Thanks Zebidee. I took a meter to it, and discovered the SCART plug wasn't grounded properly. Opening it up, the metal shield wasn't connected to pin 21, I guess it had jiggled out in transit. Slotting that pin back into place solved the crazy bad interference I was getting.
While open, I spotted none of the R G B grounds were connected either, and the chosen cable didn't have enough conductors to support them. Checked the VGA end and they were empty on the PCB too.
The original design says some connections are optional, but since I was still seeing and hearing interference, I figured I should just replace the whole SCART cable part wirh a fully wired one and see what happens.
That is good news!
You only need one ground from the VGA - it depends on your cable, but typically it will be connected to pin 10 or 5.
Pins 6,7,8 are often not used, but usually connected/soldered to ground at the video card and TV ends anyway.
For making a VGA-SCART video cable, you need just six wires from VGA. RGB (1,2,3) + syncH&V (13,14) + GND (5,6,7,8,10). Just twist all the available VGA grounds (video+sync, not shield) together and run them into one of the SCART RGB ground pins.
Make sure you don't have any other random wires from VGA connected to SCART ground, here I'm talking mostly about the monitor ID/data pins (VGA 11,12,15) or the 5v pin (VGA 9). The wires can be any colour, so don't assume, check with DMM.
You'll also need to link the syncs together for SCART. Exactly how, and what your cable does with sync, is another issue we can get to later.
4 hours and some sorely tested patience later, I've done that, along with belt-and-braces joining all the video grounds. The video noise was still present, albeit reduced further. Audio noise, no difference.
I've done a bunch more messing around, not sure if its video card or mains socket but fingers crossed I've eliminated all video noise. Audio is there but the level is pretty low - low enough at normal volume for it not to be audible anyway. Might try your suggestion although I would need to inject the audio into the SCART plug as none of my TVs have a separate input - should I just make sure audio and video grounds are separated in the cable and see if that makes a difference?
Separating the video/audio grounding points may make a notable difference. Even with all the grounds connected back at TV/PC, the electrons still seek the shortest path of least resistance. If you can give the audio a ground return that is a little more direct, it will take it, and interfere less with your video ground return.
Video and audio signals operate at very different frequency levels (Mhz vs Khz) and there is usually some frequency filtering, so providing better ground returns may help.
Having said this, you will probably still get some interference, a kind of variable hum, sometimes sounds like a Jedi light sabre, with running the game audio output through the TV. In my experience, the hum varies mostly with screen brightness, a strong indication of voltage regulation issues. There are some very powerful voltages going around in TVs, and their voltage regulation and noise filtering can only do so much.
This is why, whenever I use a TV for a cab, I usually prefer a separate audio amplifier with its own power supply. Effectively, this separates the video and audio systems. The TV speakers can be re-used for $zero if you want, or get some new ones (speaker are where prices can be whatever you are willing to pay). There are some some decent & inexpensive audio board amps available with a range of features, choose according to your taste (prices vary with features, but around $5 to $30, check your marketplace). This also makes it easier to control things like volume, bass/treble etc.
In my current Aussie lowboy build I'm using a ZX-502MT (cost <$10) like this (clicking on pic will take you to the thread discussion):
Finally, installing a ground loop isolator for the audio inputs may help. There are many simple board-type audio isolators available under $10, so worth a try. They come with a range of different input/output options for attaching audio cables, from RCA connectors to screw terminals to 3.5mm and combos. Search your favourite online marketplace. For example:
Audio ground loop isolator
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Topic: The ultimate VGA to SCART adapter (Read 39838 times)
