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

Comments Locked


View All Comments

  • prisonerX - Saturday, September 3, 2016 - link

    "What if you're trapped in a technology recycling shop situated in a Faraday cage that has a policy of not accepting USB hubs or wireless tech?"


    This is the typical response from port fixated - make up a bunch of weak special situations where you're confronted with endless wired devices and all wireless devices and connections are somehow absent, along with all USB hubs. Unsurprisingly most people don't live in this bizarro wired world, and those few that do have machines with more ports. The difference is that those few wired people seem to want to force those normal people to carry around unnecessary ports on their machines all day.
  • inighthawki - Saturday, September 3, 2016 - link

    Can't tell if you're just trolling at this point or if you're really as dumb as what you just wrote...
  • AnnonymousCoward - Friday, September 2, 2016 - link

    Are you a proud owner of that foolish macbook that has 1 Type C port or something?

    I need power, a mouse, flash drive, and headphone jack.
  • StrangerGuy - Monday, September 5, 2016 - link

    Shhh, here's some top secret information: Normal people don't wish they have less USB ports on their computers, or replace perfectly functional old school HDMI/DP/USB cables just because greedy vendors said so.
  • Meteor2 - Thursday, September 8, 2016 - link

    I do, and I thought I'm normal.

    I don't like messing around with different cables and ports. One cable, one connector. Seems normal and natural to me.
  • Zan Lynx - Friday, September 2, 2016 - link

    And the hubs have real problems with alt-mode. A lot of them don't support it.
  • prisonerX - Friday, September 2, 2016 - link

    That's not good. You'd think after 10 years of USB-C they'd have fixed that by now.
  • Ryan Smith - Friday, September 2, 2016 - link

    It can't be fixed. Alt mode is simply multiplexing a different signal on to the cable. That can't work through a hub, because the hub only transmits USB. It's a bit like expecting Power over Ethernet to work over a regular hub/switch.
  • prisonerX - Saturday, September 3, 2016 - link

    Of course you can fix it. If you detect it you can pass it through to a dedicated port used for alt mode output (where the screen would always be plugged in) or otherwise switch the USB signal. With your POE analogy you'd have a dedicated ethernet port that accepts power which routes it to a dedicated ethernet port that outputs power.

    The actual solution would obviously be according to what users require or find useful but it's hardly impossible. Even if you arbitrarily restrict it to a general solution for all ports it not impossible just horrendously expensive and unnecessary.
  • LukaP - Saturday, September 3, 2016 - link

    Hubs by definition work on the physical layer, so they should be able to accept and retransmit any signal they get no? its just splitting each wire of the USB port into more wires, one for each out-port.

Log in

Don't have an account? Sign up now