train in output from a neuron with or without the use of neuronal spontaneous activity. The ISI variations were caused by an input sinusoidal signal of unknown frequency and small amplitude, i.e. not sufficient, in absence of noise to excite the firing state. The frequency of the sinusoidal signal was recovered by using homomorphic filtering based procedures. See also Digital image processing Image processing Digital imaging Image editing References Further reading A.V. Oppenheim, R.W. Schafer, T.G. Stockham "Nonlinear Filtering of Multiplied and Convolved Signals" Proceedings of the IEEE Volume 56 No. 8 August 1968 pages 1264-1291 External links Overview of homomorphic filtering Image processing Signal processing