Abstract: A surround sound processor for presentation of a stereophonic source program on a multiple loudspeaker array surrounding the listening area. The processor includes input signal conditioning and matrixing circuits, a control voltage generator responsive to the directional information contained in the stereophonic source signals, and a variable matrixing circuit for generating appropriate loudspeaker feed signals to create the illusion of the sound field wrapping around the listening area. The control voltage generator includes a servologic circuit employing variable time constant smoothing circuits each responsive to the difference signal between its input and its output and to a pulse signal from a one-shot triggered by threshold detectors which compare the difference signals with fixed threshold voltages. In one aspect, suitable for incorporation into an integrated circuit, a multiplier circuit and a controlled bidirectional current source charging and discharging a capacitor provides the smoothing function.