Frap Tools Turns the Tables: Feeding Filters into Oscillators for Sonic Mayhem

Most of us have patched an oscillator into a filter more times than we care to admit, but Frap Tools asks the forbidden question: what if you patch a filter into an oscillator instead? This video kicks off with a simple yet subversive setup, where the CUNSA filter’s output is sent directly into the BRENSO oscillator, turning the usual subtractive workflow on its head.

The patch is built around two sequences—a bass and a lead—each handled by one half of a BRENSO oscillator. Both voices are routed through separate filters on the CUNSA, each set to low pass mode, with the bass using a 12dB/octave slope and the lead a 24dB/octave. The result is a foundation for exploring how filters can shape and even generate modulation signals when fed into an FM-capable oscillator.

BRENSO’s reputation as a true zero analog FM oscillator is on full display here. The video highlights how both its oscillators can be independently modulated in true zero linear FM, opening up a playground for cross-modulation and complex timbral shifts.

Instead of sticking to the usual sine-to-sine FM, the patcher demonstrates that BRENSO’s FM circuit is flexible enough to accept other signals—like those coming from the CUNSA filters—making it possible to treat the module as two independent FM engines. This flexibility is key to the patch’s experimental edge.

A clever detail in this patch is the use of two CUNSA filters, both set to low pass, but with different slopes and in combo mode. Each filter is pinged by gate tracks from a Whistler sequencer, eliminating the need for envelopes and allowing for percussive, plucky responses straight from the filters.

The real magic happens when the sine-like outputs from these filters are used to modulate the BRENSO oscillators’ frequencies. By cross-patching these signals, the video demonstrates how to achieve intricate FM textures and shifting harmonics. This approach turns the filter from a mere tone shaper into an active participant in the synthesis process, capable of generating its own modulation content.

Once the basic patch is established, the video dives into more creative workflows. The volt-per-octave signals are duplicated and routed to both the BRENSO and the CUNSA filters, ensuring that pitch tracking remains consistent across the system. This careful routing is essential for keeping the resulting sounds musical, even as the patch grows more complex.

To add further movement, modulation sources like the FALISTRI (set to LFO mode) are brought in to control the amount of FM applied to both oscillators. The BAGÀI module’s fluctuating random voltages are also introduced, targeting the bass for extra unpredictability. By tweaking ratios and parameters, the patcher showcases how easy it is to morph between different timbral landscapes, from subtle FM to outright chaos.

A final touch of reverb rounds out the sound, demonstrating how this unconventional patch can yield rich, evolving textures. The workflow encourages experimentation, showing that filters and oscillators can interact in far more adventurous ways than tradition suggests.