 By a similar token, analog synthesizers such as the Serge synthesizer and many modern Eurorack synthesizers can produce undertone series as a side effect of the solid state timing circuits (e.g.the 555 timer IC) in their envelope generators not being able to re-trigger until their cycle is complete.As an example, sending a clock of period into an envelope generator where the sum of the rise and fall time is greater than and less than would result in an output waveform that tracks at of the frequency of the input clock.