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?

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    \$\begingroup\$ 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. \$\endgroup\$
    – user39382
    Commented Jun 17, 2016 at 22:49
  • 1
    \$\begingroup\$ Is your colorburst frequency correct for your monitor? \$\endgroup\$ Commented Jun 17, 2016 at 23:42
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    \$\begingroup\$ 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. \$\endgroup\$
    – Mark
    Commented Jun 17, 2016 at 23:52
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    \$\begingroup\$ This is what an NTSC composite video signal should look like. That first, fast "blip" is the color information, which is almost completely drowned by noise. More info here. \$\endgroup\$
    – rdtsc
    Commented Jun 23, 2016 at 18:26
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    \$\begingroup\$ 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. \$\endgroup\$
    – FiddyOhm
    Commented Aug 15, 2016 at 10:14

1 Answer 1


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.


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