In my previous post, I had revised my converter to use the LM1881 dedicated sync separator. Results, unfortunately, were poor.
So, I went back to the data sheets for the AD724 RGB to NTSC/PAL Encoder and the LM1881 Video Sync Separator.
Poring over them revealed I’d benefit from a few more capacitors. I also switched from a 3.58mhz color burst oscillator to an oscillator that runs at 14.318mhz, or four times as fast. The AD724 can take either signal, so I thought I’d go with the higher one. With the lower frequency oscillator, the AD724 uses an internal phase-locked loop to generate a 14.318mhz clock. If you use the 14.318mhz oscillator, that PLL is bypassed.
I also added some jumpers to the red, green and blue inputs from the computer, in case I wanted to adjust the input impedance. I did the same to the composite output.
Here are the resulting schematic and PCB.
My first attempt at building this didn’t go well: it didn’t work at all. I’m not entirely sure what wasn’t working, so I built a second and took a little more time to test along the way.
Note: there was an iteration before this that used a 78M05 connected to the computer’s +15v to generate on-board 5v. Turns out that wasn’t necessary.
And here are the before and after results. I think this is about as good as I can get it. The signal is a little hot and has some ghosting, so adjusting monitor brightness and contrast is required.
