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HDMI Alt Mode for USB Type-C Connector

21 points by marcopolis 3y ago

mithrilskeleton 3y ago
How is this relevant considering DisplayPort alt mode has been a thing for a while?
- bastawhiz 3y ago
  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.
  - crote 3y ago
    Virtually all USB-C-to-HDMI cables use DisplayPort Alt Mode and internally convert from DP to HDMI.
  - seba_dos1 3y ago
    It's irrelevant. The linked page is several years old; HDMI alt-mode didn't catch on and virtually nothing implements it.
  - lxgr 3y ago
    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.
Gigachad 3y ago
Is there anything interesting here or is this just the spec that we have been using on laptops for ages now?
- crote 3y ago
  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.
  - Gigachad 3y ago
    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.
eigen 3y ago
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."
- bastawhiz 3y ago
  I'd suspect "simple" is doing a lot of work here.
  - lxgr 3y ago
    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.
- derefr 3y ago
  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.
- dmitrygr 3y ago
  > 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?
  - lxgr 3y ago
rektide 3y ago
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.
crote 3y ago
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.
- nyanpasu64 3y ago
  It's not the real deal, but https://www.quantumdata.com/assets/essentials_displayport_usb-c_dp_alt_mode_protocols_webinar.pdf 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).
- MBCook 3y ago
  > 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?
  - hocuspocus 3y ago
    > 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.
startupsfail 3y ago
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.
- teaearlgraycold 3y ago
  This has not been my experience at all with DP in lightning and USB ports.
  - lxgr 3y ago
    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.
    - teaearlgraycold 3y ago
      Oops. I meant Thunderbolt. Terrible naming decisions were made.
      - lxgr 3y ago
        Never underestimate the amount of pain engineers will subject themselves and others to, all in service of a clever pun.
- stevefan1999 3y ago
  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
  - crooked-v 3y ago
    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.
amstan 3y ago
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 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.
MBCook 3y ago
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.
- lxgr 3y ago
  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.
seba_dos1 3y ago
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-axed-as-displayport-alt-mode-reigns-supreme/ https://arstechnica.com/gadgets/2023/01/hdmi-to-usb-c-spec-a...

21 points by marcopolis 3y ago

mithrilskeleton 3y ago

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

bastawhiz 3y ago
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.
- crote 3y ago
  Virtually all USB-C-to-HDMI cables use DisplayPort Alt Mode and internally convert from DP to HDMI.
- seba_dos1 3y ago
  It's irrelevant. The linked page is several years old; HDMI alt-mode didn't catch on and virtually nothing implements it.
- lxgr 3y ago
  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.

Gigachad 3y ago

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

crote 3y ago
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.
- Gigachad 3y ago
  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.

eigen 3y ago

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."

bastawhiz 3y ago
I'd suspect "simple" is doing a lot of work here.
- lxgr 3y ago
  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.
derefr 3y ago
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.
dmitrygr 3y ago
> 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?
- lxgr 3y ago

rektide 3y ago

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.

crote 3y ago

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.

nyanpasu64 3y ago
It's not the real deal, but https://www.quantumdata.com/assets/essentials_displayport_usb-c_dp_alt_mode_protocols_webinar.pdf 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).
MBCook 3y ago
> 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?
- hocuspocus 3y ago
  > 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.

startupsfail 3y ago

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.

teaearlgraycold 3y ago
This has not been my experience at all with DP in lightning and USB ports.
- lxgr 3y ago
  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.
  - teaearlgraycold 3y ago
    Oops. I meant Thunderbolt. Terrible naming decisions were made.
    - lxgr 3y ago
      Never underestimate the amount of pain engineers will subject themselves and others to, all in service of a clever pun.
stevefan1999 3y ago
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
- crooked-v 3y ago
  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.

amstan 3y ago

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 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.

MBCook 3y ago

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.

lxgr 3y ago
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.

seba_dos1 3y ago

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-axed-as-displayport-alt-mode-reigns-supreme/ https://arstechnica.com/gadgets/2023/01/hdmi-to-usb-c-spec-a...