AV Nuggets - The Quest to Send HDMI Signal to a USB C Display

My (soon to be previous) job used these portable 17inch LED monitors as confidence monitors. The monitors were intended to be used as a portable external display for a laptop. As such they have two inputs: Mini HDMI and USB C. Obviously, the USB C input is to provide power and provide display port over USB C. The HDMI input is our primary interface. We run SDI from the tech table to the confidence monitor and use run signal into the confidence monitor via HDMI.

For those with any experience in working with mini HDMI, you will know that it is a very fragile interface and breaks constantly. We did not avoid fate and at least one of the confidence monitor’s HDMI input broke. Not to worry! The USB input still works! We can still get our HDMI input into the USB port via an adapter. Just need to grab a dongle…

For those who cannot tell, this is sarcasm. HDMI ports are not bidirectional. They can be either an input or output, they can never be both. That’s why you can’t just plug an xbox into your computer and expect it to pop up on your display. HDMI just doesn’t work that way. Or I guess no AV equipment works like that…

I got into a massive argument with one of the freelancers at my job over it. He INSISTED it was possible to get HDMI into the confidence monitor via usb c and I dongle. Despite my explaining, he still stubbornly attempted to get it to work. He failed.

Through the grapevine another freelancer heard about this argument and he, again, insisted there must be a purpose made device for this. The problem must have been because of the dongle. Once we get an HDMI to USB C adapter THEN it will work… Right??? Well what is a device that takes an HDMI signal and converts it to USB? Ah that’s right, a capture card! If I just plug a capture card into the confidence monitor’s USB port then I can send it and HDMI signal!!!

Again, sarcasm….

The method described above would not work due to differences in technology. HDMI capture cards operate on the USB video class or UVC standard. This is the standard used for webcams. A capture card is just a webcam that uses an HDMI source instead of an actual camera built into the device. USB C displays do not use that protocol. They use Display Port over USB C. This spec allows for USB C to HDMI adapters to easily add extra displays to you device. Alternatively, it also allows these portable monitors to receive video and power via a single USB C cable from your device. It is pretty convenient and it has given the laymen an opportunity to use these monitors with ease. However, it has cause PLENTY of confusion regarding USB C video devices.

So what is the solution? The answer lies in the protocol name Display Port over USB C. We need to use a bidiretional USB C to Displayport cable to get video into the monitor without actually using a computer. The specific brand of portable monitors we were using, Lilliput, had two USB ports. One for power and one for video. And while the monitor can receive video and power from a computer over the same cable, in the solution above we would need to power it separately as the displayport cable would not carry power. This is where the two USB ports came in handy. This is not a feature on all portable monitor so be wary if you are trying to solve this issue. We can power the monitor like we normally do, through the USB DC port, and then send video via displayport to the other USB port.

To intergrate this workaround into our normal system, we will need a variety of dongles. Of course we will need an SDI to HDMI adapter, and then a female HDMI to female DP or male DP with a female to female adapter to plug into the USB C to Displayport cable. You may need a host of other power and adapters if the HDMI/Displayport adapter requires external power. Which it most likely will. I do not think the Displayport interface will provide power to the adapter via the USB C port. Additionally, you will have to make sure that the HDMI/Displayport adapter is bidirectional or signaled in the correct direction, or else the HDMI will not be received.

So after all of that was this solution implemented and the broken confidence monitor put back into the field?

No of course not.

The amount of adapters and power necessary to run this set up, with so many potential points of failure, this setup seems to be more trouble than its worth. Additionally, having so many parts to this work around, its not something really feasible in a pinch. Replacement is not really a cost prohibit action, at least with the department’s budget. The monitors themselves only cost a few hundred dollars at most.

All in all, HDMI to USB C input exists. It’s just not really worth it.