Op Amp Contest: Go down an octave, no FFT, no PLL, no oscillator!

We love a project that gets us thinking, and that was certainly the case with [MS-BOSS]'s Octave Downshifter which is an entry in our current op-amp competition. Instead of resorting to an FFT or PLL, it uses a technique best described as a custom analog computer to implement the octave downshift calculations. It's an extremely clever approach, and we admit it took us more than one reading to figure out how it works.

As you would with any math problem, it divides the task of halving the frequency into its constituent math functions. The square root computing circuit is probably the one that required us to dredge up the most dredged up undergrad courses on vaguely remembered analog circuits.

The result is a fascinating read that's well worth taking the time to understand if you're interested in analog electronics. It's by no means the easiest way to create this particular effect in 2023, as we're much more accustomed to seeing our community create digital effects, but if you imagine yourself as an op-amp designer, you really have to give it a look.