They probably use a LM1881 ( http://www.national.com/pf/LM/LM1881.html ) composite sync seperator chip to track which video line they are on. Then they probably take timed ADC samples on a block of lines and average them. The values are probably then stored in a 2d array which is then used to be compared with the next frames.
This is not necessarily a difficult process or project other then the fact that you probably have to set a PIC ADC to the fastest conversion time, which might be fast enough for what you need.
You will most likely have to use ASM or maybe a very very very efficent c compiler. You will also have to use some really really tight loops to track video lines and read adc values.
If it was me, I would have prolly used some sort of faster processor like a SX. This would allow better timing for video processing.
I was actually going to try this once, but I never got around to placing an order for a LM1881 (I'm just a poor high school student).
Harrison
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