Audiotool - Phase/Frequency Modulation Synth (Heisenberg)
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This might be the most powerful free instrument available today (and I really mean that). You can choose to work with Heisenberg like a traditional subtractive synth, or if you're feeling ambitious, dig into the phase/frequency modulation options. Or combine them and do a bit of both.
We could very easily dedicate an entire course to this instrument, sadly we only have time to make a couple of patches using the alternative synthesis methods mentioned above. In the future we will be dedicating a lot more time to sound synthesis and phase/frequency modulation. Consider this to be just an appetizer for what is to come.