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I did some research on the Internet, what I found are all about using the USB port on the laptop as a display out, to something like an external monitor, which aren't my concern.

Is it possible to use the USB 3.0 as display in for laptop? i.e. Take the video signal from somewhere, and display the video output on the display of the laptop, via the USB 3.0 port. An example will be like take the video output from a console and display the video on a laptop's monitor.

The required bandwidth, let's say for a 8 bit colour 1080p 60Hz signal, is equal to 1920*1080*8*3*60 bit/s = 356 MB/s < 625 MB/s (the theoretical max bandwidth of USB 3.0). So in in terms of bandwidth it's possible.

In terms of latency, I read from somewhere else that it is really small, so this shouldn't be a concern either.

Oh, and I know this isn't practical, or I better found a laptop with a HDMI-in port, or whatever. I just want to know if this is possible. Thanks in advance.

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    You need an HDMI capture device, but they aren't cheap (£100 UK upwards) - you can buy an HDMI display for a similar price. You will also find that certain media won't play, because of copy protection.
    – AFH
    Aug 30, 2018 at 11:32
  • Why do I need a HDMI capture device? Plugging the source directly into the USB port, and with some software on the laptop (which I have no idea if it exists lol), is this possible?
    – user161070
    Aug 30, 2018 at 11:42
  • "is this possible" - no, not without a capture device...
    – Attie
    Aug 30, 2018 at 11:58
  • @Attie Why not possible? USB 3.0 should provide adequate bandwidth
    – user161070
    Aug 30, 2018 at 12:01
  • You need an HDMI capture device to convert the electrical signals and to give you the software to interpret the data stream as video frames.
    – AFH
    Aug 30, 2018 at 12:09

3 Answers 3

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Is it possible to use the USB 3.0 as display in for laptop? i.e. Take the video signal from somewhere, and display the video output on the display of the laptop, via the USB 3.0 port. An example will be like take the video output from a console and display the video on a laptop's monitor.

In other words, you want to turn your laptop into a display with USB interface. Theoretically, yes, it is possible. Under several conditions however. But practically, no.

Typically a USB display must conform to a special Video Class device (AV-class) capable of image rendering. The original proprietary interface was pioneered by DisplayLink. The USB device therefore must conform to certain endpoint structure, descriptor content, and a USB host must have a Virtual Graphics Card software/driver installed, which would create a virtual display, and transmit the image data over USB to the DisplayLink device.

So, the laptop port must be a Dual-Role-Port first, and be able to act as USB device. Many tablets and smartphones have a single DRP. When connected to USB host, they act as MTP (media transfer) or Mass Storage devices. They can act as webcam and stream video from laptop to host PC, but not in the opposite direction.

Therefore, the laptop must pretend to be a video streaming device. AFAIK, no DRP gadgets have implemented the Video rendering class. More, it is unclear if the standard implementations of OTG device controllers are capable of implementing of AV rendering class, although it is theoretically possible if SoC designers decided to configure the USB device controller IP to support AV class at silicon level. Which practically doesn't happen, unless somebody can correct me.

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  • Can you elaborate more on DRP? I can't find it anywhere in the pdf you've linked / can't find anything that I can understand with Google. Thanks.
    – user161070
    Aug 31, 2018 at 2:44
  • @user161070, DRP a term from Type-C specification and indicates a USB port that can be either USB host (normal state of desktop PC and laptops), or can become a USB device. For classic micro-AB connectors it was known as "OTG". A normal Type-A receptacle can't be turned into device mode.
    – Ale..chenski
    Aug 31, 2018 at 3:31
  • Note that a Video Class Device is an adaption of the DisplayLink protocol. Legacy DisplayLink isn't an USB protocol. USB master/slave isn't an issue in the DisplayLink protocol, because it's not USB in the first place (though it's also asymmetric). Duel role/OTG laptops are extremely rare, but again, that's not the main issue. It's extremely important to keep the distinction between USB 3.0 alternate modes and normal USB in mind.
    – dirkt
    Aug 31, 2018 at 15:41
  • @dirkt, DisplayLink works over standard USB ports and therefore uses the standard USB protocol. It just designates itself as "vendor-specific" USB class. It is important to realize that USB itself doesn't have any "alternate modes". It is an option in Type-C connector specifications, allowing to re-use pins for alternative purposes, including DisplayPort video.
    – Ale..chenski
    Aug 31, 2018 at 17:21
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You can't understand why this isn't possible unless you look under the cover at the actual technology involved. "But I can plug one side of a cable into the USB port, an the other side into a monitor, and it works, so why can't I do it the other way round?" isn't going to help.

Your USB 3.0 port consists of a number of high-speed serial lines. Normally they would speak the USB 3.0 port protocol, which has nothing to do with graphics whatsoever.

Now some people though it would be nice if you could use the USB 3.0 port for some other purposes, so they invented alternate modes. These use some or all of the high-speed serial links for other purposes, with a different protocol. And just because you have some device somewhere with an USB 3.0 port doesn't mean this is possible.

So there are laptops where you can use the USB 3.0 port to send DisplayPort data: The framebuffer scanning output of the GPU is wired up to the USB 3.0 controller, and the USB 3.0 controller can be switched to output this data on the serial lines, using the DisplayPort protocol.

This means you can plug the other end into a monitor, and it will display the data.

It doesn't mean you can plug in some other DisplayPort data source to your laptop, and your laptop will be able to read the data: The DisplayPort protocol is not symmetric.

If you wanted this, in addition to the extra hardware that allows you to output the GPU framebuffer scanout via the USB 3.0 controller, you need even more hardware that also allows the USB 3.0 controller to read the DisplayPort data, and then you'd have to invent a way to store this data somewhere where the GPU can get at it. And such hardware doesn't exist (at the moment).

So, no, you cannot use an USB 3.0 to read DisplayPort data. Not until someone invents the hardware for it, and puts it in your laptop, and makes the laptop more expensive because of that.

You can buy various other hardware parts that will read various forms of video signals, and transfer that to your computer in various ways, but that will be additional hardware.

Edit

1) Yes, you misunderstood. The GPU has parts that read out the frame buffer (called "CRTCs", "Pipes", or other names), and produce a monitor signal. This signal is connected to the USB controller. Not the framebuffer. And because the hardware outputs a signal, it can't input a signal. You can't reverse electronics.

2) The form of the input connector is not important. The signal (and the direction of the signal) is. You can have an input connecter with an USB Plug, a DisplayPort plug, or just some wires hanging around if you fancy doing this. What's important is the hardware behind that: In a monitor, it reads the signal. It couldn't output any signal.

3) A webcam just uses the normal USB port protocol to transfer data. What the computer does with the data is up to the computer: It can save it to a file, or overlay it on the framebuffer so it gets output on the monitor. All this is done by software, and it's fast enough you perceive it as realtime (there's actually a short delay).

Yes, you could have a piece of hardware that reads a DisplayPort/HDMI/VGA signal from a second computer, and is connected in the same way to the first computer as a webcam. You can buy this hardware. See above.

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  • The output of the GPU is connected to a frame buffer, which is connected to the USB 3.0 controller, which uses the DisplayPort protocol to send the signal out. However, since this protocol is not symmetric, one cannot do the vice verca. Did I misunderstand the answer?
    – user161070
    Aug 30, 2018 at 14:15
  • Excuse me, I should say which switches itself to use the DisplayPort protocol instead of the usual one. One more thing, if this is the case, why can I see some monitors on the market, which use USB-C as their input connector? Do they have some sort of hardware/buffer behind the port that laptops lack so that they can achieve accepting the video signal while laptops cannot? If that same hardware behind the port get replicated to a laptop, can the target be achieved? Or are there still other technical limitations?
    – user161070
    Aug 30, 2018 at 14:24
  • Also, someone above mentioned that webcams use USB port to connect to a computer, but for some reason the footage that the webcam is capturing can get viewed on that computer's display in real time. Why is this even possible?
    – user161070
    Aug 30, 2018 at 14:29
  • @user161070, a webcam is a DEVICE, and computer is HOST. The host can read the webcam datastream and can do with the stream whatever is wants.
    – Ale..chenski
    Aug 31, 2018 at 3:36
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    @user161070, no, the GPU and its frame buffer is not connected to USB 3.0 controller. The GPU buffer goes into a dedicated DisplayPort or HDMI port, and outputs the data in DP or HDMI formats. The USB port gets simply multiplexed between USB path and DP path, reusing the same differential wires. DP over Type-C has nothing to do with USB.
    – Ale..chenski
    Aug 31, 2018 at 3:41
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The screens in laptops are taking the signal from the GPU on the board (dGPU or iGPU) and from what I know there is no laptop with something like "HDMI in" just becouse they aren't made for that. Ofc you can get usb->hdmi/vga etc. dongles but they are still OUTPUTS. The only way to do something like that would be to get pcb with inputs and connect it to the screen of laptop (you can DIY an monitor with inputs, but it's something else than laptop with video in).

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  • But is it possible to send the video signal to the USB port of the laptop, then use some software to display the video on it? Like without actually taking the laptop apart and disconnect the connection between the motherboard and the monitor.
    – user161070
    Aug 30, 2018 at 11:16
  • Like the way web-cams or DSLR cameras work ? ye but with things like consoles etc. not really. You can try with some sort of elgato capture cards, but it will add massive lag and if the laptop itself is not powerfull enought it will just fail to decode that signal
    – Tankers
    Aug 30, 2018 at 11:57
  • Yea like how webcams work where video signal is sent to the laptop via USB and display via its monitor. But why "with things like consoles etc. not really?" I'd imagine with some special software/drivers it should be possible.
    – user161070
    Aug 30, 2018 at 12:04
  • It's all hardware based, you will need something like elgato card, but you will still need laptop that will have some power to decode the signal from them, but still there is the lag etc. So ye it's plausibble but most of the time it's still the hardware that will do the work. link here you can see it works, but A) it's expensive B) it add's lag and with fast paced games in such a small preview it will just be cluncky to focus C) there are now ways for PS4 and XO to be streamed to the PC (remote play and something with xgames from M$Store)
    – Tankers
    Aug 30, 2018 at 12:29
  • The elgato card is a capture card, and from what I know it's for replicating the video signal so one can have video output for one monitor, and stream with another machine, at the same time. Yes, the video you linked show that one can work this out using the elgato, but I think this is unnecessary if the method I described works. For wireless streaming, you know, there are a whole bunch of issues, like latency. So the question remains, will plugging the video signal into the USB port (using some adapters ofc), and with some software, works?
    – user161070
    Aug 30, 2018 at 12:41

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