The term Duophonic describes a synthesizer's ability to produce two musical notes simultaneously. Historically, this technology emerged as a middle ground between monophonic synthesizers (one note at a time) and polyphonic ones (multiple notes). In classic synthesizers like the ARP Odyssey, the system works by detecting the lowest and highest keys pressed at the same time and sending a different control voltage to each of the machine's two oscillators. It is important to note that, although two notes are sounding, most duophonic synthesizers operate in a "paraphonic" regime, meaning the two notes share the same filter and amplitude envelope, lacking complete independent articulation as found in a true polyphonic synth.