I have a circuit in which a fixed voltage is passed via a potentiometer connected between +9v and 0v, is then buffered and determines the start frequency of a voltage controlled filter.

I want to replace the connection to +9v with a voltage proportional to the strength of the input signal, so that instead of the sweep ramping up to a point fixed by the potentiometer, it will ramp up to a point determined by how hard a note is played on the guitar.

The reason for this is that the decay portion of the sweep is smooth and ripple free, as it is independent of any signal from the guitar note fading away.

Any ideas? The voltage extracted may need to be just a short pulse, or perhaps some sort of sample and hold circuit would be needed???