Hi,
I'd like to output an RGB video signal on an old CRT TV. The resolution would be at least 160x200, 320x200 if possible. So I need to output three analog values, with value changing at approximately 5MHz but they need fast transitions (e.g. if I use PWM, I guess I need at least 4 PWM periods in one pixel).
I could use dedicated video DAC, the problem is that they seems to be harder and harder to source and since this is for an open source project that I hope may live for a few years and the price is apparently higher than the fpga I need (probably a gw1n 1k)... Is there any interesting chip I missed ?
I have seen several people using r-2r ladders for this, but they only use 3 to 5 bits per components, I need 8. Can I use an r/2r or r ladder with trimpots on the higher bit to adjust the DAC, or will other factors (such a temperature drift ?) make it a bad idea ? (I won't mass produce so calibrating each piece once is acceptable)
Should I build a PWM or PDM DAC inside the FPGA ? (in PAL, with 52us per line, 320 columns, 256 possible values and 4 PWM cycles per pixel, that would mean 6gHz clock... probably cheaper to save on the FPGA and use a DAC, but maybe PDM would be doable ?)
Another solution I can think of is a 10 bits r/2r ladder, and and hardcoded calibration table to compensate for the resistors variation... any better solution ?
Thanks,
J.