Frequency monitor and switcher... How to?

[malc-c]

Plenty of VGA to CGA converters on the market

http://cgi.ebay.co.uk/Arcade-VGA-to-...b3b0a3ff442aa2

for example

[xnihilo]

Yes, sure, but for what price.
I could use a LM1881N to get sync from composite video but I'm not sure RGB out and composite video are enabled at the same time on the VGA card. moreover, the level will still be 0.7VPP