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I'm stuck with this project (Patreon)

Published:
2019-10-05 20:21:17
Imported:
2026-11

Content

You remember the two player version of the console I mentioned ages ago in the last video?
I'm stuck... It feels like back then when I said on video color Pal wouldn't be possible.
It feels really cursed since I thought It'll be easy going porting it to the external resistor ladder DAC. What happens now is that the colors are wrong and it looks like the resistor ladder acts differently compared to the internal DAC.
If that is really true I can basically trash the project and post a video how it failed. But I'm not really able to grasp what the actual cause is. When the signal is connected to the tv the signal voltage drops by a certain amount. That doesn't happen to the internal dac. Compensating for that drop does not solve the problem of the hue not working.
Any suggestions?

The last thing that I will try is to store the complete I2S signal buffer contents to SD of the old code for the internal DAC and fill the buffer in the new code to be sure it's no silly mistake.

Files

Comments

ElectronAsh

Are you able to set up a DMA to output from the internal DAC at the same time, so you can directly compare? Maybe there's a phase shift of the colour burst that's happening? I tried to generate Composite (and Chroma for S-Vid) a while ago on a small CPLD, for a Game Gear TV-out board, using a resistor DAC, and never got it to work. lol

ElectronAsh

As you say, even a slight voltage drop probably isn't going to cause the colour to shift that much. So I'd guess it's probably a phase issue. Maybe try shifting the colour burst one or two samples either way, and see how the colours change.

ElectronAsh

Oh, how is the data being transferred to the output pins for the external DAC? Is it using only one instruction to set a group of pins at once? Perhaps there's a delay there for updating each pin state, and that's enough of a phase shift to upset the colour burst / colours.

ElectronAsh

You're probably using a lot of extra clock cycles just for the colour modulation, too. Maybe you could just have some external latches to do RGB instead? Then convert that to Composite with an external chip, a bit like a combination of the Euzebox, and the NTSC version, which uses a colour encoder chip... https://www.hwhardsoft.de/english/projects/euzebox/

Frank Hellmann

With the internal DAC the expected signal impedance of 75Ohm may not mater that much, but might do with your resistor ladder. Timing issues could also cause that. Does the color shift vary if you change the write order of the output pins?

Alexander Gräf

Are you buffering the signal through a video op amp after the resistor ladder?

bitluni

No op amp, timing should be ok since grey scale works and line is 64µs. It's not a simple phase shift since the hue creates a pattern when I draw a solid color block. It could really be the signal impedance. I try to draw the fixed buffer first and then try to compensate for the impedance...

123

Your cable adds capacitance and resistance to the line, so you definitively have phase shift due to different time constant. The impedance of the line changes, you'll get a bunch of miraculously created stationary and reflected waves...