Once upon a time desktop PCs had a 2D graphics card and a 3D graphics card. A pass through cable connected the 2D into the 3D card, which combined and outputted the two signals over layed. If I had, say, two outputs, both the composite signal for old analogue TV, can you just combine the two signals together and, as long as they don't draw over the same location, get the combined output? Timing is crucial, obviously, they both have to send the vblank at the same time. I guess in the desktop PC days they would read the timing off of the 2D signal and use that to time the 3D signal.

Can you just sum the two signals? Or will you have to attenuate by 50% first?

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    $\begingroup$ That's not how they worked so there is no answer - and this is really not a signal processing question (I get that video signal are signals and they are being processed but that's different). The video outputs of 2D video card and 3D accelerator were never overlaid on top of each other or combined in any way. The 3D card just can select if the video of 3D card output to monitor is taken from the video input connector generated by the 2D card or from the video generated onboard the 3D card. So do you want to know (a) how 3D cards worked or (b) how analog video is combined regardless of 3D cards $\endgroup$
    – Justme
    Aug 18 at 7:27
  • $\begingroup$ Yes, I was expecting to have got this all wrong! Thankyou for the assist, could you perhaps point me towards the correct Stack to all this question? I'm still hoping that I can sum the signals together in some way with just correct timing and some attenuation if required. or maybe just have a software switch between the two outputs. $\endgroup$
    – RoboJ1M
    Aug 18 at 15:59
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    $\begingroup$ Well, it's about retro video cards so retrocomputing.se could work for that. For the analog video merging, not sure, it could be electronics.se maybe. $\endgroup$
    – Justme
    Aug 18 at 16:12
  • $\begingroup$ I've asked in the electronics meta exchange first, hopefully that's the right thing to do, ask if it's the right place to ask. Many Thanks. 👍 $\endgroup$
    – RoboJ1M
    Aug 18 at 16:16


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