i geus it should be fast enough, i mean, why not? the frequenty is way below the avg frequentie right?
No it is not fast enough.
You have to look for a line sync pulse, then you have to delay by a precise amount, then you have to capture the pixel.
Lets suppose you want to capture a whole line.
First you wait for the frame sync pulse, then you have a short delay and then take 640 digital samples and put them somewhere. This requires quite a lot of memory, processing power and an external digitiser.
What you can do is sample one pixel per line in the same X position, this gives you 480 samples and a bit of time to store them somewhere. You still need an external flash A/D converter.
Way back in the 80's I made several TV video digitisers using this technique. With one column taking one frame to sample the 640 samples at 60 frames per second will take 10.6 seconds to read in a whole frame. Assuming you have external memory to put it in and some time to do the actual processing, your response time would be quite slow.