This circuit is a octave shifting that is used for electric guitar is done by rectifying the original signal, just like AC to DC conversion inside your AC-DC power supply adapter. This circuit is use single supply instead of symmetric power supply. This is the figure of the circuit.
The rectifying is done by four 1N4148 silicon diodes, configured as full-wave rectifier bridges. Because the bridge is inserted inside the negative feedback of the operational amplifier (op-amp) U1B, the nonlinear characteristic of the diodes around the turn-on point (the forward bias voltage) is compensated by the op-amp’s feedback mechanism. As the result, the output of the rectifier looks like coming from ideal diode with no bias voltage needed. [...]
Source: http://www.zimbio.com/Power+Supply+Circuits/articles/xhIg6f2AwK0
/Frequency+Doubler+Effect+Electric+Guitar