Filter Sweeps

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, though not every composer trusted 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

In the routines

In a table-driven player the sweep is an instrument effect like pulse modulation: Cadaver describes filter tables with commands of the form “add m to cutoff frequency for n frames”. The complication is that the chip has one filter for three voices, so either only one voice controls it at a time — set by a pattern command — or the player has to arbitrate.

Unreliable by design

The 6581’s filter varies from chip to chip, and period composers knew it. Asked by Zzap!64 in February 1986 whether he had reservations about using the filters, Martin Galway said he did: “the filters are too unreliable. A brilliant result on one machine is no guarantee of satisfactory sound on another.” He expected to use them only once the C128, whose SID he thought “greatly improved”, was common. In the same interview he rejected the magazine’s earlier claim that Rob Hubbard had “stretched the SID chip to its limits” — “there are plenty of things you can do with it yet”.

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