bass-foundry/docs/ideas.md

# Plugin ideas

A running list of plugin ideas for riddim, dubstep and bass-music sound design.
Each entry names the problem first. Entries are grouped as released or idea. Names follow the metalwork theme of the Bass Foundry suite.

## Problems seen in bass-music production

1. Every bass patch brings its own sub level and phase, so the weight of a drop
   jumps between switch-ups, and layered patches cancel in the sub.
2. Patches at the same peak level still differ in perceived loudness and
   brightness. The usual fix is hand-set gain or clipping per patch.
3. Resampling is repetitive: bounce, chop, trim, declick, normalise, find the
   root key, name, save, load into a sampler.
4. Writing wobbles: LFO shapes are drawn, synced, copied and edited for every
   variation. Triplet and dotted patterns plus per-note changes take long.
5. Unison, OTT and stereo distortion widen the mids. Parts disappear in mono or
   on club systems.
6. Kick and bass clash. Sidechain helps, but the sub phase at each note start
   changes how the kick hits.
7. Distortion chains build up harsh resonances. (Patina covers this.)
8. Comparing a drop with a reference: the difference is usually sub/mid balance
   over time, which generic meters do not show clearly.
9. Fixed-frequency tools act differently on every note. An EQ boost at 300 Hz
   lifts the 4th harmonic of one note and the 7th of another; a distortion with
   a fixed tone filter sounds brighter on low notes than on high notes.
10. Layered bass patches partly cancel in the mids, not only in the sub.
11. Texture makes a riddim patch: grit, rattle, metal, vowel, noise. It takes long
    chains (distortion, comb, formant, OTT, resample, repeat), and the result
    changes from note to note.
12. Grit and tone cannot be shaped separately. More distortion for more grit
    also changes the body, the level and the low end.
13. Resampled textures sound printed: a loop of one moment, with no movement
    inside the note.
14. Texture changes inside a note (the switch-up within one held note) need
    automation lanes or chopping.
15. There is no meter for texture. Two switch-ups can sound "the same" without
    a clear reason, and generic analysers show only level and spectrum.
16. OTT and multiband compression use fixed band edges, so the same setting
    squashes different harmonics on every note.
17. Riddim masters rely on clipping. A clipper that also clips the sub turns
    the low end into distortion.
18. Resampled basslines have no MIDI, so tools that need the notes have nothing
    to follow.
19. A clean sub carries the note on a club system but vanishes on phones and
    laptops. Distortion makes the note readable there, but also changes the
    patch.
20. Detuned layers (reese) beat at a rate that changes with every note and every
    harmonic, so the movement is never in time with the track.
21. Kick and sub on the same MIDI tick are not heard at the same moment: the
    heard hit depends on the attack shapes and on the offset between them.

## Signal flow and suite audit (2026-11)

How the released plugins fit into one production:

```text
WRITE      Rivet (motif → storyboard) ──MIDI──► synth
DESIGN     synth → Lathe → Temper → Knurl → Patina
RESAMPLE   Crucible (capture → slice → export) ──► sampler / wavetable synth ──► DESIGN
LAYER      layer A + layer B → Weld (mids) → bass bus → Anvil (sub)
MIX        gap: loudness, clipping, OTT, mono check, kick against sub, reference
```

Writing and sound design are covered. Mixing and mastering have only Anvil and
Patina. Findings from the audit:

- **Latency adds up.** A full chain (Knurl 71 + Temper 67 + Lathe 67 + Patina
  about 45 + Anvil 80 ms) is about 330 ms. FL Studio compensates it for
  playback, but a chain this long cannot be played live.
- **MIDI mode keeps the lookahead.** `Tracker::latency()` depends on the pitch
  range only. With MIDI as the source, the note is known at its attack, so the
  lookahead could be close to zero. This needs a latency change at runtime,
  which must be checked in FL Studio first.
- **Knurl confuses every tracker after it.** Measured: Knurl output fed into
  `TrackerConfig::BASS`. From C2 (65 Hz) upwards, Double, Fry (the default),
  Gallop and Triple make the next tracker read the note one octave (Triple:
  an octave and a fifth) too low for about 85 % of the note. Below C2 the false
  sub-octave is under 30 Hz, out of range, so low notes are safe. Level and time
  jitter also cause 5–25 false note starts per second. Anvil on the bus after
  Knurl then plays the sub an octave low and restarts its phase. Until there is
  a shared tracker: put Knurl after the other tracked plugins, and use MIDI as
  the source for any tracked plugin after Knurl. See [Signal chain](chain.md).
- **Each tracked plugin needs its own MIDI route.** Four plugins in MIDI mode
  are four FL routing setups.
- **Weld and delay compensation are untested together.** If the main layer's
  chain has more latency than the sidechain layer and FL does not compensate
  the sidechain route, the main layer arrives late, which an allpass cannot
  fix. Put Weld after every plugin with latency on that track.
- **Crucible lost takes on reload.** Takes are not saved with the project.
  Fixed: Keep takes writes every capture's loop to disk at once.
- **Settings handling differs.** Patina has presets and A/B; the others do not.
  Lathe's drawn gains and Knurl's drawn pattern cannot be automated.

## Research findings (2026-11)

From the papers in `research/` (local, not committed). Each finding names where
it goes in the suite. They are starting points to test by ear, not rules.

- **A published bass fatness model.** Schmedeke's dissertation (Hamburg 2021,
  "BASS DROP!", section 5.4.3) models momentary fatness of dance-music excerpts
  over 50 ms windows from two inputs only, with adjusted R² 0.773:
  `l = 10 / (1 + 4.5487·exp(0.13·(62.9366 − SPL<200)))`,
  `h = 10 / (1 + 1.4317·exp(2.5575·(0.3299 − HR<200)))`,
  `Fat = 0.14689·l·h` (0 to 14.7). SPL<200 is the C-weighted level below
  200 Hz in dB(C) with the material normalised to −23 LUFS; HR<200 is the
  MPEG-7 harmonic ratio (how periodic) of the same band. Chapter 4's earlier
  linear model on synthetic tones (R² 0.597) points the same way: a tonal low
  band is fatter, a noisy one is not. Anvil's clean sine maximises HR; Quench
  keeps it when the top is clipped. The thesis does not state the dB(C) of its
  reference, so the dBFS-to-dB(C) offset is an assumption. Measured with
  `scripts/fatness.py` on the seven Devath references (offset 100 dB): drops
  sit at 4–6, breakdowns near 0, kick gaps show as 50 ms dips; SPL<200 medians
  are 67–69 dB(C), a few dB under the thesis's "mostly 70–80", so the offset
  may be 2–5 dB low. Goes into Assay as one lane, never as an overall score.
- **Virtual bass from known harmonics.** Moliner, Rämö and Välimäki (DAFx 2020)
  boost the harmonics that exist with weights that follow the sound's spectral
  envelope (limits 10 dB/oct below, 3 dB/oct above) and generate the missing
  ones, so a small speaker suggests the fundamental. Their nonlinear stage fails
  intermodulation tests, so they use it on transients only. The suite knows f0,
  so it can add harmonics at exactly k·f0 with no intermodulation (problem 19).
  Anvil's sub is a pure sine, so Lathe has no harmonics there to boost.
- **Group-delay audibility.** Liski, Mäkivirta and Välimäki (2021): between
  500 Hz and 4 kHz, listeners hear a group-delay peak from about 0.64 ms
  (positive) or 0.56 ms (negative) on clicks over headphones; a fast-decaying
  hi-hat hides it. Weld's allpass sections add positive group delay there.
  Done: Weld shows the delay spread (released in 0.3.0). Measured: common fits from
  Weld's own tests spread 0.82–1.03 ms, above the click threshold, so the
  display gives a note, not a warning.
- **Timbral tension.** Farbood and Price (2017): more roughness, inharmonicity
  and spectral flatness raise judged tension; brightness (centroid) gives no
  clear result. These are the measures planned for Loupe, and the controls a
  tension macro would move.
- **Coupled one-knob controls.** A 2026 distance renderer derives five distance
  cues from one control, so they always agree; in its listening test it roughly
  halved placement error and every participant preferred it. A pattern for
  macros in Forge.
- **Shared motion binds layers.** Temporal coherence (Elhilali et al. 2009) is
  a grouping cue: sub, body and top sound like one sound when their main
  movements line up. One modulation source inside Forge gives that by
  construction; across separate `.clap` files it needs the shared memory
  rejected below.
- **Heard timing of layered hits.** Danielsen et al. (2026): attack shape and
  the offset between two layers together set when the compound hit is heard
  and whether it fuses. Hove et al. (2014): timing deviations are heard more
  clearly in lower voices, so the bass sets the grid (problem 21).
- **Loopback FM.** Smyth (DAFx 2026): an oscillator that frequency-modulates
  itself shifts its own sounding pitch as feedback rises. Her closed-form
  solution gives the sounding frequency, so a growl can stay on the tracked
  note at any feedback.
- **Beating depends on note and harmonic.** For two layers detuned by ratio r,
  harmonic k beats at k·f0·(r − 1) Hz (Plomp and Levelt 1965 for the roughness
  context). Fixed cents beat faster up the keyboard; fixed Hz beats the same on
  every harmonic (problem 20).
- **Do not encode as rules:** a best sub frequency, crossover or kick/bass
  offset; one number for loudness, roughness, groove and preference; random
  jitter sold as humanisation; more harmonics sold as more physical bass;
  largest correlation sold as the best alignment.

## Released

### Anvil — sub anchor (problems 1, 6)

Replaces the band below a split frequency with a clean mono sine that follows
the bass pitch, at one level and one start phase for every note. Pitch comes
from detection with 80 ms lookahead or from MIDI. See [Anvil](anvil/README.md).

### Crucible — resampler (problem 3)

Records the input continuously. One click captures the last bars on the host
bar grid, slices on a grid or on hits, trims, declicks, normalises, detects the
root key and exports labelled WAVs, a full loop and an optional Serum/Vital
wavetable. See [Crucible](crucible/README.md).

### Lathe — harmonic EQ (problem 9)

EQ bands placed on harmonics 1–32 of the detected note instead of fixed
frequencies. "+4 dB on harmonic 3, −6 dB on harmonic 7" sounds the same on F1
and on C2. Odd/even balance, tilt over harmonic number, and a Focus control
that lifts or cuts everything between the harmonics. Uses Anvil's pitch
tracker, or MIDI. See [Lathe](lathe/README.md).

### Temper — note-tracked distortion (problem 9)

A waveshaper whose tone filters before and after the shaper follow the note, so
every note distorts with the same character and the same harmonic content.
Input level is normalised before the shaper, so drive does not depend on how
loud the patch is, and the output is gain-matched. Oversampled to keep aliasing
(which is never harmonic) out. See [Temper](temper/README.md).

### Weld — layer phase alignment (problem 10)

Takes a second bass layer through the sidechain. It fits polarity and a chain
of up to four allpass sections to the measured cross-spectrum (no added
delay), so the layers sum instead of cancel. A display shows how much level the
cancellation cost and how much Weld recovered. Anvil solves the sub band; Weld
solves the mids. See [Weld](weld/README.md).

### Knurl — cycle texture (problems 11, 13)

Follows the played note and treats every cycle on its own: a repeating level
pattern (in-tune subharmonics), random level and timing per cycle, and a noise
burst per cycle tuned to the note. Amounts are counted in cycles, so the texture
is the same on every note. Slow per-harmonic drift is postponed. See
[Knurl](knurl/README.md).

### Planish — harmonic OTT (problem 16)

Upward and downward compression in four bands whose edges are harmonic numbers
of the tracked note (default 1, 2–4, 5–16, 17 and up), so the same setting
squashes the same harmonics on every note. Each band's level is measured over
one period of the note, so heavy settings do not ripple at f0. See [Planish](planish/README.md).

## Next for released plugins

- **Anvil**: *Kick lock* and the *offset audition* are done (Anvil 0.3.0, see
  [Anvil](anvil/README.md)): the kick through the sidechain ducks the sub and
  restarts its phase at an offset (problem 6); the audition steps −30 to +30 ms,
  4 kicks each, Keep takes the offset heard, and the low-band sum per offset is
  shown apart from that choice (problem 21). Measured on a 55 Hz kick burst
  against a 55 Hz sub: +2.4 dB at 0 ms, −4.6 dB at −10 ms, −5.9 dB at +10 ms,
  and +1.8 to +2.2 dB again one period away (±20 ms), so the sum alone cannot
  pick the hit. Open: a finer second sweep around the kept offset; a gate or
  sensitivity for kicks with long tails or a kick bus with other drums.
  A sub shape from sine to rounded
  triangle. A note attack counted in cycles. *Ghost harmonics* and the
  *Phone* listen are done (Anvil 0.3.0, see [Anvil](anvil/README.md)): harmonics
  2–10 above a "from" frequency, filled only where the patch lacks them, with
  the sub level unchanged (problem 19). Open for Ghost: weights that follow the
  patch's own harmonic slope instead of a fixed 1/k; measuring a new note's
  harmonics from the lookahead, so its first 60 ms do not count them as
  missing; a listening check on an 808 over a real phone.
- **Lathe**: A/B gain snapshots with one automatable Morph parameter. A
  keyswitch note that selects the drawn shape (EQ switch-ups without
  automation). A dynamic mode per harmonic.
- **Temper**: a harmonic split: harmonics 1–2 clean, 3 and up driven. More than
  one Push band. A bias that changes per cycle from Knurl's cycle clock.
- **Knurl**: velocity or note selects the pattern. Stereo grit above a split
  only. Drift (planned for 0.3).
- **Patina**: Tone guard (done, Patina 0.4.0). MIDI as its pitch source. M/S
  processing. Threshold, Ratio and Soft/Hard knee from tester feedback (done,
  unreleased 0.5.0). Next, from the same feedback round:
  - An absolute level gate (dBFS) so quiet passages are never cut, if the
    tester meant a loudness threshold rather than a prominence threshold.
  - Narrow rings on dense or noisy material are under-read: a +12 dB ring
    (Q 25) on noise reads only 3–6 dB above the floor, so it is cut by 1–3 dB.
    Two causes, measured: the joined-tops envelope lifts the floor to the noise
    peaks (about 5 dB above the noise mean), and envelope smoothing loses about
    3 dB on a narrow ring. A floor that follows Sharpness (1/3–2 octaves) was
    tried and rejected: it lost a resonant lowpass, absorbed broad bumps and cut
    plain harmonic tones. Better candidates: join tops only across deep gaps
    (harmonic spacing), or a mean-based floor for noise-like regions.
  - A finer analysis grid (1024 cells) for narrow rings at high frequencies.
- **Weld**: delay spread display (done, Weld 0.3.0). A *gentle* fit option
  that keeps the delay spread under a limit, for plucky layers. Alignment slots
  per note or per switch-up. The External
  sender/receiver fallback if FL's sidechain does not arrive.
- **Crucible**: a heard-onset marker per slice next to the technical cut, set
  by ear and written to the WAV (problem 21). A switch-up slicer that cuts where the spectrum changes, for
  legato basses. Optional retune of slices to one root. Reopening saved takes
  from the `takes` folder in the take list.
- **Rivet**: more than two sound roles, one per switch-up patch. Its MIDI
  output as the one note source for the tracked plugins.
- **Suite**: presets and A/B in `common-ui` for every plugin.

## Ideas

### Order

1. Small fixes from the audit. Done: chain-order guide
   ([Signal chain](chain.md)), Crucible Keep takes. Open: Weld test with delay
   compensation (in FL), MIDI mode without lookahead (needs an FL check that a
   latency change at runtime is accepted).
2. Patina pitch-aware mode. Done as **Tone guard** (Patina 0.4.0): bumps that
   follow the note are protected, bumps fixed in Hz are cut. Measured first:
   Patina already cut fixed formants on every note, but it also cut a boosted
   harmonic as hard (5.7 dB), at a position that moved with the note.
   Open: MIDI as the pitch source.
3. Planish (done, released 0.1.0), then Quench and Ballast (both done,
   released 0.1.0): they fill the mix and master gap.
   - In parallel, from the research findings, in this order: Weld delay
     spread (done, Weld 0.3.0); the fatness lane, as the first Assay piece
     (prototype `scripts/fatness.py`); Anvil Ghost harmonics and Phone listen
     (done, Anvil 0.3.0); Braze (done, 0.1.0); Anvil Kick lock with offset
     audition (done, Anvil 0.3.0).
4. Forge (done, unreleased 0.1.0): Lathe, Temper and Knurl behind one tracker.
5. Assay, one plugin for the display-only ideas.
6. Texture plugins (Anneal, Swarf, Etch, Bellows) and the note-keyed sequencer.

Merges: Gauge, Loupe and Drop compare share one analysis core and one display,
so they become one plugin, **Assay**. The wobble sequencer, Damascus and Chase
are all "step grid, note selects the pattern", so they become one sequencer.

### Forge — one tracked channel strip (problems 9, 11, latency)

One tracker, one latency, and Lathe, Temper, Knurl and later Anneal as modules
in an order you choose. Two further reasons from the research findings: one
modulation source for all modules moves sub, body and top together, which
binds them into one sound; and coupled macros become possible, for example a
*Tension* knob that raises roughness (Knurl), inharmonicity and noise (Anneal)
together at matched loudness, measured by the same descriptors as Loupe. It removes the stacked lookahead, the trackers that
disagree (Knurl's sub-octave cannot confuse a tracker that reads the input
before Knurl) and the repeated MIDI routing. The standalone plugins stay.
Sharing one tracker between separate `.clap` files would need named shared
memory across DLLs, which is fragile; one strip is simpler.

Built (unreleased 0.1.0, see [Forge](forge/README.md)). Lathe, Temper and Knurl
became modules (a `Tracked` trait in common-dsp: `ahead`, `event`, `process`);
each standalone plugin is now its tracker plus its module, Lathe and Temper
bit-exact against the old engines, Knurl one sample later (its interpolation
needs one newer sample) and equal within float rounding. In the strip one
tracker reads the input; a per-sample record of the tracker's events, note state
and input is replayed to each module, delayed by the latency of the modules in
front of it. Findings:

- One module alone in Forge matches its standalone plugin sample for sample, in
  all six orders (tested).
- Knurl's Double-style pattern (0, −12 dB) in front of Lathe: standalone Lathe
  after Knurl read the note an octave low in 149 of 149 frames on an A2 saw;
  Lathe inside Forge read A2 in 149 of 149.
- Latency 72 ms against 206 ms for the three in a row (48 kHz).
- Temper's Normalize looks one lookahead ahead; in the strip it reads the strip
  input, not the modules in front of it. Fine while those change every note's
  level alike.

Next for Forge: the *Tension* macro over Knurl (and later Anneal) at matched
loudness; Planish and Braze as modules (both are tracked, and Braze after
Knurl is the next tracker that hears the sub-octave); a short crossfade when
the order changes; one shared modulation source (per-note envelope or LFO in
cycles) that every module can read.

### Braze — tempo-locked beating (problem 20)

Brazing joins two metals with a filler. Braze adds a detuned copy of the bass
whose beating is in time with the track on every note. With the tracked f0, it
sets the detune ratio for each note so that a chosen harmonic beats at a
tempo-synced rate: r = 1 + B/(k·f0), for example an eighth at 140 BPM is
B = 4.67 Hz. A second mode adds a frequency-shifted copy, so every harmonic
beats at the same rate. Controls: rate (synced), target harmonic, depth,
spread across harmonics, mode. It reuses the tracker, and Weld should not
"fix" this beating: put Weld before Braze.

Built (released 0.1.0, see [Braze](braze/README.md)). The detune is a delay
line whose delay falls one period per beat cycle and jumps back one period,
which is seamless on a periodic note, and the delay follows the host's beat
position, so the peaks land on the grid. Harmonic n then beats at n/k · B: every
harmonic is on the grid, so the planned spread control was not needed. A Low
cut (LR4 on the copy, the bass split by the same crossover) keeps the sub out.
Open: the frequency-shift mode. Mixed with the dry bass, a plain single-sideband
shift gives every harmonic the same envelope at the same moment, which is
tremolo; it needs a dispersive allpass on the copy to stagger the harmonics
before it sounds like movement. Also open: a listening check on a real reese,
and how Weld reads a layer after Braze.

### Arc — self-FM growl (research spike)

An arc welds with a sustained spark. Arc runs loopback FM (Smyth, DAFx 2026)
on the tracked note: a time-varying delay line modulated by its own output.
The closed-form sounding frequency lets Arc retune the carrier, so the growl
stays in key while feedback sweeps from clean to torn. Needs listening and an
aliasing check before it becomes a plugin.

### Quench — sub-safe clipper (problem 17)

Quenching hardens hot metal in one fast step. Quench clips only above a split
frequency and adds the clean sub back with its own headroom, so the master can
be clipped hard without distorting the low end. Pairs with Anvil's split.

Built (released 0.1.0, see [Quench](quench/README.md)). Design checks, made
in a prototype before the plugin:

- Splitting and giving the top the instantaneous room, clamp(top, −C − sub,
  C − sub), is exactly a plain clip of the sum: the error is the same signal.
  The sub-safe part has to come from somewhere else.
- Feeding the clip error back through 1 − lowpass diverges: the complement of
  an LR4 lowpass peaks at 1.5× near the split.
- What works: clip the top symmetrically at the ceiling minus the sub's held
  peak (no pumping at the sub's rate, the sum stays under the ceiling), then
  high-pass the clip error at the split so its products below the split go.
  Two clip-and-high-pass passes and an oversampled hard clip hold the ceiling.
  Measured: sub level +0.04 dB against −2.2 dB for a plain clip; low-band
  products −35 dB against −16 dB. Cost: 0.5–1.2 dB less RMS at the same
  ceiling.
- Where it does not help: a top whose fundamental lies within an octave of the
  split is partly in the sub band (saw at 98 Hz over a 49 Hz sub: about the same
  as a plain clip).
- Next ideas: link the channels' sub peak and offer mono below the split
  (with Gauge); give the top the room on the sub's far side too (asymmetric)
  with the error still high-passed, and measure whether that wins back the
  loudness without pumping; a true-peak ceiling.

### Scribe — audio to MIDI (problem 18)

A scribe marks lines on metal. Scribe runs the note tracker on a bass and
outputs MIDI notes and a `.mid` file. A resample from Crucible then gets its
notes back, so the other plugins can use MIDI as their source.

### Bevel — mono-safe harmonic widener (problem 5)

Pans harmonics of the note alternately (for example odd left, even right) with
complementary filters, so the mono sum stays flat. Pairs with Assay.

### Peen — note-start transient shaper

Peening hardens a surface with hammer blows. Attack and sustain shaping on
every note start, with times counted in cycles, so each switch-up gets the same
punch on every note. Could become part of Ballast.

### Ballast — switch-up loudness leveller (problem 2)

Measures short-term perceived loudness per note or per switch-up against a
target or a reference patch and adjusts gain only, so each switch-up lands at
the same loudness without compression changing its character. Earlier working
name: Hit Leveler. Pairs with Anvil: Anvil levels the sub, Ballast the whole
patch.

Built (released 0.1.0, see [Ballast](ballast/README.md)): one gain per part,
parts from note starts or the host grid (a patch change on the same note is not
a new note to the tracker), K-weighted loudness of the first 100 ms, Learn for
the reference. Found while building it:

- A note start from a pitch change comes about 20 ms early, in the gap before
  the note; a plain mean over the window then reads the part ~0.9 dB quiet.
  Fixed with 10 ms blocks and a relative gate (−20 dB under the loudest block).
- The K-weighting filters ring the last part into the next one: a near-silent
  part after a loud one read −18 LUFS instead of under −60. Fixed by weighting
  every part from rest.
- Next ideas: a brightness match next to loudness (problem 2 names both; with
  Lathe or Patina doing the EQ), a "follow" option for long notes that swell,
  and switch-up detection from timbre change instead of the grid.

### Bellows — pitch-tracked resonator

A comb or Karplus-style resonator tuned to the detected note. It turns noise,
distortion or drums into pitched metallic tone that is in key on every note.
Damping, harmonic spread, single or multiple combs.

### Gauge — low-end mono check (problem 5)

Shows phase correlation and mono-sum loss per band and per note in the bass
range, and marks the notes where widening breaks the mono sum. Optional fixes:
mono below a chosen frequency and phase rotation on the mids. Display-first, so
it is a small build. Earlier working name: Bass Mono Check.

### Wobble sequencer (problem 4)

Step patterns for volume, filter, formant and pitch with triplet and dotted
grids built in. Each MIDI note selects a different pattern, so a switch-up is a
different note in the piano roll. ShaperBox overlaps, but it has no note-keyed
patterns. No name yet.

### Fatness lane (problems 2, 8)

Momentary fatness per 50 ms from Schmedeke's model (see Research findings),
drawn as one lane next to sub/mid energy, for a patch and a reference. It
explains why two drops at the same LUFS feel different in weight: a quieter or
noisier low band lowers the value. Prototype: `scripts/fatness.py` (offline,
on rendered files). The first piece of Assay. Open: the dB(C) offset, and how
the lane reads on a single bass patch rather than a full mix.

### Drop compare (problem 8)

Loudness-matched A/B of a drop against a reference, with sub, low-mid, mid and
high energy over time: for example "your sub is 3 dB under the reference in
bars 2–4". No name yet.

## Texture ideas

These ideas target problems 11–15; Knurl, the first of them, is released above.
Most of them reuse the note tracker in
`common-dsp`, so a texture setting means the same thing on every note, the same
way Lathe and Temper work.

### Anneal — tone and grit split (problem 12)

Annealing changes how a metal behaves without changing its shape. Anneal
splits the signal into three parts: the harmonic part (energy on the harmonics
of the tracked note), the residual noise and grit between the harmonics, and
the transients. Each part has a level fader, a drive and a tilt.

- Turn the grit up 6 dB and keep the body and the sub unchanged.
- Take all the grit out of a layer and leave a clean tone for further design.
- Listen to each part solo, or send the grit to a second output for its own
  chain.

DSP: an STFT with the note tracker; harmonic masks at k·f0, with a width that
follows the pitch; a transient mask from spectral flux. Latency is one FFT
frame. The display shows the three parts in different colours on Lathe's
harmonic strip.

### Swarf — in-key granular shavings (problems 11, 13)

Swarf is the shavings that machining leaves. Swarf records the bass itself and
sprays short grains of it back as a layer. Grain lengths are whole pitch
periods, and every grain is retuned to the note playing now, so the shredded
layer stays in key across a riddim line. Controls are density, spray (position
spread back in time), grain size in cycles, reverse probability, and a
harmonic-only filter for the layer. It is useful for the "shredded metal" top
layer, and works on a resample from Crucible as well.

### Etch — texture imprint (problem 11)

Etching transfers a pattern onto metal with acid. Etch takes a texture source
through the sidechain or from a loaded sample (a metal scrape, a vocal, a field
recording) and imprints its fine structure on the bass. The bass keeps its
pitch and its harmonic levels; between and on the harmonics, the source's
noise, roughness and movement come through. A "harmonic lock" mode samples the
source's spectral envelope only at the bass harmonics, so the imprint is always
in key. It differs from a vocoder: a vocoder transfers the envelope, and Etch
transfers the surface.

### Chase — talking formants that hit harmonics (problems 11, 14)

Chasing shapes metal by hammering it from the front. Chase is a formant filter
with a 2D vowel pad (a, e, i, o, u, plus "yoi" and "wub" shapes). You draw a
path over the pad and sync it to the tempo, or let each MIDI note select a path.
The formants stay in Hz, so a vowel stays a vowel on every note. A **snap**
control moves each resonant peak onto the nearest harmonic of the tracked
note, so a narrow formant never falls into the gap between two harmonics and
vanishes on some notes. Gain-matched, so a vowel change does not change the
level.

### Damascus — texture step sequencer (problem 14)

Damascus steel is folded from layers into a visible pattern. Damascus holds up
to four texture slots (for example a formant, a comb, a sample-rate reducer and
a Knurl-type rattle) and a tempo-synced step grid that chooses a slot, or a mix
of two, per step. Steps have their own crossfade time, so changes can be hard
cuts or smears. Each slot is gain-matched, so the steps change texture and not
level. Each MIDI note can select its own pattern, which is a texture switch-up
inside one held note. This overlaps with the wobble sequencer above; both could
become one plugin.

### Loupe — texture meter (problem 15)

A loupe is the lens used to inspect engraving. Loupe measures texture per note
over time:

- roughness and fluctuation strength (psychoacoustic models);
- harmonic-to-noise ratio, which shows how gritty a sound is;
- inharmonicity, which shows how metallic a sound is;
- brightness relative to the note (harmonic centroid, not centroid in Hz).

Roughness, inharmonicity and flatness are also the three timbre measures that
raise judged tension (Farbood and Price 2017), so the same lanes show a build-up
getting tenser without a note change.

It places every switch-up of a drop as a point on a texture map. Two patches
that sit on the same point are why a drop sounds repetitive. A reference track
can be loaded for comparison. Display only, so it is a small build, and it fits
with Gauge and Drop compare.

### Further leads

- **Creative extrema stretch** for Crucible: Nielsen (DAFx 2026) drives an
  overlap-add stretch by the spacing of local extrema, a free measure of
  detail. Cheap and gritty, not transparent; a bass has few extrema per cycle,
  so the stretch stays close to cycle-synchronous.
- **Reference to preset:** an offline search over a bounded set of Lathe and
  Temper parameters that matches a reference switch-up (ST-ITO, TAPE). Large;
  needs learned audio features.