Polyphase channelizers. I got interested in software defined radios, which lead me to getting a HackRF one, and that lead to learning how to build SDRs, and that lead to joining a company that was building cutting edge SDRs for really diverse tasks, and that lead me to diving into DSP and the mathematics of radios, which lead me into modern protocols and modulation schemes which lead me to various people doing experimental RF work and started noodling on what an SPU (Spectrum Processing Unit analog to a GPU) might look like and that lead me to Dr. Fred Harris' work on polyphase filters and channelizers and now I'm internalizing all of that so that I can build a device that processes spectrum in new and novel ways.