The Octavia was a guitar effects pedal designed for Jimi Hendrix by sound technician Roger Mayer. It utilized an analog circuit comprised of a frequency doubler, envelope generator, and amplitude modulator, together with additional frequency-shaping filter circuitry. The effect generated based on the incoming signal/performance. For example, some notes would produce a signal an octave up from the input guitar signal, while others would include ring-modulated overtones (as heard on the guitar solo in “Who Knows” by Hendrix).
Today, Octavia-style effects are manufactured and modeled by a variety of companies.