HELM is an open source synth built by Matt Tytel.

HELM includes,

  • 32 voice polyphony
  • Interactive visual interface
  • Powerful modulation system with live visual feedback
  • Dual oscillators with cross modulation and up to 15 oscillators each
  • Unison and Harmony mode for oscillators
  • Oscillator feedback and saturation for waveshaping
  • 12 different waveforms
  • 7 filter types with keytracking
  • 2 monophonic and 1 polyphonic LFO
  • Step sequencer
  • Lots of modulation sources including polyphonic aftertouch
  • Simple arpeggiator
  • Effects: Formant filter, stutter, delay

Even more interesting though HELM is a a free software and consequently you can examine and investigate the code online at (github)[https://github.com/mtytel/helm].

More details online.