[Coco] GBS-8200 RGB to VGA

[Barry Nelson]

Another thought that might stabilize the sync from the composite video line…

At 15khz a .1 uf capacitor has an effective impedance of 106 ohms but at the color burst clock frequency of 3.58 mhz, the same capacitor has an impedance of 0.444 ohms. Since the CoCo 3's composite video output circuit has a 10 ohm resistor on the collector of the output transistor and a 120 ohm resistor on the base, putting a .1 uf capacitor from the video output to ground should effectively ground out the high frequency color burst signal and part of the black and white video, but should leave the horizontal and vertical sync pulses virtually untouched. This should eliminate the jitter on the 80 column screen. If the capacitor doesn't do it by itself you could a small 10 ohm in series. This has the advantage of providing sync as well as a proper black level reference signal.