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simple average isn't good enough. you need to be sensitive to a pixel changing at all (within some tolerance) over a certain time, because the station logo doesn't change at all, while the rest of the picture usually changes, but may not change much, or may be visible for a long time but not all the time (that recognizable frame/border within view)

look into the background subtraction/segmentation module within OpenCV. tweak the parameters.

if that doesn't help, you'll probably need to come up with your own approach.