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Thresholding behaviour

The results reported in this section all relate to a chain consisting of N=12 cells, with , , and for i > 1. For these values, , which is only coupled to itself, generates a chaotic time series; would exhibit period two behaviour if they were not self-coupled, oscillating between .

For these parameter values, the actual output () is approximately a two-level, aperiodic signal --- see figure 3(a). Furthermore, for these parameter values at least, the overall effect of the CMC on its input can be well described by a simple thresholding operation: see figure 3(b), in which the output is equal to if the input is and is otherwise. The threshold appears to be , an unstable fixed point of the mapping of .

This type of behaviour would be expected if for i>1, and such that period-2 behaviour resulted ( e.g. 3.2). In that case, none of the cells except the first is self-coupled. Hence, in travelling down the chain, an initial value iterates towards one fixed point or the other, and hence thresholding takes place. Less easily explained is the occurrence of this behaviour when .

  
Figure 3: (a) A typical sample of and (respectively input and output) waveforms. (b) A comparison of the actual output with the input thresholded at 0.730.

Furthermore, the output () is an approximately one-dimensional function of the input () as can be seen from figure 4.

  
Figure 4: The transfer function vs. . The relationship for these parameter values is one-dimensional for some ranges of the input, fractal for others -- see inset.



Jonathan Deane, and David Jefferies
Tue May 28 11:26:02 BST 1996