Green Ringer (Musitronics Version)

“If a single note is played into the Ringer it will
produce a second tone — one octave above the original note. If two
notes are played — depending on the musical interval between the notes the additional tones produced will be harmonically
related to the original notes (such as sub-octaves) or dissonant. The
Ringer gives the musician much better control over the effect produced
than a conventional ring modulator since he is no longer playing
against an arbitrary modulator frequency.”