Frequency Shifting With Tonal Control

FAC Shifter is a multiband stereo frequency shifter designed for tonal control, combining selective frequency shifting across the spectrum with advanced MIDI note tracking. Sharing the same foundation as FAC Gravity, it brings a different creative perspective, making the two plugins natural complements.

The plugin can process the full spectrum or target a movable band-pass region defined by low and high crossover frequencies. This lets you precisely control which frequency range is shifted while leaving the rest untouched. You can preserve a bass fundamental while reshaping its upper harmonics, or add subtle phasey, chorus-like textures without transforming the entire sound.

To keep frequency shifting tonally coherent, two distinct MIDI key tracking modes determine how the shifter responds to the notes you play. You can anchor played notes to a fixed root note or let them follow the frequency shift directly. Both modes work alongside the main shifter param to open up more playable and musically controlled spectral transformations.

And there's still more to explore. You can infuse the shifter's internal quadrature processing with fuzz, ranging from subtle vibrato to chaotic distortion. The final sound can also be shaped by choosing which sidebands make it through, selecting positive or negative sidebands or combining both independently for the left and right channels.

The core frequency shifting controls are complemented by a feedback control for building resonance and spectral complexity, along with a dedicated width control for shaping the stereo image. A Dry/Wet control lets you blend the processed sound with the original, while an optional limiter helps keep the output under control.

FAC Shifter works as an Audio Unit FX, featuring a clean, touch optimized interface with precise control and real-time spectrum visualization. It’s available as an AUv3 for iOS (iPad/iPhone), macOS, and Vision Pro, and requires a compatible AUv3 host app to run.

Show/Hide Help Section

Ⓐ Frequency Shifting Shifter Knob: Controls the frequency shift amount and direction. At the center position, the shift is 0 Hz. Turning clockwise shifts frequencies upward, while turning counter-clockwise shifts them downward Range: Sets the available frequency shift range of the Shifter Knob. Available settings are 10 Hz, 50 Hz, 250 Hz, 1 kHz and 5 kHz, as well as host-tempo-synchronized rhythmic divisions ranging from 8 bars to 1/32 note. Zero Snap: When enabled, the quadrature phasor returns to its nominal position whenever the frequency shift reaches 0 Hz, ensuring a consistent stereo image and sound at the zero-shift position Spectrum: Displays a real-time stereo output spectrum Key Track: Enables MIDI key tracking and reveals additional controls for Anchor and Follow modes (see section B: MIDI Key Tracking)
Ⓑ MIDI Key Tracking Mode: Selects between two MIDI tracking modes, each with its own dedicated Operator control Anchor: Uses a fixed reference note (Root) to interpret incoming notes. The frequency shift responds to the played notes relative to this reference, maintaining a consistent tonal relationship as you play Follow: Uses incoming notes to directly influence the frequency shift, the depth and direction are determined by the Transposition setting Operator: Provides the dedicated control associated with the selected tracking mode Root (Anchor mode): Establishes a fixed tonal reference for frequency shifting. Tap to capture and lock the currently received MIDI note as the Root. Tap again to unlock it. To find your reference, unlock the Root, play a sound at the desired MIDI note, and adjust the Shifter frequency until you find an interesting transformation. Then lock the Root to maintain a consistent tonal relationship as you play other notes. Returning to the Root note reproduces the same frequency shifting behaviour as with key tracking disabled Transposition (Follow mode): Controls how closely the shifter's base frequency follows the played MIDI note. At 0 semitones, MIDI notes have no influence. Increasing the value progressively brings the shifter's base frequency towards the played note, with +12 semitones corresponding to the exact frequency of the MIDI note. Negative values apply the same relationship in the opposite direction. The control ranges from -48 to +48 semitones
Ⓒ Multiband or Fullband Target: Enables or disables multiband processing. When enabled, the frequency spectrum is divided into three bands using two adjustable crossover frequencies. The frequency shifter is applied only to the middle band, while the low and high bands remain unaffected. When disabled, the multiband filter is bypassed and the frequency shifting is applied to the full frequency spectrum Solo: Mutes the low and high bands, allowing you to hear only the middle band with the effect active. (Multiband target only) Multiband Control Bar: Provides visual and interactive control over the multiband filter. The two crossover sliders set the lower and upper frequency boundaries of the middle band, defining the frequency range processed by the shifter. The filter slope determines the steepness of the crossover transitions, with adjustable settings ranging from 6 dB to 192 dB. (Multiband target only)
Ⓓ Feedback and Quadrature Fuzz Feedback Knob: Controls the amount and direction of the signal fed back into itself. Positive values drive the frequency shift upward, while negative values drive it downward. Higher feedback amounts produce more pronounced frequency movements, resonance, and evolving spectral textures. QFuzz Slider: Controls the amount of fuzz applied to the internal quadrature processing of the frequency shifter. At 0%, the quadrature processing remains untouched. Increasing the amount introduces progressively stronger fuzz and distortion, ranging from subtle vibrato textures to more aggressive and chaotic sonic transformation	Ⓔ Sidebands and Width Dual Sliders: Independently select which frequency-shift sidebands are retained for the left and right channels. Each channel can select Positive, Negative or Both sidebands, allowing different sideband combinations between the two channels Width: Sets the width of the final stereo image. 0% for mono output Ⓕ Output Mix: Controls the balance between the dry and processed signals Phase: Sets the phase position of the frequency shift
Ⓘ Limiter Optional output limiter, choose from soft, safe, or Off	Ⓖ Preset Button Ⓗ Info Button Preset manager (Tap or long press) Review the app, Tip Jar to support FAC, documentation...
Additional Notes on MIDI Key Tracking A working MIDI input must be connected to FAC Shifter to receive external MIDI notes for key tracking When no MIDI input is connected, the Mode display shows -C2-, indicating that C2 is being used as the default note When MIDI input is received, the display shows the currently played note Follow mode can operate without an external MIDI input. In this case, C2 is used as the default note value Anchor mode requires an incoming MIDI note to capture and establish its Root reference MIDI key tracking operates in conjunction with the main Shifter parameter MIDI note handling is monophonic, using Last Note Priority (Last Note Wins)

Always check your audio levels

In general, use the gain in parsimony, to avoid any excessive levels that may damage your ears or your system. FAC Shifter shall not be liable for any loss or damage of whatever nature (direct, indirect, consequential, or other) to your material (software, hardware, or other)

As with all FAC products, the interface has a unique sober look and provides a great user experience. The value of each knob can be monitored at any time. The knobs are very responsive, natural and follow a specific curve according to the parameter type

Preset Management

FAC Shifter also comes with factory presets to provide you a source of inspiration to satisfy your needs in any situation you might require

The preset button provides common management features: loading, organization, saving, importing and exporting. The left and right arrows allow fast navigation shortcuts among all the presets (factory + user), a short press on the center label provides a detailed list of all the available presets, select the one you want to work with. On the other hand a long touch (hold) provides contextual operation