  Local periodic perturbations induce frequency-dependent propagation waves in
an excitable spatio-temporally chaotic system. We show how segments of
noise-contaminated and chaotic perturbations induce characteristic sequences of
excitations in the model system. Using a set of tuned excitable systems, it is
possible to characterize signals by their spectral composition of excitation
pattern. As an example we analyze an epileptic spike-and-wave time series.
