Filter Sweeps
Moving through the frequency spectrum
Filter sweeps create dynamic timbral changes by gradually adjusting filter cutoff frequency, a signature effect of the C64's SID chip.
Overview
Filter sweeps move a synthesizer’s filter cutoff frequency over time, creating timbral motion from bright to dark or vice versa. The SID chip’s multimode resonant filter made sweeps a signature sound of C64 music. Composers like Rob Hubbard exploited filter modulation extensively, creating sounds impossible on chips without filtering.
Fast facts
- Definition: gradual filter cutoff change.
- Effect: timbral brightness variation.
- SID advantage: built-in resonant filter.
- Programming: update filter registers over time.
- Uses: bass growls, pad evolution, transitions.
SID implementation
Programming filter sweeps:
- Register $D418: filter cutoff low bits.
- Register $D416: filter cutoff high bits.
- Resonance: $D417 controls filter resonance.
- Timing: update each frame for smooth sweep.
Musical uses
When composers used sweeps:
- Bass sounds: growling, evolving bass lines.
- Pad textures: slowly changing atmospheres.
- Transitions: filtering in or out.
- Emphasis: dramatic moments.