The results described here represent the state of the art in the exploration of radically new territories of design space. The circuit is small, but definitely not trivial. For a human designer to solve this problem using only 32 cells, with no clock or external components, would be very difficult indeed (if feasible at all).
The circuit vividly demonstrates the power of unconstrained evolution. With a freedom to explore rich structures and dynamics, evolution has been able to exploit the natural behaviours arising from the physics of the device. Analysis of such an evolved circuit enhances its utility, but requires novel approaches. There are numerous tactics that can be used to piece together answers to analysis questions even for a seemingly impenetrable circuit. We still do not understand fully how it works: the core of the timing mechanism is a subtle property of the VLSI medium. We have ruled out most possibilities: circuit activity (including glitch-transients and beat-frequencies), metastability [41], and thermal time constants from self-heating. Whatever this small effect, we understand that it is amplified by alterations in bistable and transient dynamics of oscillatory loops, and in detail how this is used to derive an orderly near-optimal output. Certain peripheral cells fine-tune particularly sensitive time delays. On the key question of long-term consistency of behaviour, we know that the entire FPGA circuitry is strongly reset to a deterministic stable logic state for every high half-cycle of the input waveform. Long-term pathologies are therefore highly unlikely, demonstrating that analysis effort can sometimes remove worries related to the use of highly unconventional circuits in practical applications.
It now seems indisputable that hypotheses H1 and H2 are true: `Evolutionary algorithms can explore some of the regions in design space that are beyond the scope of conventional methods.' The fascinatingly alien tone-discriminator circuit was produced using a very basic evolutionary method, with no great difficulty other than to leave behind preconceptions of how electronics should be. The circuit gives a tantalising glimpse of the theoretically-possible engineering attractions, such as small size by finding forms and processes that are natural to the VLSI medium. However, it is invalid to make a direct comparison with conventionally-designed circuits: these have a much larger operational envelope.