HDMI Alt Mode for USB Type-C Connector | Hacker News Reader

HDMI Alt Mode for USB Type-C Connector

hdmi.org

21 pointsmarcopolis52 comments

HDMI Alt Mode was basically dead on arrival.

The computer world pretty much standardized on DisplayPort already, so USB-C devices would already have to support DisplayPort Alt Mode anyways. Why add support for yet another Alt Mode when DP-to-HDMI converter chips are dirt cheap, and pretty much all USB-C-to-HDMI cables will be using those anyways due to the aforementioned DP ubiquity.

One thing which really bothers me is that all those Alt Mode specifications are proprietary. USB-C itself is a completely open spec and anyone can download the PDFs from their website - but the Alt Modes require you to join the VESA / HDMI consortium, with an associated member fee and NDA.

It's not the real deal, but https://www.quantumdata.com/assets/essentials_displayport_us... looks to be quite useful for understanding DP Alt Mode. I've used their DP regular protocol PDFs, though because this is a transcript of an animated webinar, there are images covering text which you'll need to remove in a PDF editor or LibreOffice Draw (which can import PDFs).

> The computer world pretty much standardized on DisplayPort already

Do you know why that is? I live in the Apple world do I didn’t follow the changes.

Last time I had a PC it was VGA and DVI, which gives you a sense of the timeline. Then it seems to have been HDMI for a while (?) and now seems to be moving to DisplayPort.

Is there a reason DisplayPort over Thunderbolt didn’t catch on? Maybe something about how common add-in GPUs are making implementing TB harder?

> Then it seems to have been HDMI for a while (?)

Not really. HDMI is basically DVI++ but it's first and foremost a TV and video interface.

Once DVI started to be limiting, both PCs and high-end monitors adopted DP. HDMI came to PCs and monitors later, sometimes with limitations, for instance my Dell U2711 couldn't run at its native resolution over HDMI. A few years later UHD 4:4:4 was literally impossible from a PC GPU before HDMI 2.0, while not a problem even on older DP 1.2.

Also DP can do cool things like carrying the signals for multiple monitors (daisy chaining)... I have one usb type c cable that does power display and usb for my 2 dell monitors... it is awesome... sadly Apple never implemented DP daisy chaining this and when you try it with their hardware you only get a mirrored output.

Yes, it’s a pity they never implemented that.

Presumably some deal with Intel to push Thunderbolt :/ It’s an open spec now, but at the time Apple was pushing Thunderbolt hard, only Intel was building the chips for it, both for computers and peripherals (ie monitors, docks etc.)

DisplayPort is royalty-free while HDMI charges licensing per unit. Additionally, Intel strongly supported DP so many boards came with support even if there was nothing to connect it to, and Intel ensured support in USB specs. Eventually enough display manufacturers added support* and DP became common enough, this generated demand and we ended with a self-reinforcing system. There's still a lot of HDMI, so I wouldn't say the computer world is standardized. In my experience, people just use whatever port is available...

* Display turnaround time is significant. Displays can survive a decade comfortably. A few people brag about their 10000K display with 10000Hz refresh rates, but many people just keep their display until it drops or until they buy a new system. So DP took a long time to be universally supported and that hampered adoption for a long time.

HDMI was designed by the entertainment industry to connect HD televisions. Its initial version was electronically basically half of a DVI connector, with some audio data thrown into the video stream. Plenty for a 1080p 60Hz stream, but not much beyond that.

DisplayPort was developed by VESA for computer use. It supported a higher bandwidth than HDMI and rapidly gained support for features like variable refresh rates, compression, and feeding multiple monitors through a single cable.

Nowadays it doesn't really matter that much, and they both essentially have the same capabilities. It's now HDMI for televisions and DP for computers because History.

So, maybe I'm missing something here, but who is talking about computers? Most things that need to be connected to with HDMI, are TVs. I have 5, none of them take display port or USB-C. The all take HDMI. I have no "monitors" because... Why would I need one (I only "game" things that have to run on dosbox)?

But let's talk computers. And phones. All mine, only have USB-C. I have an old one that has micro, and I chromecast the screen from that one. I connect them all to TVs. With HDMI on the TV end.

So literally your dead on arrival example is not even present in the life of my household, while what's talked about in the article is widely used.

It's not used at all. HDMI alt-mode is several years old and nothing implements it. Even HDMI LA considers it dead now, they closed their certification program for it.

In practice, video over USB-C uses either DP alt-mode, Thunderbolt or USB4 (and the latter ones encapsulate DP over their own protocols). All USB-C to HDMI cables and adapters actually use DisplayPort and converters. HDMI alt-mode was indeed dead on arrival.

I'm not addressing the Alt-mode. Please read what was written, and reply to what was written. The comment to which I replied said you connect devices with display port, not HDMI, because all your devices are monitors. The real-world use case is USB-C to HDMI, to connect to a TV. I have no idea what my cables do, but they have USB-C on one end, and HDMI, not DP on the other, and I plug them into the TV.

The comment stated you don't need HDMI on the other end. I am not arguing for "Alt-mode" protocol, nor did I write "Alt-mode" anywhere in my comment. I am specifically replying to the wrong fact that most people use USB-C to connect to and HDMI port, not a DP port, on the other end of the cable.

From a quick google, the reason Alt mode is "dead" is because most devices natively support HDMI and you don't need alt-mode. https://www.notebookcheck.net/The-demise-of-HDMI-over-USB-C-...

You are missing that there is a chip in HDMI side of the cable that converts DisplayPort to HDMI. DisplayPort is what goes over the cable and what the device knows about.

You seem pretty confused.

> From a quick google, the reason Alt mode is "dead" is because most devices natively support HDMI and you don't need alt-mode.

If you do video over USB-C, you need alt-mode. Everything uses DisplayPort alt-mode and converts it to HDMI if needed. These days most devices have no other form of video connectivity than USB-C.

You link to an article that literally states "DisplayPort has won".

> The real-world use case is USB-C to HDMI, to connect to a TV.

The real world use case is to connect a PC or phone to an external display. Usually that's going to be a PC monitor, where physical DisplayPort actually is ubiquitous these days. Personally, I don't even have a TV - I'm not exactly a gamer so I don't see why would I need one.

(I actually connect my laptop to a screen using HDMI connector on the screen side, but that's only because it's about 10 years old :P)

I recently picked up a $100 external display on amazon. It's essentially a laptop screen with a magnetic cover a la surface tablet.

It's just a screen, no touch capabilities or any real processing onboard.

This screen has 3 usbc ports and a mini hdmi port. It also has an audio out jack.

In theory, it can be driven with a usbc to usbc cable from a phone or computer, but I don't have a device that can drive it this way.

The second usbc port is auxiliary power for when your phone can't power the whole screen.

The third usbc port is for daisy chaining another device.

The way I use this thing is from my (fairly old) laptop that only has display port out. I have an adapter that changes the DP port on my laptop to full side hdmi and then use the hdmi to hdmi mini cable that came with the screen.

Unfortunately, this setup does not pass sound to the audio jack. I wish my laptop had an hdmi port instead.

I have a similar portable screen with almost the same set connectors (only two USB-C ports and miniHDMI, no daisy chaining). That "(fairly old)" is the key there, as pretty much all modern laptops support DisplayPort over USB-C. I can plug that screen to my 4 year old laptop or my phone using USB-C to USB-C cable and it works, including audio.

> while what's talked about in the article is widely used

Except it's not, because that functionality is basically nonexistent. All the USB-C to HDMI connectors out there are actually doing the USB-C DisplayPort mode with the equivalent of a DP->HDMI hardware dongle built in.

Also, there is MHL alt mode which sends the signal over one pair like DP.mi don’t know if this gets used but at least it makes sense unlike HDMI mode.

The reason why HDMI alt mode is awful is that it uses most the wires for HDMI. That doesn’t leave any wires for other uses like USB3. It may even use USB2 wires. The UsB-C docks would be impossible. The only use is for passive adapters.

It may be worth adding "(2016)" to the title, otherwise people are going to think like this dead tech is somehow relevant today ;)

If you're confused, read https://arstechnica.com/gadgets/2023/01/hdmi-to-usb-c-spec-a...

Yeah, this is what a lot of people don't get: HDMI Alt mode has failed to launch, a lot of devices out there just use the DP alt mode when trying to output video out of a type C port.

When you have a C-to-HDMI (eg https://frame.work/products/hdmi-expansion-card) it's more like .... C-to-DP(implicit in type C)-to-HDMI. If you have a monitor that has both DP and HDMI you probably want to get yourself a simple C-to-DP cable as there's no active conversion going on so it's more likely to work.

It's alas unlikely but oh how grand it would be to not need HDMI.

USB4 ports mandatorily have DisplayPort output, on each port. HDMI is not going to catch up. It is an expensive proprietary system & won't ever match the availability.

not a lot of details in this but I'm curious how the AC-coupled USB-C interface works with the DC-coupled HDMI. typically there would be a DP-to-HDMI convertor in the cable plug and just use DP alt-mode but this specifically says:

"This allows native HDMI features and capabilities to be utilized in a source device and to be directly connected to any HDMI display. Other Alt Mode solutions do not enable HDMI-specific features and also require converters and connector adapters to connect to HDMI-enabled displays. A simple HDMI to USB Type-C cable can be used to connect USB Type-C devices to native HDMI displays."

I'd suspect "simple" is doing a lot of work here.

Simple as in “not requiring protocol level conversion”.

There is a similar situation with DP++ to HDMI adapters; they need to only shift voltage levels (3.3V for DP vs. 5V for HDMI) since the DP++ port can output “HDMI on DP wires”, but actual DP to HDMI conversion involves actually translating things at the pixel level.

Maybe an opto-isolator on each signal line at the HDMI end? HDMI inputs are generally on devices that have their own power source, so it's not like they need the power coming over the cable; and they're also generally on devices that have the room for active components like an opto-isolator inside them.

> native HDMI features

Please tell me that this is not marketing speak for HDCP! What other "native HDMI" features are there that DisplayPort does not already support?

Maybe HDMI-CEC? But at least some versions of DisplayPort also support that, including DP alt mode. Some USB-C to HDMI (via DP alt mode) adapters even advertise that capability.

Non-PCM audio (i.e. Dolby Atmos/DTS-X) and HDR10+/Dolby Vision do not appear to be supported by DisplayPort.

(Nor ARC/eARC, but I think those would be of limited utility for the USB-C alt mode.)

DisplayPort supports HDCP, it's not HDMI-only.

How is this relevant considering DisplayPort alt mode has been a thing for a while?

It's relevant because the display just needs an HDMI port. You can plug a HDMI-to-USC-C cable into a computer and a TV or projector and it'll work using this. If your device doesn't do displayport then displayport support is moot.

Virtually all USB-C-to-HDMI cables use DisplayPort Alt Mode and internally convert from DP to HDMI.

It's irrelevant. The linked page is several years old; HDMI alt-mode didn't catch on and virtually nothing implements it.

Were there ever any (source) devices with HDMI alt mode shipped, though? I vaguely remember hearing about one smartphone, but that seemed more like a showcase for the technology.

Is there anything interesting here or is this just the spec that we have been using on laptops for ages now?

You probably haven't been using it for ages, because virtually nobody bothered to implement it. Everyone is using DisplayPort Alt Mode instead - even for C-to-HDMI adapters.

Ah, that's an interesting distinction. Makes this spec seem pretty much useless. Guess they had to put something out rather than give up control of their proprietary spec.

So… the same connector as Thunderbolt, and it can deliver video like Thunderbolt. And in both cases it’s running DisplayPort over the connector.

So there were two different ways to run DP over a cable with a USB-C connector.

Sounds like it’s good it’s dead.

No, HDMI alt mode was (is?) actually sending HDMI over the USB-C wire.

Thunderbolt also doesn’t really natively send DP over USB; it encapsulates it (plus PCI express) in a custom wire format.

Only DP alt mode actually uses DisplayPort at the wire level over USB-C cables or ports.

USB Type-C is such a mistake for video connectivity. It just doesn’t work most of the time and when it works, any tiny move of the USB Type-C cable breaks it down.

This has not been my experience at all with DP in lightning and USB ports.

Lightning doesn’t support DP, does it?

There are lightning to HDMI adapters, but these fascinatingly run a full SoC that receives and decodes a compressed H.264 video stream over lightning, which I wouldn’t really call HDMI over lightning.

Oops. I meant Thunderbolt. Terrible naming decisions were made.

Never underestimate the amount of pain engineers will subject themselves and others to, all in service of a clever pun.

That's why you should buy an expensive cable that comes with shielding. Most likely the Type-C cables that said USB4 compilant would do

Monoprice has relatively cheap USB4 cables in 3 ft (40Gbps) and 6 ft (20Gbps) versions. I've gotten a bunch to replace my previous mess of different cable types.

I use USB-C-to-DP cables as well as full Thunderbolt and haven’t had issues with them.

USB-C cables can be garbage, so you probably need to get a better one.

Well, here is one example. I use the cables that came with the portable monitor. When I use a miniHDMI cable, no problems. With USB-C the monitor drops the connection and blinks every so often.

Here is another example. A coworker came crying that her monitor is dropping connection. We’ve tried an HDMI cable (this is a full size monitor connected to a laptop), problem disappears.

It’s a difference between working reliably (like an HDMI cable) vs working sometimes, if you are lucky.

I've had the same experience. Especially when you're trying to daisy chain or use anything very high bandwidth. I've tried different cables too but the mating just isn't solid enough and small bumps kill the link.

it's 2023 and I can't plug my phone into a dock or monitor without fancy bits. At least I can plug in a USB and and it can act as a host.

The _only_ reason I want a linux based phone is so I can just use my phone like a normal device.

The software implementations (who remembers MTP?) for phones kills its usability as a general purpose device.

What do you mean by "fancy bits?". I can plug my Galaxy S23 into the same dock as my Macbook Pro and use DeX with video out and my keyboard and mouse without installing or doing anything other than clicking "ok".

I‘ve personally had similar issues with both “real” mini DP as well as USB-C to DP cables, adapters, or USB-C native monitors. Conversely, I’ve also used USB-C video without any hiccups in other device constellations.

It depends much more on the combination of monitor, source, and cable, than on native DP vs USB-C, in my experience.

I have a portable USB C monitor that gets power and video from one cable. Why is it a mistake?

This is my “universal” cable;

https://www.amazon.com/dp/B093YVRHMB

- USB A and USB C on one end.

- USB C, micro USB and lightning on the other end.

- 100 W charging support

- 10Gbps data transfer

- supports video over USB-C

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