Modifying an HDMI dummy plug's EDID using a Raspberry Pi
41 comments
·June 15, 2025avidiax
One caveat of these dummy plugs is that they don't do HDCP. They handle the typical use case of forcing a specific resolution output for headless machines rather well, but fail for the use case that you need to run something that expects HDCP.
This seems a good place to ask: does anyone know of a good solution like this HDMI dummy plug, but that negotiates HDCP? I need to test video streaming apps that require HDCP to play at full resolution, but it is inconvenient to have a full TV for every test.
The one solution I've found is an HDMI multiviewer, which seems to negotiate HDCP to each port individually.
ndiddy
I use this HDMI splitter. It lets you either set a preprogrammed EDID, or learns an EDID from whatever you plug into HDMI output 1, and then shows up as a connected monitor for as long as the splitter's plugged in without having to connect anything to the outputs. I believe it negotiaties HDCP between the computer/console/whatever and the splitter, then sends the signal to the output monitor without HDCP. https://www.amazon.com/dp/B07VP37KMB
hedora
Aliexpress sells things that claim to terminate hdcp and forward hdmi. Caveat emptor.
kachapopopow
I find it crazy that the signal between our monitors and desktop computers is encrypted when these exist.
dcan
Terminating HDCP is difficult, you’d have to downgrade it to HDCP 1.4 and then have a 1.4 ‘compliant’ (see: device on the end for it to be a dummy monitor. If you need anything newer than HDCP 1.4, it’s likely not possible.
tverbeure
I did a tear down of this Monoprice dongle: https://tomverbeure.github.io/2023/11/26/Monoprice-Blackbird....
It terminates as an HDCP 2.0 endpoint and converts to HDCP 1.4. You’d still need an HDCP 1.4 sink to make it work though.
avidiax
I'm using the Monoprice multiviewer. It negotiates HDCP without a display attached. Other than being a bit big and expensive, and being unable to strip HDCP, it's a good solution.
I found the same device in generic packaging on AliExpress, but haven't had the chance to order that version, yet.
There are lots of professional SDI converters and such, but they are either $3k+ or "call for price".
kevin_b_er
That was written by you?
I don't agree with this section:
> The HDCP converter simply announces itself as a final video endpoint… yet still repeats the content to its output port. Without a very expensive HDMI protocol analyzer, we can’t check if the source is tagging the content as type 0 or type 1, but there is no reason now to think that it’s not type 1.
There's no magic in the HDMI protocol that says type 1 vs type 0. Its just another HDCP message over DDC, but it is only sent to repeaters. In this case, since the HDCP Repeater is lying about not being a repeater, it isn't getting sent the StreamID Type information.
Sporktacular
Great teardown. Can these things remove HDCP altogether? It seems like if it can report that the sink is HDCP2.x then it can do so even if it has no compliance at all right? So that would mean it streams an encrypted stream to something that needs to then still do the decryption? These devices seem like they'd be underpowered to do that in real time at 18 Gb/s.
amelius
I have a different usecase. I have an embedded system that sends out HDMI. However, its boot screen is something I want to replace by another HDMI stream (a static image would suffice). I specifically don't want to change anything on the embedded system for a thousand reasons I won't go into. How do I cheaply and robustly do this?
mschuster91
Try one of these HDMI splitters that are more or less openly advertised as "HDCP strippers" on Amazon.
crazysim
I think multiviewer might have been a synonym for that.
I wonder if the chips on these dummy plugs are powerful enough to hack in HDCP support though.
aappleby
The dummy plugs are literally just a 256-byte eeprom hooked to the I2C lines, there's nothing else inside the shell.
null
slipheen
Relatedly, is there a good archive of EDID binaries somewhere, or a better program to make them?
I have an nice programmable EDID emulator plug, and I can clone my monitor and others, but there's some times there's a specific resolution or feature I want to set and I don't have a way to. (Like 8K with DSC, etc)
I'm aware of https://github.com/bsdhw/EDID but it's rather limited in terms of modern monitors.
I've also written by own using https://www.analogway.com/products/aw-edid-editor but getting all the nuances of various modes, and setting preference order, etc, is rather difficult, at least for me :)
3abiton
I had a similar problem recently: I got this cheap 5.1 surround system soundbar that support up to Dolby TrueHD via HDMI. But here is the catch, it only works with eArc enabled devices (new gen TV). If you plug your PC you need to use SPIDF or aux which hamper the quality. One solution beside buying an audio ectractor/splitter is to fake the PC edid to be reconized as eARc by the soundbar. Still yet working on this, no strict guidelines existing unfortunately.
zdw
You can also buy these with a passthrough, which is useful for older systems that balk at higher resolution monitors.
I have a 2011 era AMD FX8350 system where the onboard 880G northbridge+video chipset doesn't output video over HDMI correctly with a 4k display. Hooking up one of these inline to tell it to send a 1080p image works great, which the monitor does a 2x integer upscale to 4k.
dougg3
I also have a couple of passthroughs -- I probably should have mentioned them in the post as another option. The one I have is fancy -- it can read the EDID from a monitor, save it, and use it as an override for another monitor.
Another awesome thing is it can force the monitor to always be detected. One of my monitors virtually unplugs itself when I shut it off, which causes a bunch of issues for me, and the passthrough completely solved it. The one I use is the HD-EWB by THWT.
gadiyar
Doug, thanks for mentioning this. I didn’t realize pass-through ones existed. I’ll check out the one you have. Nice article btw.
dougg3
Thank you! Hopefully it works for you.
I guess I should reword the way I said something in the previous message: when I said "it can force the monitor to always be detected", I really should have said "it forces the monitor to always be detected".
dishsoap
Fun tip: the same process works to modify the edid stored on a typical monitor or laptop screen. Sometimes you can even change various settings on the tcon by writing to other i2c addresses. You also don't need a raspberry pi, any computer works.
crtasm
The author recommends using a Pi while noting it's not a requirement
>If you attempt these commands on a PC, it’s possible that you could accidentally flash hardware that isn’t an EDID, like a RAM module’s SPD EEPROM.
dishsoap
True, although the i2c controller that the dimms are connected to is an entirely separate device from the i2c controller in the gpu that's connected to the display ports. As long as you know what you're doing the risk is not significant.
dougg3
Yeah, if you are 100% confident you're using your GPU's I2C controller it's probably fine, but the reason I warned about it repeatedly in the post was because I stumbled upon this GitHub issue where two people accidentally flashed their RAM SPD:
ajb
The flash chip usually has a write enable/disable pin and most monitors and TVs will wire it to prevent writes to the EDID. I would guess only cheap ones don't bother. It's risky as without protection, a voltage glitch during a read can turn it into a write and trash the flash.
Aurornis
> the same process works to modify the edid stored on a typical monitor
That would be a strange oversight by the hardware developers.
Typically they would buy pre-programmed EPPROMs and then place it on to a board where the write enable pin is never pulled high.
It would be strange to put an EEPROM into a product like a monitor and leave it writable, but I’ve seen stranger things on shipping hardware.
crote
Modern monitors don't even use an EEPROM chip for EDID anymore. The I2C bus is hooked up to a microcontroller inside the monitor, which allows it to implement Display Data Channel. This way you can tune things like display brightness and color profile from an application running on the computer, instead of messing around with the monitor's OSD.
Tools like ddcutil aren't very well-known, but they can be quite useful if you want to do something like DIYing a KVM switch by just having the PC tell the monitor to switch to a different input!
ajb
Yeah,it shouldn't happen - but I've seen it happen. What's worse, the first batch we got from that place weren't flashed with an EDID at all - and were shipped directly to customers (who mostly didn't notice, because the main product it connected to had default that worked, but it wasn't optimal. Meant none of those screens could be used with a normal laptop though). Ironically the combination of the two issues meant we could have fixed the EDID in the field, but we didn't dare in case we bricked someone's $x000 TV.
ashirviskas
Why are dummy plugs a thing? What can you do with them that you cannot do in software? (asking as a person who had no issues with having 18 virtual displays and no dummies).
ndiddy
One example: I use software called Looking Glass on my PC for interacting with a Windows virtual machine. I have two GPUs in my computer, an AMD one for the Linux host and an NVidia one that gets passed through to the Windows guest. Looking Glass then captures the NVidia GPU's output and displays it in a window on my desktop. This allows me to use Windows software in the VM and get acceptable performance (Windows has basically required graphics acceleration to run acceptably after 7). The problem is that the NVidia GPU will not do anything without having a display connected. NVidia Quadro GPUs support dumping a monitor's EDID and then mapping that file to an output (so the GPU always thinks that monitor is connected to that output), but their consumer-grade GPUs don't support this. That's where the dummy plug comes in.
SXX
A lot of OS / GPU / driver combinations dont actually let you setup virtual displays with arbitrary settings. And you might want it for setting streaming with OBS or games streamings via Steam / Parsec / etc.
Some years ago it's kind a worked for me on Linux with Xorg and open source drivers and Windows with Nvidia, but when it comes to MacOS or Windows+AMD or Intel GPU it simply doesn't work that well.
dd_xplore
I have a moded chromebox(booting windows and linux), which refuses to boot without any video device attached to hdmi port. So I had to use a dummy plug.
pfych
Dummy plugs are a lot easier for most people. I added a fake 4K monitor to my desktop via software for remote game streaming, and it was a lot more complicated than I expected[^1].
lyu07282
What gpu/driver were you using?
TheJoeMan
Raspberry Pi’s with remote desktop won’t render the desktop unless a monitor is physically plugged in… easiest solution for say a PhD student.
tshaddox
Presumably it’s for devices which do not have easily modifiable software.
01100011
Does anyone know of a cheap DisplayPort EDID emulator to fix issues with a KVM and linux? Last time I checked, they were much more expensive than HDMI, to the point where it would be better to buy a new KVM.
crote
The issue here is that DisplayPort doesn't use a basic EEPROM hooked up to an I2C bus for EDID. Instead it uses the high-speed DisplayPort-specific AUX bus, which is significantly more complicated to mess around with. Heck, I don't think you can even find any decent documentation about it without joining VESA and signing a bunch of NDAs.
awaymazdacx5
hex dump for the USB ibus2 plug extract is concatenated to EDID
Minor note for those wanting to try this at home - these cheap dummy plugs only have a 256 byte eeprom, which is not enough for storing the various extended EDID blocks needed to specify high-refresh high-resolution configs. If you just want 1080p60 they're fine, but you won't be able to simulate a 4k240 monitor with them.
Also, some of them have the write-protect line pulled high (or low? don't remember) and you'll need a bit of surgery to actually write to them.