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Purely for hobby, I am working on an RGBHV (VGA) to YPbPr (Component) converter circuit for old games consoles modified to output RGBHV on a VGA cable. From the game consoles, the RGB signal is taken from before the composite encoder chip and the HV sync signals are generated by a TI LM1881 chip being fed with composite video from after the composite encoder chip. This creates valid 15khz VGA that works with my VGA capture card for high quality video capture.

Now, the converter I am working on needs to take this VGA signal and convert it to YPbPr to display on a CRT TV. In my inexperienced attempt at scrapping this circuit together, I have used two LT6550 chips and Linear's application note. Here is the circuit: Exported circuit with annotations (Import & Simulate in http://www.falstad.com/circuit/). I have also attached a picture. The top part (RGB->YPbPr) is pretty much verbatim from the application note, the only thing I have added was the bottom part for adding the sync information to Y.

My two questions are:

What are the waveforms supposed to look like for YPbPr? I can find heaps of information on google about composite video, VGA, and some about the Y component and understand what those waveforms are supposed to look like on-the-wire in an actual application, but PbPr information seems scarce as far as actual oscilloscope style displays go, even though I know Pb is 0.565(B-Y) and Pr is 0.713(R-Y). In a month or two I will be able to look at the waveform with an osc. myself.

Are there any glaring problems with the circuit? Not knowing what the waveform is like, I find it odd that Y is -300mV sync 0mV to 700mV signaling but Pr and Pb are -350mV to 350mV signaling. Does this mean I need to AC couple Pr and Pb? Or do I need to level shift Pr and Pb up by 350mV? Or are these voltages valid? I prefer DC coupling wherever possible, I have had crappy black levels with AC coupling in the past somewhat mitigated by larger capacitorsScreenshot of circuit in falstad circuit simulator.

2nd screenshot with some displays turned off for more clarity

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  • \$\begingroup\$ A real circuit might help some folk read it. At the moment it is incoherent to my eyes. \$\endgroup\$ – Andy aka Oct 21 '13 at 22:25
  • \$\begingroup\$ I have added another picture for clarity. Please note that in some browsers you might have to right click and view image or right click and open image in tab to see it in more detail. I can model it in Eagle as well if you like. \$\endgroup\$ – Lt.Derpington Oct 21 '13 at 22:53
  • \$\begingroup\$ From what I read in "Digital Video and DSP: Instant Access" it looks like the voltage levels are correct, there is allowed to be a DC bias of +-1V anyhow and things are typically AC coupled in the receiving end. My only remaining question is if the sync adding circuit at the bottom I came up with is an OK way of doing it. The simulated results seem fine... \$\endgroup\$ – Lt.Derpington Oct 22 '13 at 5:41
  • \$\begingroup\$ I am changing the circuit to use a TTL-level composite sync coming out of the LM1881 instead of NAND H and V syncs since I don't think that will work. \$\endgroup\$ – Lt.Derpington Oct 22 '13 at 21:02
  • \$\begingroup\$ Did you ever get this to work with csync? I am trying to adapt this to the signal coming out of an old gaming console, which has a csync signal of 1Vpp. \$\endgroup\$ – jaredkwright Jan 18 '17 at 22:47
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The negative sync tip is easily done using AC coupling using an active clamp circuit during the sync positive edge with a 1 shot to short the output during the back porch. Although your is DC coupled and relies on RGB being at 0V to make Sync Tip with R ratios, it shud be ok.

You can make the video either 0.7V or 1.0V with negative sync at -0.3V. I prefer 1V for video.

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