The ES18 performs non-linear wave shaping known as full-wave rectification, but with sophisticated level-compensating conditioning as well. Actually the circuit uses three full-wave rectifier sections linked in a very refined controllable format. Each section can double the frequency of a sine or triangle wave applied to its input. Thus sweeping the [CV IN] input over its range will produce a smooth timbral transition using the even harmonics (second, fourth, and eighth). Many other partials are present in this basic sound, however, and the sonorities are very rich and varied.
A notable feature of this multiplier is that, the full-wave rectification is not accompanied by a reduction in the output amplitude. There is no alteration of the essential level of the sound. There are two inputs to provide mixing before processing, and two outputs. The [DC] output is a "squared up" version of the [AC] output. This [DC] resembles voltage controlled pulse width modulation (only much more interesting).