Camera switch with ov7670 to output standard A/V signal - composite
An arduino uno is way too slow to output composite video. If you want to generate a PAL video signal you can see here the signal specification.

Most important, one line is output in 52us. If the line is 720px long (720x576 is the common resolution), then you will have to generate one pixel in 72ns. This means that


you will need a very fast DAC (a flash one)
you will need a VERY fast microcontroller. If you have a 160MHz uC you will have to send one output byte every 11 clock cycles, which means that you will barely be able to do any more processing.


Result: it is practically unfeasable. I suggest you, if you want to use it, to look for hardware encoders. But... Probably you will need some faster processing units: maybe a raspberry PI, which already have an hardware encoder...