the only way to offer HDMI via USB-C was to first pipe out DisplayPort, and then covert that to HDMI, which requires a more complex full protocol adapter.

Digging into the announcement, it’s interesting to note that the alt mode specification is for HDMI 1.4b, and not HDMI 2.0, which means that the maximum resolution with full chroma subsampling is 4Kp30. The latter 2.0 specification uses the same pins, just at a higher data rate, so I’m not sure if there’s some kind of technical limitation in play here, or if the consortium had other reasons to favor 1.4b. Few mobile devices can output 4Kp60 video right now, however laptops with dGPUs are already there, and eventually iGPUs will get there as well. Otherwise the full HDMI feature set is supported, including the audio return channel, CEC, and the Ethernet channel.

Meanwhile the HDMI consortium hasn’t released too much in the way of technical details for how the pin configuration works, so there are a few holes here. As we’ve already seen with DisplayPort’s alt mode, you can typically use 11 pins for an alt mode – the 8 SuperSpeed pins, the 2 SBU pins, and one of the CC pins – and ignoring the 4 shield pins of the HDMI connector, I’m not sure how this maps to the remaining 15 pins of an HDMI connector. The consortium notes that this is meant to enable HDMI over a “simple cable,” so if a chip ends up being required, I’d expect it to be equally simple.

One thing to note though that compared to the existing DisplayPort alt mode, HDMI requires all 4 of its high speed data/clock lanes to operate, so it doesn’t appear that there won’t be an option to have a cable carry a mix of HDMI video and USB SuperSpeed data. This means that manufacturers that make multi-port adapters with both USB 3.0 and HDMI – like Apple’s Digital AV Multiport Adapter – will still need to utilize DisplayPort-to-HDMI conversion to make the necessary lane allocations work. The HDMI alt mode, in this respect, seems far more focused on just directly connecting devices with HDMI displays, with maybe a USB-C pass-through for power.

In any case, the HDMI consortium expects the first HDMI alt mode capable devices to be announced early next year, possibly in time for CES 2017. Like the other alt modes, manufacturers do need to build in support for the HDMI alt mode – typically using a simple mux – so whether a device supports this alt mode will vary on a case-by-case basis. But given how popular HDMI is, if it’s easy (and cheap) to implement I wouldn’t be surprised to see pretty wide adoption for this alt mode in laptops and other devices that already have HDMI capabilities.

Source: HDMI Consortium

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  • stephenbrooks - Friday, September 2, 2016 - link

    Hoping that devices with multiple USB-C ports will incorporate a crossbar switch so that any of the USB-C ports can map to any of the internal sources (HDMI, actual USB, DP, ...) rather than having the user remember which port can do what.
  • Ryan Smith - Friday, September 2, 2016 - link

    Nothing is set in stone yet, but according to the announcement the Consortium will be working on this very issue.

    "In some cases manufacturers will be displaying HDMI logos. We are also working with USB-IF to make sure consumers can recognize when HDMI Alt Mode is supported on USB Type-C devices."
  • aj700 - Friday, September 2, 2016 - link

    Hmm not sure if I understand this, isnt easier if display manufacturers use Usb type c connector on their device. They can internally convert to whatever they want, Right? We could get rid of hdmi connector itself. I though standards makes life easy...
  • prisonerX - Friday, September 2, 2016 - link

    That's the idea. USB C to your display and you get the signal + power, no other cables required.
  • Mr Perfect - Friday, September 2, 2016 - link

    OEMs: "Now that you can do everything through USB-c, we've made our devices slimmer by putting nothing but a single USB-c port on them! You don't do more the one thing at a time, right?"
  • prisonerX - Friday, September 2, 2016 - link

    Ever heard of a USB hub, genius?
  • AnnonymousCoward - Friday, September 2, 2016 - link

    You want to carry that around?
  • prisonerX - Friday, September 2, 2016 - link

    Carry what around? All these devices I'm plugging in? Let's assume we're talking about a laptop: either you're walking around and you don't carry much of anything and one port is fine, or you're at your desk and you only want to plug in one cable to connect all attached devices including power and your monitor.

    It makes perfect sense if you stop and think about it for a moment.
  • inighthawki - Friday, September 2, 2016 - link

    1) USB port + flash drive is not an uncommon portable scenario.
    2) In meeting, need to connect external display (e.g. projector) using this new usb display tech, and want to keep a mouse
    3) At a colleague's desk and they don't have a spare USB hub (because why would you need two, right?) - whoops!
    3b) Someone brings their laptop somewhere they don't normally bring it - no USB hub available.
    4) Some people do not like USB hubs, or it just adds clutter compared to what the user needs. Wireless mouse receiver + flash drive, ah yes let me setup a usb hub so my laptop is now tethered to my desk, and possibly even requires power for the hub.

    Don't be daft. Stop and think for a moment and you might realize there are scenarios which don't fall into your scenario. I cannot count the number of times I've run into one of these exact situations where the single port on my laptop has become a burden.
  • HomeworldFound - Friday, September 2, 2016 - link

    The ports are tiny, they could easily have three or four of them in the space dedicated to a single full size HDMI port.

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