This segment of Bitwig's Polymer tutorial covers phase modulation and feedback, with the host opening up the expanded device view to walk through the internal patch routing before demonstrating the technique in action. The sub oscillator feeds into the primary oscillator's phase modulation input, with an optional scalar letting you choose between a static modulation amount or a filter envelope that shapes the PM signal dynamically over each note.
As modulation depth increases from a sine wave source, sidebands and harmonics appear in a non-linear way that differs from a standard filter sweep or phase distortion. The fundamental itself can drop out at certain modulation depths, which opens up timbral territory that a cutoff filter sweep simply cannot reach. Changing the sub's tuning relative to the carrier shifts the spacing between those harmonics, moving from tight, muddy clusters to wider, brighter spreads.
The feedback path works differently: it routes the oscillator's own output back into its phase modulation input. With a sine wave, this produces a more controlled, predictable harmonic result compared to cross-oscillator PM, making it a stable starting point for dialing in a tone. Pushing the feedback knob further introduces instability and noise, which is where a filter becomes useful for taming the result.