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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  • ImSpartacus - Friday, September 2, 2016 - link

    Yeah, exactly what is the intended value proposition? Honestly, I wouldn't mind leading an adaptor on my tv for the times when I want to use usb c to connect to it. It's simply not that much of an inconvenience, lol.
  • FLHerne - Friday, September 2, 2016 - link

    Active adapters are expensive and add latency - you need a chip to interpret one signal (presumably DisplayPort), buffer enough data to cover differences in the protocols, and then output an HDMI signal.

    Not to mention the nightmares of DRM copy protection (https://xkcd.com/129/).

    With this, all you need is a plain cable with some stringy copper in it.
  • inighthawki - Saturday, September 3, 2016 - link

    I don't get your point - this cable can easily be created with a passive adapter.
  • FLHerne - Saturday, September 3, 2016 - link

    The feeling's mutal then, because it can't.

    The existing graphics alt-mode for USB-C is DisplayPort. The protocol isn't compatible with HDMI, there's no such thing as a passive DP protocol -> HDMI adapter.

    You're probably confused by DisplayPort++, which is alt-mode HDMI for DP sockets - i.e. the socket detects a passive 'adapter' and uses the HDMI protocol instead of DisplayPort.

    This is required for dedicated DP 1.3+ sockets, and fairly common on older ones, but *isn't* part of the DP alt-mode spec for USB-C which only includes the real DP protocol.
  • FLHerne - Saturday, September 3, 2016 - link

    Oh, and DisplayPort++ only covers the video segnal - even with such a port, a passive adapter can't do HDMI audio, let alone ethernet.
  • inighthawki - Saturday, September 3, 2016 - link

    Sorry perhaps I'm a bit confused about how all this works - I'm not very familiar with alt mode or any of the usb protocols, but my understanding from the article made it sound like this allows hdmi to be supported over USB-c as long as the device supports it, by simply repurposing the lines in the cable, and thus from the cable's perspective, passive (no active converter needed on/in the cable itself), and as hinted by the first image in the article - "no adapters, no converters".
  • FLHerne - Saturday, September 3, 2016 - link

    I see - you're correct about what this does, I was describing the alternatives.

    Without this mode:
    - To drive an HDMI display from a USB-C port you need an active DisplayPort-over-USB-C to HDMI adapter, which are bulky/expensive/power-hungry [pick two].

    With this mode:
    - To drive an HDMI display from a USB-C port you need a simple, totally passive cable. No power, very compact, should be just as cheap as any other cable.

    Hence "the intended value proposition" (per the comment I responded to) is that you can use a simple £4 cable instead of a £20+ adapter. Neither serves any other purpose, but at least this is cheaper and tidier.
  • mkozakewich - Tuesday, September 6, 2016 - link

    You keep on missing his point. With this new signalling, you can use a cheap passive USB-to-HDMI adapter and then use all the normal HDMI-to-HDMI cables. It's really annoying needing new cables for everything.
  • ltcommanderdata - Friday, September 2, 2016 - link

    Do they have a new logo associated with this alt mode? With so many alt modes now and the ability of manufacturers to pick and choose which to support it seems like it'll be difficult to know what a given USB-C port supports without looking at a detailed spec sheet.
  • ImSpartacus - Friday, September 2, 2016 - link

    Yeah, it really defeats the point of usb...

    Maybe it'll consolidate in a couple years.

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