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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  • FLHerne - Sunday, September 4, 2016 - link

    USB isn't really a bus, and its 'hubs' - particularly in USB 3 - aren't really hubs.

    The 'hubs' don't split the wires or even directly repeat signals - they read incoming packets, convert them between versions (e.g. a USB 1.1 mouse sharing a hub with USB 3 storage), and resend them when the upstream connection is free. USB 3 hubs can track the location of devices and only send on the relevant port, like Ethernet switches.

    Needless to say, alt-modes can't possibly work with the current design of hubs.

    (Not that a simple split-the-wires hub would work for video signals either...)
  • Mr Perfect - Friday, September 2, 2016 - link

    Well that was an awfully aggressive post for a reasonable concern.
  • prisonerX - Saturday, September 3, 2016 - link

    Maybe you're too sensitive for the internet, or maybe you should examine the tone of your own post before you criticize others.
  • AnnonymousCoward - Friday, September 2, 2016 - link

    So will there be HDMI-only Type C ports on devices?

    Can the HDMI sink charge your phone that's driving it?
  • Zan Lynx - Friday, September 2, 2016 - link

    USB-C can have arguments about who charges who. I read about it on some Linux discussion list. Pretty funny. A phone might end up trying to charge a laptop. Or with both not sending power at all.

    You might have a powered monitor charge the laptop, but an unpowered monitor has to have the laptop send it power. And you could plug two laptops into each other, what happens then... And if you have a plugged in laptop and unplug it, all the devices have to renegotiate power distribution.
  • prisonerX - Saturday, September 3, 2016 - link

    You're probably going to give the user some options as to when to charge other devices and when to accept charging.
  • Ryan Smith - Friday, September 2, 2016 - link

    There will be HDMI-capable Type-C ports on devices. Using an HDMI-only port would defeat the purpose of USB-C. In practice there is no shortage of USB ports available on chipsets, and muxes are easy.
  • FLHerne - Sunday, September 4, 2016 - link

    I read that as replacing HDMI ports on displays/projectors with HDMI-only Type-C ports, so they can be connected via standard USB-C cables instead of the weird Type-C -> HDMI one.

    Does the spec allow for that?
  • Ryan Smith - Sunday, September 4, 2016 - link

    That's a good question; I don't know if you can do that. The point is a bit moot since this standard only allows HDMI 1.4 anyhow - and the HDMI consortium is about the last group who will want to give up their connector - but check back in a few years with the next DisplayPort standard and see if we still have the DP connector.
  • AnnonymousCoward - Sunday, September 4, 2016 - link

    I was thinking, an HDMI-only Type C port would give a thickness advantage compared to an HDMI-only HDMI port. However, it wouldn't be as simple as slapping on the Type C port to output your HDMI source, because Type C has a reversable connector. So, you might be right that as long as you'd add reversable capability, you'd probably be able to mux USB on that port.

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