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The ED139 is a sawtooth-based VCO capable of frequency modulation (FM) extending past zero frequency into the negative-frequency regime. Thru-zero FM provides a much wider and richer variety of sounds than “ordinary” (positive frequency only) FM.

The negative-frequency version of a waveform is simply a time-reversed replica of the original waveform.  When a VCO is modulated through zero frequency, the waveform slows down to a stop and then speeds back up in the reverse direction.

The ED139 ’s output waveforms also include triangle and sine waves, for producing sounds with fewer high harmonics, as typically used for bell sounds, train whistles, and so on. The unit also features a variable synchronization control that can be adjusted over a range of settings from hard sync to a fairly loose soft sync.

The module can also serve as a highly accurate and stable “ordinary” VCO (i.e., without the deep FM), with sawtooth, triangle and sine output waveforms

The ED139 is based on the Electronotes TZVCO by Bernie Hutchins, itself from a Douglas Kraul design from Electronotes EN#62.

Build Guide