1. A method of monitoring an operating state of an engine, comprising the steps of: measuring data from a cylinder of the engine during the cylinder's ignition cycle by sampling data during equal time periods of a predefined time window of the cylinder's ignition cycle, said data being indicative of the operating state of the engine; processing said data according to a distributed discrete Fourier transform that transforms only preselected frequencies during the cylinder's ignition cycle so that the transform is completed prior to the ignition cycle of another cylinder of the engine; and monitoring the operating state of the engine by processing said discrete Fourier transform.