Filter Sweeps

Overview

A filter sweep moves a synthesiser's cutoff frequency over time, producing timbral motion from bright to dark or vice versa. The SID's state-variable analogue filter — running on the same die as the digital oscillators — made sweeps a signature sound of C64 music. Composers like Rob Hubbard, Martin Galway, and Tim Follin built their identity around modulating it.

Fast facts

Definition: gradual change of filter cutoff frequency.
Effect: timbral brightness variation, often with a resonant peak.
SID advantage: built-in resonant state-variable filter (LP/BP/HP) per chip.
Programming: rewrite cutoff registers each frame from an IRQ.
Uses: bass growls, evolving pads, transitions, percussion fakes.

SID filter registers

The SID's filter is configured by four registers; the cutoff is not at $D418. Per the 6581/8580 register map:

Register	Field	Purpose
`$D415` (FCLO)	bits 2:0	Filter cutoff, low 3 bits
`$D416` (FCHI)	bits 7:0	Filter cutoff, high 8 bits
`$D417` (RES/FILT)	bits 7:4	Resonance (0–15)
	bit 3	Route external audio in through filter
	bit 2	Route voice 3 through filter
	bit 1	Route voice 2 through filter
	bit 0	Route voice 1 through filter
`$D418` (MODE/VOL)	bits 6:4	LP/BP/HP mode bits (mix-selectable)
	bit 7	Voice 3 off (cuts V3 from output)
	bits 3:0	Master volume (0–15)

The cutoff is an 11-bit value: cutoff = (FCHI << 3) | (FCLO & 7). Voices with their $D417 bit clear bypass the filter and go straight to the mixer.

Filter modes

$D418 bits 4-6 are mix-selectable, so combinations are legal:

Bits set	Result
LP only (bit 4)	Low-pass — bright → dark
BP only (bit 5)	Band-pass — emphasises a frequency range
HP only (bit 6)	High-pass — dark → bright
LP + HP	Notch (band-reject)
LP + BP + HP	Approximately all-pass — filter effectively bypassed

Setting all bits clear silences any voice routed through the filter.

A simple downward sweep

Update the cutoff each frame from an IRQ:

sweep_cutoff:    .word $07F8        ; start near max cutoff
sweep_step:      .word $0030        ; per-frame decrement

update_filter:
    ; cutoff -= step
    sec
    lda sweep_cutoff
    sbc sweep_step
    sta sweep_cutoff
    lda sweep_cutoff+1
    sbc sweep_step+1
    sta sweep_cutoff+1

    ; bottom 3 bits → FCLO, top 8 bits → FCHI
    lda sweep_cutoff
    and #$07
    sta $d415
    lda sweep_cutoff+1
    sta $d416
    rts

For a routed-and-resonant sweep, set $D417 once with the desired voice routes and resonance:

    lda #$F1          ; resonance %1111, voice 1 routed through filter
    sta $d417
    lda #%00010000    ; LP only, master volume 0 — set volume separately
    ora #$0F          ; volume = 15
    sta $d418

Musical uses

Application	Effect
Bass growls	Slow LP cutoff sweep over a sustained bass note
Pad textures	Gentle BP sweep across a held chord
Transitions	Cutoff ramp synced to a section change
Percussion fakes	Snap the cutoff down with high resonance to imitate hi-hats and snares
Wobble bass	LFO-style triangle-wave sweep — the precursor to dubstep wobble

6581 vs 8580

The two SID variants behave very differently in the filter:

	6581 (12V, original)	8580 (9V, revised)
Cutoff curve	Wildly per-chip non-linear	Near-linear, predictable
Distortion	Audible at high signal levels	Mostly clean
Resonance	Less pronounced	Stronger, more peaky
Sound	"Warm", "dirty"	"Clean", "crisp"

A piece written for the 6581 — most pre-1986 SID music — often sounds wrong on an 8580 because the cutoff curve and distortion are fundamental to how the music was tuned. Modern players (reSIDfp, libsidplayfp) approximate per-chip cutoff curves to compensate.

Moving through the frequency spectrum