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I'm trying to generate PAL tv signals using a PIC24FJ64GB002 using a similar method to the circuit described half way down this page. The only difference is that my circuit needs to run at 3v instead of 5v due to using a PIC24 so I've recalculated the necessary resistor values and the closest ones I can find seem to be 220 and 470 ohms.

I'm connecting to the TV input via a scart lead connecting the video signal to pins 17 and 20 on the diagram on this page.

But I get no picture at all on the TV. It might as well not be connected.

I'm thinking one of three things could be wrong :-

  1. My timing is out. I've posted the diagram at the end of this post showing the timing on the two pins for my test and as far as I can tell I'm generating the correct sync timing and the other signal is just some stripes for testing. How accurate does the timing need to be? I'm using the timer interrupt on the PIC to start each line 64uS apart.
  2. I'm using the wrong resistors to mix the signal. Can someone check if I calculated the correct values to use at 3volts? How accurate do the voltage levels need to be? I chose the closest standard values, is this accurate enough?
  3. I'm connecting to the wrong inputs on my scart lead? Everything I've read shows this as being correct though...

I am generating what I believe to be the correct frame sync pulses too. Although again, everything I've read seems to indicate that even if I get this wrong I should see something and as every line is the same loss of vertical sync shouldn't matter for this test?

Anyway, any input and suggestions would be welcome.

Signal timing http://this.domain.name/wp-content/uploads/2010/06/tv.png

Edit and additional question

If I want to use RGB signals on a scart it looks like I need to have 3 connections for R, G, B and one for the sync on the composite video pin. And then each one has it's own separate GND connection... Is there any reason I can't just connect those 4 together?

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  • \$\begingroup\$ you got your +1 just for using the logic analyzer feature of the PICKIT2. I have never seen it used as everywhere I work we have O-Scopes everywhere. It looks sexy. the questions was great also. \$\endgroup\$ – Kortuk Jun 18 '10 at 16:48
  • \$\begingroup\$ I'd love a proper oscilloscope then I could see the voltages on the "mixed" signals here. But I don't have one and they cost more than I can justify spending for a hobby project. The pickit2 logic analyzer seems to work very well indeed though for many digital signals. I'd almost buy one just for that! \$\endgroup\$ – John Burton Jun 18 '10 at 17:37
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    \$\begingroup\$ I've tried the suggestions below and no had success at all. Are my timings and resistors correct? I have a pic16something somewhere too. I think I'll build one of the projects I've seen around that use one of those then at least i won't be trying to debug my own hardware, my own software AND an unknown connection to the TV all at the same time. And when that works restart this project. \$\endgroup\$ – John Burton Jun 18 '10 at 17:46
  • \$\begingroup\$ I cannot say I know better than you do, but you have peaked my interest. \$\endgroup\$ – Kortuk Jun 18 '10 at 18:41
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It could be a SCART switching issue. With SCART, some TVs need a low signal on pin 16 to select RGB rather than composite.

http://freespace.virgin.net/matt.waite/resource/av/scart.htm#Note%202%20:

You could also try putting your composite signal into one of the RGB channels to see if it's picked up properly.

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  • \$\begingroup\$ Thanks for the reply. It looks like you are generating RGB signals though, not composite video? I tried connecting that pin both ways though and unfortunatly it made no difference. \$\endgroup\$ – John Burton Jun 18 '10 at 14:25
  • \$\begingroup\$ Oops, I misread, updated. \$\endgroup\$ – Toby Jaffey Jun 18 '10 at 14:28
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    \$\begingroup\$ Yeah, doing a PAL colourburst on a PIC would be a "challenge" \$\endgroup\$ – Toby Jaffey Jun 18 '10 at 14:37
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    \$\begingroup\$ Just do finish up this post - I tried using a different TV and it worked perfectly and yet I still can't get it to work on the original one. Clearly some difference there I need to investigate some time in the future, \$\endgroup\$ – John Burton Jun 21 '10 at 18:08
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    \$\begingroup\$ I found in the end that I had a badly wired scart lead with the ground wired to the wrong lead even though I checked it about 100 times! For some reason is worked on one TV but not another. I now have it working well on both. \$\endgroup\$ – John Burton Jun 29 '10 at 7:05
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Did you turn it to channel 3? or do you have a composite input and need to select the correct video source setting on the TV/Monitor?

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  • \$\begingroup\$ I have a composite input on the TV. Well, a composite input on the SCART connection anyway. \$\endgroup\$ – John Burton Jul 1 '10 at 8:06

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