External 15khz to 31khz video converter?

[humppa]

It will work, but picture quality using Composite will be very poor. You might want to try the S-Video input since it has better signal quality, but you will need to convert the RGB-signal for that purpose (only CD32 has S-Video). The cheapest solution would be to buy a Genlock on Ebay that has S-Video output (Genlocks are very cheap nowadays).

The best, but somewhat more expensive solution is the CM-345S (aka "Presenter 1"). It has great signal quality since it accepts RGB-signal over a SCART connector. Much better than AverTV and other cheap Composite/S-Video "TV" boxes. There is a lengthy thread here and several photos and screenshots in the Image section (search for "CM345S").

I actually tried one of those cheap VGA/TV-boxes first, but returned it immediately. The image was not only extremely fuzzy with colour bleeding (typical Composite), but the picture was also jumpy and not stable.

[humppa]

I don't have to use a composite out from the computer for my signal though. It supports VGA in.

The VGA on these boxes is only used for pass-through. You could for example connect a Voodoo to VGA pass-through and Amiga-AGA to Composite.

Well, that and the VGA in would need to have the ability to recognize a 15khz signal in.

It doesn't. The only way to keep the original RGB signal quality is to use a real scandoubler or something like the CM-345S. The latter is also a flickerfixer by the way and besides RGB/SCART, you can also connect a Composite device to it. Using remote control, I can select between Amiga AGA, Amiga Voodoo and C64 - all on a 21" CRT.