I have two RGB video outputs that I want to combine simply by merging the signals, is the EE forum the correct place to ask about this?

The pinouts of the RGB connector is:

  1. Red
  2. Green
  3. Blue
  4. Sync
  5. 0V
  6. +5V

I've already asked in Signal Processing and I've been told that was the wrong place to ask.

  • \$\begingroup\$ RoboJ1M - Hi, Please stop adding the technical details here. I have reversed your last edit to remove them. Meta is the place to ask about the site only e.g. about site policies. So it was correct for you to ask here whether the question would be on-topic on the main site. However it is misusing the site to actually ask the technical question here. Please ask the technical question on the main site instead. Thanks. \$\endgroup\$
    – SamGibson Mod
    Commented Aug 24, 2023 at 17:34
  • 1
    \$\begingroup\$ Sorry, I was just trying to get the question correct before asking it. I'll write on the main site next. Thanks. \$\endgroup\$
    – RoboJ1M
    Commented Aug 27, 2023 at 19:34

4 Answers 4



Yes, the topic is on-topic here. You will have to add a lot more info, though!

The full picture


Yep, asking about how to build something that combines electrical signals is on-topic.

Problems with Lack of Information

You will need to give a much more in-depth characteristic of your signals – after all, this is analog video, and that doesn't say anything about the number of rows you send, or how wide they; whether both signal sources use the same resolution, row and frame rate, length of sync periods… There's a lot that you leave undefined, and the first dozen comments will be trying to clarify all these points (if you're not getting closed lacking necessary detail), or you find someone with much time and they write a lengthy answer on what you need to recover from both signals, and how 1990's video signal processors worked, and why they had to be so complex in the absence of upfront knowledge about the signal.

Though I do recommend making very clear what the technical question is: Because what you're asking here ("I want to combine by simply merging") is plain impossible (as pointed out in the comments over at DSP.SE), because as you rightfully pointed out, you would first have to match timing.

So, if you mean to ask how to build such a complete signal combiner, you need to ask how to do that, and list your considerations so far¹ for the whole system are.

If you are really just asking about how to sum two signals, which you already assume to be synchronous, giving you a useful output signal, then do please say that².

Other considerations

It usually is much easier for experts in the field to answer a specific question and then to generalize from that answer for a beginner, than the other way around. So, even a small sentence like "I want to combine the video from the VGA port of my Segantendo XGameStation with the output from the SCART port of my PAL VCR; the output should go to the VGA capture card in my PC" will help them a lot giving you a better starting point.


¹ you mention aligning vsync, but that's not sufficient; also you assumed in the question over at DSP.SE that you could modify one signal source to match the timing of the other, and while I doubt you can, that'd be different for a signal combiner, as that needs to just take the signals you have

² and please make sure you mention what the desired output signal properties are. Bonus points, probably, if the readers of the question can understand how you keep the frames and lines in sync, because, hinthint, that's going to be the hard part and everyone is going to ask you about that, not to waste your time describing a solution to you that won't work in practice at all.

  • \$\begingroup\$ Wow! Thankyou! OK, I'll bounce a few details off you here if I may, I have very detailed manuals and reference guides and they are all online as PDFs too. First off, the device is an Acorn BBC Master microcomputer. Here are an outline of the chips that generate the video signal from the reference manual: \$\endgroup\$
    – RoboJ1M
    Commented Aug 24, 2023 at 16:42
  • \$\begingroup\$ Screen Output Three chips are primarily responsible for providing the screen output:- a) Acorn VIDPROC ULA chip b) 6845 Cathode Ray Tube Controller c) Acorn CHROMA MSI video matrixing chip The video processor takes a byte-wide data stream from memory, serialises it according to the screen mode in use, passes it through a palette to provide logical to physical colour transformation and on to the RGB outputs. From here the video data is buffered for connection to an RGB monitor and mixed for use with the composite video and colour television outputs. \$\endgroup\$
    – RoboJ1M
    Commented Aug 24, 2023 at 16:42
  • \$\begingroup\$ And this should be a link to an image of more detailed information: 1drv.ms/i/s!AqS0hawrZ6-Ogoc7OALpRStUBNceaw?e=CAs9Hn \$\endgroup\$
    – RoboJ1M
    Commented Aug 24, 2023 at 16:43
  • \$\begingroup\$ Is that useful and should I add that to the question? \$\endgroup\$
    – RoboJ1M
    Commented Aug 24, 2023 at 16:44
  • 1
    \$\begingroup\$ anything you've been considering can and probably should be part of such a question \$\endgroup\$ Commented Aug 24, 2023 at 16:50
  • \$\begingroup\$ Thanks, ok, last question: What's a genlock and is it useful for what I'm attempting? \$\endgroup\$
    – RoboJ1M
    Commented Aug 24, 2023 at 16:58
  • 2
    \$\begingroup\$ technical question, please don't ask that here in meta \$\endgroup\$ Commented Aug 24, 2023 at 17:30

If you're looking for a product to solve your problem, that would be off topic here.


Adding to other input:

This would be a good question on SE EE if properly put.
It it's framed as a buying question it will be closed.

It needs to be mainly a design question.
eg maybe "How would I go about making a device which xxx. .... . Aside: Do existing devices that do this exist in the real world."

You need to define sources and output as well as possible.
Are all identical in technical detail? Are you able to frame and line sync both sources?
How should combination work? (average per pixel, stronger dominates, XOR, add, ...)

If you cannot fully sync sources you could build a frame buffer per input and output the desired combination of the two buffer at a given time. ie it's doable, but not overly trivial.

I look forward to the question :-)


It should be on-topic, however the video signals cannot simply be "merged." VGA is a pretty-deep rabbit-hole, involving horizontal and vertical sync pulses, luminance data for each scan-line, colorburst data, front and back-porch, etc. There are also several close cousins between composite, VGA, component, S-Video, etc. Combining two signals is complex because the timing of these are almost certainly never in sync, and even if they were, then only certain aspects of the luma and color data may be combined to keep the signal within specifications. Signals that violate specifications even a little bit tend to cause the video to drop-out altogether.

To get analog video in sync, a "genlock" is used. This isn't a complete solution however as it just allows switching between the two signals, not mixing them at all. If you don't want to redesign the stone wheel (then wooden wheel, then steel wheel, then rubber tire...) consider finding a video mixer that accepts two of your video input types. Some vendors sell these as standalone units (like Newegg), but they are generally quite expensive due to their complexity.

  • 3
    \$\begingroup\$ Meta is the wrong place to answer the yet to be posted question \$\endgroup\$ Commented Aug 20, 2023 at 0:25

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