FM AID is a module designed to perform through-zero linear Frequency Modulation (FM) in the analog modular world. Any arbitrary signals can be used as a Carrier and Modulator. The module copies FM in how it is done in digital implementation, but in a completely analog circuit free of digital aliasing artifacts. Digital or software FM oscillators are mainly based on a ramp waveform which is then shaped to other waveforms. This is also represented in the FM AID module: sawtooth signal is expected on the Carrier input to give the indicated sine, triangle, sawtooth and square waveforms on the respective outputs. However the user is not limited to only use sawtooth for the Carrier and any other signal can be plugged giving many complex waveforms at the outputs.
All known FM tricks from digital implementation are also applied – you can endlessly experiment with the Carrier/Modulator frequency ratios, Modulation index (FM knob), Modulator’s amplitude shaping (CV knob), feedback FM (output is fed back to the Modulator’s input) and many other.
Carrier input has normalled connection to the Modulator’s input. This internal routing is disconnected each time a plug is inserted in the Modulator’s input. That is a handy option which allows the module to be used with only one signal source – this single signal will act as Carrier and Modulator at the same time. There won’t be any frequency differences between Carrier and Modulator in this case, but many interesting wave folding sounds will be available anyway.
Technically speaking FM AID uses Phase Modulation, not Frequency Modulation. The sonic difference is that the produces sound is brighter.