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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  • jann5s - Sunday, September 4, 2016 - link

    Lol, I didn't know about lmgtfy, loving it
  • Zingam - Friday, September 2, 2016 - link

    Does this mean that USB-C will bog down the system one day because everything will use it and MB manufacturers will put cheap sub-par controllers that will underperform and also bandwidth conflicts will arise?.
  • DanNeely - Friday, September 2, 2016 - link

    The alternate modes are done by switching the connection from USB to something else on the mainboard. So we won't have the problem you're worried about. What we will have instead is the problem that without reading the manuals (HA!!) it will be impossible to know if any given pair of devices can talk to each other because everything will be carried over USB-C cables but not everything will support the same set of protocols. "Universal" as in used for everything not "universal" as in any pair of USB devices will always work together. *BLEGH*
  • Fergy - Sunday, September 4, 2016 - link

    Or it means like with ethernet that the 2 devices talk to each other to choose what kind of connection they will use.
  • saratoga4 - Thursday, September 8, 2016 - link

    Ethernet devices negotiate speed, same as USB.

    Problem here is that because HDMI and USB use the same connector, it will be possible to plug in devices that only support HDMI (for example your TV) to devices that only support USB (data only ports on a laptop).
  • mkozakewich - Tuesday, September 6, 2016 - link

    "Universal Serial Bus"
    At least it would live up to its name!
  • Gunbuster - Friday, September 2, 2016 - link

    Wait what? I thought the HDMI consortium just got done with having their panties in a bunch over X to HDMI cables, banning any cable that was not HDMI on both ends from web based retailers.
  • prisonerX - Friday, September 2, 2016 - link

    The drugs you're taking are making you think that the HDMI consortium has magical powers. Unless it's a trademark issue, they can't ban squat.
  • Gunbuster - Tuesday, September 6, 2016 - link

    Check someplace like Monoprice, they cant call them Mini DP to HDMI. http://www.monoprice.com/product?c_id=102&cp_i...

  • inighthawki - Friday, September 2, 2016 - link

    "No adapter, no converter"

    Awesome, instead we just need a whole new cable that doesn't work with anything else!

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