My goal is to get the video signal and a produce a fade off using arduino. I am reading the signal using analogRead and writing it with analogWrite
You can't read 5MHz+ bandwidth composite video with an analogRead on any Arduino, and even if you're on a Due with true analogue output, you can't write video either for pretty much the same reason.
You need to separate the positive video signal from the negative-going syncs do the fade on the video, but leave the syncs intact.
This is not a job for any Arduino.