NardJ:
Thx for your replies!Any suggestions which don't need many extra components? (A few resistors/diodes is ok.)
The generation of a PAL/NTSC video signal requires a specific relationship between the phase of the colour sub carrier and the frame/line synchronising signals (measured in nanoseconds).
It is hopeless to do that with a micro controller.
Custom chips deal with all that for you if they are still around.
As it seems you have little understanding of this i would suggest that a board similar to the link i gave would be the best solution.
Chip solution possibly.
http://uk.farnell.com/analog-devices/ad723aruz/video-encoder-rgb-ntsc-pal-tssop/dp/2312995
Decoders are much more available encoders it seems these days.
Buying a custom shield i would suggest would be a faster and cheaper option.