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From the Cyndustries website:

The Zeroscillator is a full-featured electronic-music-quality analog VCO capable of linear frequency modulation through zero hertz and into negative frequencies. This makes it capable of the FM Synthesis stuff, wild and crazy sounds simply not available from traditional VCOs, and timbres not available from digital FM synthesizers. Of course, Zeroscillators also make insane LFOs!

Traditional FM Synthesis oscillator implementations, such as the DX-7, are sine wave only. The Zeroscillator also makes available sawtooth, reverse saw, triangle, pulse (with PWM), and a special set of four quadrature outputs capable of unique bi-phase wave morphing, (more on that later).

FM sounds take on their vibrant life when the amount of modulation is varied dynamically. In the analog world, this requires a special VCA to regulate the depth of frequency modulation, (often called the “modulation index”). Since this is required almost universally for Linear FM patches, the Zeroscillator has this VCA built-in. We decided however, to take this feature one extra step beyond and provide you with a four-quadrant multiplier instead of a traditional two-quadrant VCA to expand your modulation possibilities into the unexplored. The output of this multiplier is available at the panel, so you get a free ring-mod function and an interesting pick-off point for self-modulation patches called Mod Out.

In addition to it’s precision 10-turn tuning knob, the Zeroscillator has 1V/octave plus attenuated exponential inputs and can behave just like the VCOs you’re used to. You are not forced into any specific modality and are free to use all or any subset of its 7 different modulations:

1-Exponential FM
2-Linear Through-Zero FM
3-Linear FM Dynamic Depth Modulation
4-Bi-Phasic Waveform Morphing in Quadrature
5-Pulse Width Modulation
6-Variable Sync (from none to hard synch)
7-Time Reversal

The through-zero capability may be switched off which bars the oscillator from reaching into the negative frequency netherworld. If the linear modulation goes negative, the oscillator simply stops, restarting when the modulation returns to positive territory. This results in another class of sounds and is also very interesting and rhythmic when used at LFO rates.

Now back to the Quadrature Outputs with Bi-phase Wave Morphing. These four outputs present the oscillator waveform at four different relative phases separated by 90 degrees (0, 90, 180, 270). If the 0 and 90 degree outputs are presented to the left and right speakers respectively, a definite stereo spread effect is obtained, and when the oscillator beats with its modulator, harmonic characteristics shift back and forth across the stereo image. This effect is unique to the Zeroscillator.

At LFO rates, the quadrature outputs can be used to control quad panning or create circular illusions. Remember, when the Zeroscillator is heavily FM’ed, the waveforms fold back on themselves and create patterns that look nothing like the waveforms on the label.

You can also morph the waveform coming from the quadrature outputs from triangle to sine to square! The 0, 180 pair and the 90, 270 pair have separate morphers which can move in tandem or opposition to each other. The morph is not simply a waveform crossfade, but a true morph which creates all the in-between waveforms in its travel and it can even be modulated at full audio rates! The morph is also constant-power which means the perceived loudness does not change as the harmonic content is being radically altered. With the morph feature, youll find yourself not needing a filter in a great many patches.

We even threw in the kitchen, (Sync that is)… The Zeroscillator has Variable Synchronization from, “no sync at all… “to Totally Hard Sync”. You just dial-in the right amount for your particular patch, (for classic FM type synthesis, you’ll be glad you have this control on a dedicated knob).

Finally, there is the mysterious “Time Reversal” Input, (which is actually what occurs when the oscillator passes into negative frequencies). Your external signal here forces the waveform to reverse its direction of travel as seen on an oscilloscope. The resulting effect is most similar to hard sync, but a completely separate circuit, and they both may be used together.

Richard Brewster’s modifications:

Replaced all jack washers and nuts with Switchcraft parts for a perfect cosmetic match with MOTM.

Back side photos