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I'm attempting to output video from a TMS9118 VDP to a television's composite video port. Right now the result I'm getting is grey scale (whereas the background should be green), and I have no idea why. I have compared the output from the TMS9118 comvid line to output from a DVD player on my oscilloscope, and the signals are somewhat different in quality.compared signals

The yellow signal is the dvd player. The blue signal is the VDP.

Following is what the VDP produces on the television screen:

enter image description here

The TV displays the DVD player signal perfectly. In summary, what is wrong with the signal coming out of the TMS9118 comvid line?

Adam T.
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  • Are you sure you mean composite video? It looks to me like component video. Composite video has one 75 ohm cable, while component video has three. – Mark Jun 17 '16 at 22:43
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    Something's very wrong here. There's no way you should be measuring a >100V signal out of the DVD player. More likely you have a grounding problem. –  Jun 17 '16 at 22:49
  • Mark, yes it is composite video. There's only one video output on the 9118. Other versions do support component, but not this one. – Adam T. Jun 17 '16 at 23:01
  • duskwuff, you are quite correct. I fixed the grounding and now the voltage is similar to the signal from the VDP. Also it looks a lot cleaner than the signal from the VDP. – Adam T. Jun 17 '16 at 23:08
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    Is your colorburst frequency correct for your monitor? – Chris Stratton Jun 17 '16 at 23:42
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    It is way too noisy. Notice on the DVD trace how the color burst is cleanly sitting on the porch. Both porches and the horizontal sync are clear and flat. Notice on the bottom trace how you can barely make them out in the noise. I'm surprised it displays at all, but can understand why the TV might not recognize the color burst. – Mark Jun 17 '16 at 23:52
  • @ChrisStratton Hmm I"m not sure. I thought the monitor was supposed to detect the colorburst and then interpret the signal appropriately. – Adam T. Jun 18 '16 at 00:04
  • @Mark Thanks. I'm gonna try this with a try this with an actual lab regulated power supply instead of my crappy noisy custom made junk. – Adam T. Jun 18 '16 at 00:06
  • @Mark Do you think a low pass filter might help reduce the noise on the signal coming out of the VDP? I replaced my custom power supply with a very clean bench supply with no resulting reduction in noise. – Adam T. Jun 20 '16 at 03:55
  • Adam, if you are running NTSC, then the color-burst is around 3.58MHz, so the low-pass filter would need to be near 4MHz. That may not help much. Do you have another TMS9118 chip? have you looked at power and ground at the chip? Could a pin on the TMS9118 be shorted? – Mark Jun 20 '16 at 18:22
  • @Mark thanks again for the great pointer. Avg. Vpk - Vpk is 1V at power and ground on the TMS9118. Is that pretty bad? – Adam T. Jun 20 '16 at 21:37
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    [This is what an NTSC composite video signal](https://www.youtube.com/watch?v=MhbC_YgKvNU) should look like. That first, fast "blip" is the color information, which is almost completely drowned by noise. [More info](http://www.eetimes.com/document.asp?doc_id=1272387) here. – rdtsc Jun 23 '16 at 18:26
  • Some progress, @Mark et. al. I tried completely disconnecting the RCA cable from the VDP and found that the signal became *significantly* cleaner, though I don't know if clean enough. Going to try a shorter cable next. – Adam T. Jun 26 '16 at 15:44
  • do you have the correct 75Ohm termination on your source end? – Segfault Aug 10 '16 at 14:48
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    Please include a schematic in your question. Show all components associated with the NTSC output and power supply (e.g. bypass capacitors). A photo of your BB would help too. – FiddyOhm Aug 15 '16 at 10:14

1 Answers1

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It would appear that you have significant ground current noise between the two systems or some other large CM noise.

The VDP scale is 1/4 of the DVD yellow scale to see that same size sync tip and chroma burst. The amplitude must be double amplitude for a 75Ohm source and load and the same as the DVD player.

The output level for chroma sync if too low or noisy would disable chroma video from being displayed in the VDT.

The VDT shows a LOT of pixel clock noise on the display.

Shields up and improve grounds.

Tony Stewart EE75
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