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. Hendrix used an octave-fuzz pedal known as the octavia.