SVF Filter

Module ID: filter.svf Category: Filters Header Color: Green

The SVF Filter module

Description

The State Variable Filter (SVF) is a versatile multi-mode filter that provides simultaneous lowpass, highpass, and bandpass outputs from a single input signal. This architecture allows you to blend different filter responses or switch between them without repatching.

The SVF design offers:

• Self-oscillation at high resonance (can be used as a sine oscillator)
• Smooth cutoff sweeps without artifacts
• Stable operation across all settings
• Simultaneous multi-mode outputs

Inputs

Outputs

Parameters

Filter Modes

Lowpass (LP)

Lowpass frequency response

• Passes frequencies below the cutoff
• Removes high frequencies, creating a "darker" or "warmer" sound
• Most common filter type for synthesis
• At 12dB/octave slope (2-pole)

Use cases:

• Warming up bright oscillators
• Classic subtractive synthesis
• Bass sounds
• Removing harshness

Highpass (HP)

Highpass frequency response

• Passes frequencies above the cutoff
• Removes low frequencies, creating a "thinner" or "brighter" sound
• At 12dB/octave slope (2-pole)

Use cases:

• Removing mud/rumble
• Creating thin, airy sounds
• Hi-hat and cymbal synthesis
• Clearing space in a mix

Bandpass (BP)

Bandpass frequency response

• Passes frequencies around the cutoff
• Removes both low and high frequencies
• Width controlled by resonance

Use cases:

• Vocal/formant-like sounds
• Telephone/radio effect
• Isolating specific frequency ranges
• Wah-wah effects

Usage Tips

Basic Filtering

Connect an oscillator to soften its harmonics:

[Oscillator] ──> [Filter Input]
                 [Filter LP] ──> [VCA] ──> [Output]

• Start with cutoff around 1000 Hz
• Adjust cutoff to taste - lower = darker, higher = brighter
• Add slight resonance (0.2-0.4) for character

Filter Envelope

Create dynamic filter sweeps with an envelope:

[Keyboard] ──Gate──> [Envelope] ──> [Filter Cutoff CV]

• Set base cutoff low (200-500 Hz)
• Use positive CV Amount
• Short attack/decay creates "plucky" sounds
• Long attack creates "swelling" sounds

Filter + LFO (Wobble)

Create rhythmic filter movement:

[LFO] ──> [Filter Cutoff CV]

• Square LFO creates choppy, rhythmic effect
• Triangle/Sine LFO creates smooth wobble
• Adjust LFO rate and CV Amount for intensity

Self-Oscillation

At high resonance (near 1.0), the filter will self-oscillate, producing a sine wave at the cutoff frequency:

• Set resonance to ~0.95 or higher
• No input signal needed
• Control pitch via Cutoff CV
• Useful for pure sine tones and sound effects

Note: Self-oscillation can be loud - reduce output level first.

Tracking Keyboard

Make filter cutoff follow the keyboard:

[Keyboard] ──V/Oct──> [Oscillator V/Oct]
           ──V/Oct──> [Filter Cutoff CV]

This keeps the filter's relative brightness consistent across different pitches. Set CV Amount to achieve 1:1 tracking.

Parallel Filter Modes

Use multiple outputs simultaneously for complex sounds:

[Oscillator] ──> [Filter Input]
                 [Filter LP] ──> [Mixer Ch1]
                 [Filter BP] ──> [Mixer Ch2] ──> [Output]

Blend lowpass and bandpass for unique timbres.

Resonant Accents

High resonance emphasizes the cutoff frequency:

• Creates a "peak" or "ping" at the cutoff
• Useful for acid bass lines (TB-303 style)
• Combine with filter envelope for accent effects

Notch Filter (Advanced)

Combine highpass and lowpass outputs to create a notch:

[Filter LP] ──> [Mixer] (inverted) ──┐
[Filter HP] ──> [Mixer] ─────────────┴──> [Output]

The phase relationship creates a notch at the cutoff frequency.

Connection Examples

Classic Subtractive Synth

[Keyboard] ──V/Oct──> [Oscillator] ──> [Filter] ──> [VCA] ──> [Output]
           ──Gate───> [Envelope] ──────────┬─────────────┘
                                           └──> [Filter Cutoff CV]

Acid Bass

[Sequencer] ──CV──> [Oscillator (Saw)] ──> [Filter LP] ──> [Output]
            ──Gate──> [Envelope] ──> [Filter Cutoff CV]
                                     (Resonance: 0.7-0.9)

Wah Effect

[Guitar/Audio In] ──> [Filter BP] ──> [Output]
                      [Expression Pedal] ──> [Filter Cutoff CV]

Sound Design Tips

Related Modules

• Oscillator - Primary input source
• ADSR Envelope - Modulate cutoff over time
• LFO - Create filter wobble effects
• VCA - Control filtered output level