Audio Tools

AT20V

The semi-modular that screams.

Free and open source. For macOS and Windows. AT20V 1.0.1.

Download
The AT20V synthesizer rendered in 3D: a charcoal front panel with black knobs, a patch bay with colored cables and a 37-key keyboard.

Get the highlights.

Thirty-five live jacks.

Thirty-five live jacks. Every input and output of the original patch bay works, with cables that hang like real ones.

Two filters with attitude.

Two filters with attitude. Korg35 low-pass and high-pass, solved sample by sample, with the slopes of the 1978 circuit.

Wheel and switch, patchable.

Wheel and switch, patchable. The control wheel and the momentary switch can drive anything on the patch bay.

A vocoder in the box.

A vocoder in the box. Sing into the audio input and play chords, from 8 to 20 bands.

Up to twelve voices.

Up to twelve voices. Mono like the original, or polyphonic with unison, detune and stereo spread.

The film.

AT20V rendered in 3D, in three shots. The sound is the engine playing Random Bleeps, a self-playing preset.

A filter that screams like the original.

The MS‑20 gets its character from two small, misbehaving filters. AT20V models them from Tim Stinchcombe's circuit analysis and Korg's service manual, and solves their nonlinear feedback loop sample by sample instead of approximating it.

It was built with measurements, not guesswork: every figure below was checked against the numbers in the original documentation.

Low-pass filter
12 dB per octave, with the diode clipping that limits the resonance
High-pass filter
6 dB per octave, the real slope of the MS‑20, not the textbook 12
Calibration
cutoff at 5 and peak at 10 self-oscillates at 500 Hz, as Korg calibrated it
Filter revisions
Rev 1 (Korg35): grit grows with the mixer level. Rev 2 (LM13600): clean passband
Oscillators
band-limited, with the integrator curve, ring modulation as XOR and PW that fades to silence
Electrical conventions
Hz/V keyboard, 1 V/oct and 2 oct/V modulation, S-trig triggers, high-note priority
Oversampling
the whole voice runs at 4x or 8x
Close-up of the AT20V patch bay with yellow, red and blue cables
35
live jacks on the patch bay
2
filter revisions, Korg35 and OTA
12
voices with up to 4-voice unison
51
presets, from the 1978 charts to self-playing patches

Hear it.

Every clip is the engine on its own, playing a factory preset from a MIDI file written for it. Nothing added afterwards, only the level matched across clips. Download the MIDI and play it through the preset yourself.

  1. Acid Bass

    A 124 BPM acid line through the Korg35 low-pass.

    0:16 MIDI

  2. Scream Lead

    A slow phrase with the resonance pushed into the scream.

    0:12 MIDI

  3. Hoover Square

    An octave riff at 140 BPM.

    0:14 MIDI

  4. Glass Cathedral

    Four polyphonic chords with delay and reverb.

    0:17 MIDI

  5. Hammond Organ

    A factory chart from the 1978 manual, comping in F.

    0:20 MIDI

  6. Random Bleeps

    One key held: the patch bay plays the rest.

    0:19 MIDI

Thirty-five jacks, all of them live.

Every input and output of the original patch panel works, including the hidden default connections: plug a cable and it replaces the internal route, exactly like the hardware. Cables hang with real physics, outputs take stacked plugs, and you can build feedback loops anywhere.

AT20V front panel with patch cables connected
The front panel and patch bay. Every knob and jack was rendered in Blender from measurements of the original instrument.

What the MS‑20 never had.

A second page adds what a modern studio expects, drawn in the same visual language as the original panel.

  • Vocoder. 8 to 20 bands. Sing into the audio input and play chords; formant shift, resonance and sibilance included.
  • Polyphony. Mono like the original, or up to 12 voices with up to 4-voice unison, detune and stereo spread.
  • Tempo sync. The modulation generator locks to your DAW, so patches can play themselves in time.
  • MIDI out. Thru, or Patch mode, which turns the triggers of the patch bay into notes for other instruments.
  • Delay and reverb. A tempo-synced ping-pong delay and a plate reverb for the ambient presets.
  • AT20V FX. An effect version for audio tracks: put it on a vocal and drive it from a MIDI track.
AT20V Extensions page with vocoder, polyphony, delay, reverb and system knobs
The Extensions page: vocoder, polyphony, delay, reverb and system settings.

From the 1978 factory charts to patches that play themselves.

14

Factory

Patches transcribed from the setting charts of the original owner's manual.

13

Classics

Basses, screaming leads, drums and effects.

11

Self-playing

Generative patterns synced to your DAW. Hold a key to transpose them.

9

Ambient

Slow polyphonic pads with delay and reverb.

4

Vocoder

Robot voices, choirs and whispers for the audio input.

Tech specs.

Sound engine

  • Two VCOs with triangle, saw, pulse, noise and ring modulation; Hz/V keyboard
  • Korg35 high-pass (6 dB/oct) and low-pass (12 dB/oct), Rev 1 or Rev 2 (OTA)
  • Two envelopes, modulation generator, sample and hold, External Signal Processor
  • 4x or 8x oversampling

Patch bay

  • 35 jacks with the original normals
  • Stacked plugs on outputs, rope-physics cables

Extensions

  • Vocoder with 8 to 20 bands
  • Up to 12 voices, up to 4-voice unison
  • Tempo-synced modulation generator
  • MIDI out (Thru and Patch)
  • Delay and plate reverb

Formats

  • VST3, Audio Unit (macOS) and standalone app
  • AT20V FX: effect version for audio tracks

Minimum

  • macOS Mac with Apple silicon (M1 or later), or Intel Core i5 quad-core from 2017 or later, with macOS 11 Big Sur or later.
  • Windows PC with Intel Core i5-6500 (6th generation, 2015) or AMD Ryzen 5 1600, or better, with Windows 10 or 11, 64-bit.
  • 4 GB of RAM, a 1280 × 800 screen and a 64-bit host for VST3 or Audio Unit, or the standalone app.
  • Disk space: 168 MB on macOS (all formats), 68 MB on Windows.

Recommended

  • macOS Any Mac with Apple silicon.
  • Windows Intel Core i5-12400 or AMD Ryzen 5 5600, or better.
  • 8 GB of RAM and a 1920 × 1080 screen.
  • The 12-voice, 8x setting needs the recommended processor.

Measured CPU load

SettingApple M4AMD Ryzen 9 5900XT
Mono patch, 4x1.6 %3.6 %
8 voices with delay and reverb, 4x11.2 %32.4 %
12 voices, 8x (the heaviest setting)28.9 %78.1 %

Percentage of one CPU core at 48 kHz, sound engine only. With the editor open, the interface adds about 15 % on the M4. Hosts run each instrument on one core, so single-core speed is what counts.

Download AT20V 1.0.1.

Free, no account, no activation, no telemetry.

macOS

macOS 11 or later. Apple silicon and Intel in one universal binary. VST3, Audio Unit and standalone app. 168 MB.

Download for macOS
How to install
  1. Unzip the file.
  2. Copy AT20V.vst3 and AT20V FX.vst3 to ~/Library/Audio/Plug-Ins/VST3.
  3. Copy the two .component files to ~/Library/Audio/Plug-Ins/Components.
  4. Rescan plugins in your DAW. In Ableton Live: Settings, Plug-Ins, Rescan.

Windows

Windows 10 or 11, 64-bit. VST3 and standalone app. 57 MB.

Download for Windows
How to install
  1. Unzip the file.
  2. Copy AT20V.vst3 and AT20V FX.vst3 to C:\Program Files\Common Files\VST3.
  3. Rescan plugins in your DAW.
  4. AT20V.exe runs without a DAW.

The builds are not code-signed yet. macOS may refuse to open them until you run xattr -dr com.apple.quarantine on the installed plugins, and Windows SmartScreen may ask you to confirm the first time. Both packages include an INSTALL.txt with the exact steps.

AT20V is free software released under the GNU Affero General Public License v3, the same license as the JUCE framework it is built on. You can use it, study it, change it and share it. If you distribute a modified version, you share your changes under the same terms.

Why Audio Tools is open source