Octave effect
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Octave-effect boxes are a type of special effects unit which mix the input signal with a synthesised signal whose musical tone is an octave lower or higher than the original. The synthesised octave signal is derived from the original input signal by halving (octave-down) or doubling (octave-up) the frequency. This is possible due to the simple two-to-one relationship between the frequencies of musical notes which are separated by an octave. One of the first popular musicians to employ the octave effect was Jimi Hendrix, who also used a variety of other effects in his recordings and public performances.
Octave-up effects usually use full wave rectification using diodes to "fold up" the negative part of the waveform to make a new waveform an octave higher in pitch.
Octave-down effects are typically produced by converting the signal to a square wave, and then using flip-flop circuits to divide the frequency by two. This creates a buzzy synthesizer like tone. The MXR Blue Box used this method to create a Two Octave drop (expanded to include One Octave down in later re-issues).
The Boss OC-2 unit generates One and Two octave down tones. This is achieved by feeding the signal into a Flip-Flop, but, having the output of the flip flop changing the volume of the output signal, turning on every 2nd or 4th cycle of the waveform. This creates a tone similar to the input waveform.
