Figure 8: Channel identification in a SISO [Filter]-[van der Pol-ZCD] circuit using multiple I/O pairs. (a) Input signals are bandlimited to 50 Hz. The order of the space . (b) Biased output of the filter in response to the stimulus . (c) Downward and upward deviations of from the bias cause the oscillator to slow down and to speed up, respectively. The damping coefficient . (d) The oscillator produced a total of 56 spikes in response to 4 stimuli. Here spikes correspond to the peaks of the observable state variable . (e) A limit cycle of the van der Pol oscillator for is shown in the phase plane. In the absence of channel output, the bias resulted in a constant period of oscillation ms. The red dot denotes the zero phase (spike) of an oscillation. (f) The identified filter (red) is shown together with the original filter (dashed black) and its projection (blue). The MSE between and is −66.6 dB. (g) Fourier amplitude spectra of and . As before, but .