As for why this doesn’t work to directly mix signals, here is a diagram showing the structure of a single scanline of composite video. This signal includes sync pulses, a color reference signal, and encodes the color information with a high-frequency subcarrier. So even if your input sources were perfectly in sync, summing them together would probably wind up garbling the signal, and doing any kind of processing of the signal like attenuation or mixing with other waveforms could interfere with the sync. Modules with input decoders (Visual Cortex, TBC2, Vidiot) can decode broadcast video signals (sometimes only luma from a composite source) to a patchable video signal.
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