Cherry Audio
Cherry Audio

The (Mostly) Unfiltered Secrets of Atomika

One of the most distinguishing features of the Polivoks (and, now, Atomika) is unquestionably its unique filter. Let’s dig in to understand and see how to get the most out of it with the help of award-winning DSP developer Mark Barton and Atomika’s designer, Mitchell Sigman.

At first glance, the Polivoks synthesizer appears to be a typical compact analog synth with a handful of features that have nevertheless made it highly desirable among enthusiasts. The most notable of these features is the iconic Polivoks voltage-controlled filter. Instead of using the chips found in typical 12 dB/octave state-variable filter designs of the era, designer Vladimir Kuzmin developed a way to use programmable op-amps, resulting in the distinctive and gritty filter sound associated with the Polivoks.

Up to this point, Cherry Audio has never seen a virtual instrument that could adequately reproduce the Polivoks filter tone. However, frequent collaborator Mark Barton was able to nail it through a combination of analysis, breadboarding, and coding wizardry. Along the way, Mark discovered other methods to expand upon this remarkable filter circuit, adding Starve, Filter Drive, and Amp Drive controls that extend its twisted palette, often in radical ways.

“Kuzmin didn’t like his filter at first,” explains Mark. “He lamented that it was only 2-pole, rather rough sounding, and lacked the “sweetness” of the 4-pole Moog filter. The design, however, is brilliant. Just as there’s no oscillator like the one in the ARP Pro Soloist, there’s no filter like the one in the Formanta Polivoks. If you look over the schematics, you may miss it entirely. It has almost no parts to speak of and no capacitors. So, how can it be a filter at all, let alone a voltage-controlled filter?  This is where Kuzmin’s brilliance is evident.”

The basic design was intended to emulate the 12dB/oct slope state-variable filters seen in Oberheim synths of the 70s and 80s. These were great-sounding filters and highly flexible due to their multiple modes. Filters of this type use a component called an “operational transconductance amplifier,” or OTA, to multiply the audio with a control current and feed it into the two capacitors. As the control current is varied, the filter tunes up and down.

However, OTAs were unavailable to Kuzmin in the 1980s, so he used local programmable op-amps that were intended for something other than audio filter circuits. These chips also take a control current, but the intention was to “program” the op-amp’s speed and power draw. With a small control current, the op-amp is slow and draws less power, and vice-versa. Kuzmin cleverly used this effect to tune his filter.

“But,” Mark points out, “you still need capacitors, so where are they? They’re in the chips! Most op-amps have a compensation capacitor inside to keep them stable and prevent unwanted oscillations. This capacitor is tiny; consequently, minimal control currents get the filter’s tuning down into the audio range. As a result, the op-amp changes its characteristics drastically as the filter is tuned.”

Mark Barton nailed the Polivoks sounds through a combination of analysis, breadboarding, and coding wizardry.
Mark Barton nailed the Polivoks sounds through a combination of analysis, breadboarding, and coding wizardry.

“I knew all that going in, but I was very surprised at what I found when I built the filter on a breadboard to check it out and thoroughly analyze it,” he continues. “Surprise, surprise, the filter barely distorts. It sounds quite sweet and works very well over a wide tuning range. I found that amazing.”

Furthermore, Mark discovered several eccentric circuit behaviors along the way. “If you turn up the input signal, it starts to clip asymmetrically, so here come the harmonics. Turning up the input can also suppress the resonance. If you turn up the resonance most of the way, it self-oscillates. A well-built state-variable filter will not oscillate. The craziest characteristic of all is that the filter can “bubble.” If you get the input level just right, and the resonance just right, the filter rings in a way that sounds like dripping water.”

As for the idiosyncratic tone associated with the Polivoks sound? “Most of the aggression associated with the Polivoks sound actually comes from the overdriven VCA,” he says. “You need the VCF and the right kind of VCA to get that tone.”

Realizing this range of behaviors sounded awesome, Mark added these extended characteristics into his modeling of the filter in software (with controls originally nicknamed GRAB, GLOB, and GROT), and designer Mitchell Sigman incorporated them into the design of Atomika as the Starve and Filter Drive controls, the Amp Drive knob in the Amp section.

As Mark describes it, “If you strike a bell, it will ring. If you GRAB that bell and strike it, it will ring, but badly. Turning GRAB (Starve) up strangles the filter’s ability to ring well, yet it still rings. This is not at all the same as lowering the resonance. GLOB (Filter Drive) varies the characteristic that causes that dripping water effect and greatly alters the filter’s ringing timbre, from clean to, well, globby. GROT (Amp Drive) varies the overdrive into the VCA and makes the whole synth sound very angry indeed. These three knobs greatly add to the fun and give you the ability to tune the sound from ABBA to Anthrax.”

As Mitchell details in the Atomika User Guide:

  • Starve - This parameter is unique to the filter design, resulting from the aforementioned programmable-op-amp-incorrectly-used-as-filter architecture. If the settings are correct, the filter can take on an oscillating “bubbly” sound, for lack of better wording. Try Cutoff between 1 and 2, Resonance at max, and Starve between 5 and 6 as a starting point.

  • Filter Drive - This is not a standard filter overdrive/breakup control, but, like Starve, it is unique to how the Polivoks filter resonance circuit interacts with the power supply rails. The technical details aren’t important; what you should knows is that it tends to work best with Resonance at high settings, and the Filter Drive knob at very low or very high settings - all manner of radical distorted noises happen when Resonance is cranked and Filter Drive is set to 0!

  • Amp Drive - This sets the amount of drive into the amp section, and as you might imagine, it’s used to overdrive the amp section. Hooray for distortion!

To demonstrate one possible application of these settings, we have created a short video demonstrating how these controls can significantly impact the sonic output of the filter without even adjusting the cutoff or resonance, producing a bubbling, almost liquid timbre. To experiment with it independently, check out the “Sonic Screwdriver (MW)” patch in the SoundFX category of Atomika’s factory preset collection. Note the settings for Cutoff (relatively low) and Resonance (high), and that Starve and Filter Drive have also been mapped to the mod wheel through the MIDI Learn feature.

The team has also expanded the filter response choices beyond the lowpass and bandpass modes of the original hardware, and added highpass, notch, and peak responses. Again, from Mitchell’s Atomika User Guide:

  • Lowpass - Allows frequencies below the cutoff frequency to pass, but blocks frequencies above the cutoff frequency with a 12 dB/oct slope.

  • Bandpass - Allows a band of frequencies in the vicinity of the cutoff frequency to pass, with a 6 dB/oct slope on either side of the peak.

  • Highpass- Allows frequencies above the cutoff frequency to pass, but blocks frequencies below the cutoff frequency with a 12 dB/oct slope. Because they dramatically remove low frequencies, the highpass setting is useful for nasally tones with exaggerated high frequencies.

  • Notch - Removes a band of frequencies close to the cutoff frequency and allows all other frequencies to pass. The notch width varies dependent on the current resonance setting, with low resonance resulting in the widest Notch width. Notch filters are useful for pseudo-phaser effects when their cutoff frequency is swept.

  • Peak - A pronounced resonant peak at the cutoff frequency, and no rolloff on either side of the peak frequency. Incidentally, the Peak setting excels at really saturated, aggressive sounds when the Resonance knob is cranked.

Of course, the filter is just one aspect of how Atomika builds on the design of the Polivoks and then “goes to eleven.” Atomika enhances its capabilities by increasing polyphony from duophonic to a powerful 16-voice polyphony. It introduces unison oscillator stacking, an expanded LFO with additional waveforms, and comprehensive performance controls, including velocity, tracking, aftertouch, and pitch and mod wheel response.

And don’t forget Atomika’s second panel, which includes a versatile arpeggiator and four studio-quality effects — Phasor, Flange/Chorus, Echo, and Reverb — with a dedicated FX Modulator LFO that enables synchronized modulation of key effects parameters for even more radical and deranged sound transformations.

For more sound examples, be sure to check out the extensive playlists of Atomika demos and preset tracks. We sincerely hope you’ll enjoy exploring Atomika’s vast and, at times, madcap sonic landscape!

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