Electronic – Microcontroller with Video input preferably composite

Ok, I can disregard the sanity of this project. It is insane, by the way, with little or no practical value beyond "doing it"-- but that never stopped me before and it shouldn't stop you.

In my opinion, you need several things: A CPU, an FPGA, some SDRAM, and a USB interface. Cypress makes the EX USB FX3 that does 2 of the 4 things. It has an ARM 9 CPU and USB 3.0 interface. What it doesn't have is an SDRAM interface-- which is why you want an FPGA. The FPGA does both the SDRAM interface as well as the high-speed data interface to the USB3 logic.

What you absolutely do NOT want is for the CPU to do the bulk of the data transfer! A 100 MHz ARM 9 isn't going to be able to handle that kind of data throughput. For that matter, just about any 2 GHz CPU isn't going to be able to handle that amount of data either. The ARM9 is just there for the housekeeping, while the FPGA does the heavy-lifting.

The ideal FPGA for this project is a Xilinx Spartan-6. Relatively low-cost, easy to program, and has built-in hard cores for the DDR2 SDRAM interface.

I've done something similar with a PIC12F675 to switch (using a spare radio channel) 4 composite video signals to send to a wireless video transmitter (it was a car, not a quadcopter, but this makes no difference). But I've not used relays (altough they should work too) but a Maxim 4xSPST high bandwidth switch IC. It was the IH5352 if I recall correctly.