Filter+FX

Filter+FX is a standalone elekloader mods for the Elektron Digitakt mk1 and Digitone mk1 / Keys. The Digitakt mod adds seven per-track filter/FX modes alongside the stock filter, each using the track's own FREQ, RESO and filter envelope. The Digitone port is in filter-fx-dn/ (TYPE 4..10, with VOWEL/BANK AUX1/AUX2 on page 2). The FLTR page is redrawn with a custom TYPE-box icon and a dedicated graph for every mode.

TYPE	mode	controls
8	VOWEL	FREQ = vowel path i-e-a-o-u; RESO = formant width
9	BANK	FREQ = bank shift; RESO = Q
10	FOLD	FREQ = drive 1–8×; RESO = fold bias
11	CRUSH	FREQ = sample-hold rate; RESO = bit depth
12	RING	FREQ = carrier frequency; RESO = dry/ring depth
13	PHASER	FREQ = LFO rate; RESO = depth
14	FLANGER	FREQ = LFO rate; RESO = depth/feedback

Screenshots

Every shot is the real FLTR page from digiemu, not a mock-up.

mode	icon	FLTR page
VOWEL
BANK
FOLD
CRUSH
RING
PHASER
FLANGER

Notes

• VOWEL is SynVOW's FormantEngine model: F1–F3 from the male vowel table plus a fixed F4 at 3300 Hz, blended in the log-frequency domain.
• BANK is four fixed resonators at 250/500/1000/2000 Hz, shifted together.
• FOLD is a triangle folder; CRUSH is sample-rate reduction + 1–16 bit quantisation.
• RING is a sine carrier ring modulator.
• PHASER is four cascaded first-order allpasses with an LFO; its output saturates at full scale so transient allpass overshoot cannot wrap.
• FLANGER is a short LFO-modulated fractional delay, mixed dry + delayed, with feedback in the delay line.
• The stock filter envelope moves the FREQ position, so it also modulates the vowel position, bank shift, fold drive, crush rate, ring frequency and the phaser/flanger LFO rate.
• Stock TYPE 0–7 still run the stock filter and keep the stock curve.

Second FLTR page

VOWEL and BANK reuse the second FLTR page for two extra controls, in the same way COMB/TRASH do in the parent DigiFilter. Value 0 is the neutral/default sound.

mode	C encoder	G encoder	what it does
VOWEL	SIZE	F4	SIZE scales F1–F4 up to +1 octave; F4 sets the 3300 Hz band from 1× to 2×
BANK	SPRD	TILT	SPRD widens the bank spacing up to +1 octave on the top band; TILT goes from dark to bright

Each dial sweeps clockwise from 9:00 (value 0) through 12:00 to 7:00 (value 127).

The values are mirrored into the low bytes of Base/Width so they survive a kit save/load. Other Filter+FX modes leave the second page stock.

Cost

Per voice per 32-frame block (MCF54418, tests/analog_bench.py):

mode	static	swept	vs stock (static / swept)
VOWEL	3074 / ~3513 cyc	5082 / ~6371 cyc	+28 % / +112 %
BANK	3074 / ~3515	5376 / ~7009	+28 % / +124 %
FOLD	452-548 / ~559-799	same	-77..-81 %
CRUSH	408 / ~522	same	-83 %
RING	668 / ~930	same	-72 %
PHASER	2681 / ~3740	same	+12 %
FLANGER	2019 / ~2371	same	-16 %
stock filter (reference)	~2400 insn	-	1.00x

The vs stock column compares instructions per voice/block with the stock filter (~2400). VOWEL/BANK use four independent biquads, one eMAC accumulator per band. FOLD/CRUSH/RING/PHASER/FLANGER have no coefficient ramp. FLANGER allocates 256 samples per voice as 16-bit (4 KB).

Build

export PATH="<m68k-elf bin>:$PATH"
export ELEKLOADER_CROSS=m68k-elf-
export PYTHONPATH="<elekloader checkout>"

python -m elekloader.sdk.build . --stock <Digitakt_OS1.54.syx>
python -m elekloader.lint  out/filter-fx-1.2-os1.54.elemod \
    --stock <Digitakt_OS1.54.syx> --with <core-4.0-os1.54.elemod>

The Digitone port builds from filter-fx-dn/ against Digitone_OS1.44.syx with core-dn1-4.0-os1.44.elemod. tools/build_release.py builds, lints, patch-checks and packages both machines at once.

Install

python -m elekloader.patch --stock Digitakt_OS1.54.syx \
    --mod core-4.0-os1.54.elemod --mod releases/elemods/filter-fx-1.2-dt.elemod \
    --out Digitakt_OS1.54-filter-fx.syx --version 1.2a

Then flash the .syx with Elektron Transfer. For the Digitone use Digitone_OS1.44.syx, core-dn1-4.0-os1.44.elemod and releases/elemods/filter-fx-dn-1.2.elemod.

Sources

file
filter.c	the Digitakt adapter, models, biquad ramps and per-voice entry
fx_dsp.c	the shared Filter+FX DSP used by the DN port
filter-fx-dn/	Digitone mk1/Keys adapter, manifest, UI and notes
biquad_dsp.s	four independent biquads in one eMAC pass
filter_glue.s	TYPE label formatter and per-voice dispatch
filter_ui.c	response/transfer/modulation graphs and the TYPE-box art
biquad_tables.h	RBJ band-pass coefficients, Q30 (tools/gen_biquad_tables.py)
analog_tables.h	vowel formant positions and bank centres (tools/gen_analog_tables.py)
mod_tables.h	sine/LFO/ring-mod tables (tools/gen_mod_tables.py)
tests/analog_bench.py	instruction/cycle bench
tests/analog_smoke.py	bounded/non-silent check for all modes
tests/digiemu_filter_fx_page2.py	second-page layout/descriptor/encoder check
tests/digiemu_filter_fx_shots.py	regenerates the screenshots above
HANDOFF.md	1.1a state, verification, artifacts and open items

Licensing

Filter+FX is GPL-2.0-or-later. The code is derived from DigiFilter; the VOWEL formant data is SynVOW's own work, used with permission. The Digitakt screenshots remain Elektron's. See THIRD_PARTY.md for the full notice.

Credits

• Digital Alchemy DSP — the mod, the SVF/biquad kernels and the Digitakt UI.
• The formant model is adapted from SynVOW's FormantEngine (ER-301 MouthTowel lineage).

No Elektron firmware is included. GPL-2.0-or-later; see LICENSE.