# DeepPluck V0.3.0 Filter Engine - Implementation Report

## 1. Overall Result

Status: **partial**

The complete automatable scope of V0.3.0 is implemented and verified. DeepPluck now has one per-voice nonlinear four-pole low-pass, pre-filter Drive, resonance, key tracking, bipolar Filter Envelope amount, a second independent live envelope, parameter smoothing, and a click-free Filter bypass. Debug and Release builds and all nine CTest suites pass.

Listening acceptance, visible GUI inspection, and Cubase host validation were not performed in this automated session and remain explicitly manual. No claim of final subjective or host acceptance is made.

## 2. Starting Point

- Repository: `D:\0A_VST_Development`
- Starting commit: `d902144` (`Document V0.2.0b implementation result`)
- Confirmed implementation parent: `e2bc67f` (`DeepPluck V0.2.0b First Musical Pluck`)
- Initial tracked diff: clean
- Initially untracked and protected:
  - `CubaseTest/`
  - `Docs/DeepPluck_V0.2.0a_Master_Prompt_Rev1.txt`
  - `Docs/DeepPluck_V0.2.0b_MasterPrompt_Rev1.txt`
  - `Docs/DeepPluck_V0.3.0_Filter_Engine_MasterPrompt_Rev1.txt`
- The two historical Rev1 prompts and `CubaseTest/` were not changed or staged.
- Baseline Debug: 7/7 suites passed.
- Baseline Release: 7/7 suites passed.

The Sound Manifest, AGENTS.md, README, architecture, V0.2.0b specification/report, build file, production sources, parameter/state/preset path, and tests were reviewed before implementation.

## 3. Scope

Implemented:

- one 24 dB/octave low-pass model
- per-voice filter state and processing
- Cutoff, Resonance, pre-filter Drive, Key Tracking, and bipolar Env Amount
- independent live Filter Envelope using the existing `DeepPluckEnvelope`
- immediate Filter Release from Attack, Decay, and Sustain
- live shortening and extension of an existing Filter Release
- 20 ms continuous-parameter smoothing and 10 ms bypass crossfade
- complete APVTS/state/preset/GUI integration
- V0.1/V0.2 state migration
- expanded automated coverage, headroom, and stress validation

Deliberately not implemented:

- oscillator 2
- noise/transient generator
- unison or stereo voice spreading
- additional filter types/slopes
- LFO or general modulation matrix
- delay, reverb, chorus, distortion effect, compression, or limiting
- oversampling subsystem
- preset browser, visualizer, meters, or GUI redesign system

## 4. Files

New:

- `Source/Synth/DeepPluckFilter.h`
- `Source/Synth/DeepPluckFilter.cpp`
- `Docs/DeepPluck_V0.3.0_Filter_Engine_Implementation_Report.md`

Changed:

- `CMakeLists.txt`
- `Source/Parameters/DeepPluckParameters.h`
- `Source/Parameters/DeepPluckParameters.cpp`
- `Source/Synth/DeepPluckVoice.h`
- `Source/Synth/DeepPluckVoice.cpp`
- `Source/Presets/DeepPluckPresets.h`
- `Source/Presets/DeepPluckPresets.cpp`
- `Source/PluginEditor.h`
- `Source/PluginEditor.cpp`
- `Tests/TestMain.cpp`
- `README.md`
- `ARCHITECTURE.md`

Included as the normative milestone document:

- `Docs/DeepPluck_V0.3.0_Filter_Engine_MasterPrompt_Rev1.txt`

Intentionally untouched:

- `JUCE/`
- `CubaseTest/`
- the two older untracked Rev1 prompts
- system VST3 directories

## 5. Architecture and Signal Flow

Each of the fixed sixteen voices owns one oscillator, one filter, one Amp Envelope, and one Filter Envelope:

```text
PolyBLEP Saw
    -> dry branch ----------------------------------+
    -> pre-filter Drive -> 24 dB Low-Pass ----------+ 10 ms crossfade
          Cutoff = base + key tracking + Filter Envelope
    -> Amp Envelope -> Velocity -> 0.08 Voice Gain
voice sum -> 20 ms smoothed Output Gain -> stereo output
```

The APVTS cache binds all sixteen atomic parameters outside the hot path. The processor loads one validated trivially-copyable snapshot per block. Voices read the snapshot directly without IDs, strings, ValueTree work, or atomics.

The filter remains processed while bypassed so its state is warm when enabled. Voice ownership, sample-accurate MIDI subdivision, and the existing deterministic stealing order are unchanged.

## 6. Filter Algorithm

`DeepPluckFilter` implements a nonlinear four-stage ladder low-pass. For sample rate `fs` and cutoff `fc`:

```text
pole = exp(-2 * pi * fc / fs)
g = 1 - pole
forward = 0.76923076923 * g
history = 0.23076923076 * g
feedback = 3.8 * resonance * tanh(stage4)
```

The driven input minus bounded feedback enters four cascaded pole recurrences; the fourth-stage output is multiplied by 1.2. Four poles provide the nominal 24 dB/octave stopband. The deterministic test measured **24.2165 dB/octave**.

Cutoff is bounded to `20 Hz .. min(20 kHz, 0.45 * sampleRate)`. Resonance is normalized 0..1. The `tanh()` feedback term and bounded input transfer prevent unbounded feedback growth without a limiter or hard output clamp.

Measured isolated maximum-resonance/maximum-drive sweep peak: **1.23405**, finite and bounded.

## 7. Drive, Key Tracking and Modulation

Drive is an odd, DC-symmetric rational pre-filter transfer:

```text
linearGain = 10^(driveDb / 20)
shape = 0.35 * (linearGain - 1)
driven = input * (1 + shape) / (1 + shape * abs(input))
```

At 0 dB, `shape` is zero and Drive is exactly linear. The shape ramps for 20 ms.

Cutoff is calculated per sample in octave space:

```text
base = log2(smoothedCutoffHz)
key = ((midiNote - 60) / 12) * smoothedKeyTracking
env = smoothedEnvAmountOctaves * filterEnvelopeValue
cutoffHz = 2^(base + key + env)
```

MIDI note 60 is neutral. Key Tracking 100% moves one cutoff octave per keyboard octave. Positive Env Amount opens and negative Env Amount closes the filter. Automated tests cover all three amount polarities and key-tracking endpoints.

## 8. Filter Envelope

Each voice owns a second independent `DeepPluckEnvelope`; no DSP state machine was duplicated. Amp and Filter instances have separate state and parameters but the same recurrence, target ratios, live-update rules, and stage semantics.

Verified:

- Note-Off during Filter Attack enters Release immediately.
- Note-Off during Filter Decay enters Release immediately.
- Sustain-to-Release uses the same existing contract.
- Release starts at the current Filter Envelope value.
- A shorter live Release shortens an existing tail.
- A longer live Release extends an existing tail.
- Filter Envelope completion does not clear an Amp-active voice.
- Amp and Filter lifecycle state remain independent.

Normal Note-Off sends both envelopes to Release at the exact MIDI offset. The Amp Envelope remains the voice-lifecycle authority.

## 9. Bypass and Smoothing

- Cutoff: 20 ms, smoothed in `log2(Hz)` space
- Resonance: 20 ms linear
- Drive shape: 20 ms linear after one per-segment dB conversion
- Key Tracking: 20 ms linear
- Env Amount: 20 ms linear
- Filter Enabled: 10 ms linear dry/filter crossfade
- Tune: existing 20 ms
- Output Gain: existing 20 ms after the voice sum

Repeated bypass changes continue from the current mix value. Filter Off was verified to preserve the V0.2.0b dry signal independently of otherwise extreme filter settings. Filter state is kept current during bypass.

Host parameters remain block-snapshotted by the established architecture; MIDI remains sample accurate.

## 10. Parameters, State and Compatibility

Existing stable IDs are unchanged:

- `outputGain`, `masterTune`
- `ampAttack`, `ampDecay`, `ampSustain`, `ampRelease`

New stable IDs appended:

- `filterEnabled`
- `filterCutoff`
- `filterResonance`
- `filterDrive`
- `filterKeyTracking`
- `filterEnvAmount`
- `filterAttack`
- `filterDecay`
- `filterSustain`
- `filterRelease`

Plugin identity is unchanged:

- product/plugin: DeepPluck
- vendor: ToneTrax
- bundle: `de.tonetrax.deeppluck`
- manufacturer code: `Tntx`
- plugin code: `DpV1`
- no input, stereo output, MIDI input, no MIDI output

The numeric version is 0.3.0. The generated VST3 module manifest reports DeepPluck, ToneTrax, version 0.3.0.

State tests verify:

- V0.1 Output/Tune retention with missing ADSR/filter defaults
- V0.2 six-parameter retention with filter defaults only for missing children
- full sixteen-parameter V0.3 roundtrip
- invalid/mismatched tree safety through the existing merge contract

## 11. Presets and GUI

The four preset names remain unchanged. Each now sets all sixteen parameters explicitly:

- Deep Pluck: APVTS defaults; 1800 Hz, 18% resonance, 3 dB Drive, 35% tracking, +2.50 octaves
- Soft Pluck: 1100 Hz, 12% resonance, 2 dB Drive, 30% tracking, +1.75 octaves
- Tight Pluck: 2400 Hz, 22% resonance, 4 dB Drive, 45% tracking, +3.20 octaves
- Long Pluck: 1500 Hz, 25% resonance, 3.5 dB Drive, 40% tracking, +2.20 octaves

Filter-envelope times are individually matched to each preset role. Deep Pluck exactly equals APVTS defaults. Any changed Amp or Filter value produces `Custom`.

The editor is now 1180 x 760 with restrained sections for Output/Amp Envelope, Filter, and Filter Envelope. All controls use APVTS attachments. Bypassed filter controls remain attached and editable but are visually softened.

Visible GUI inspection: **MANUAL VERIFICATION REQUIRED**

## 12. Realtime Review

The audio path contains:

- no allocation/free
- no locks or waits
- no file, network, logging, or GUI access
- no ValueTree, strings, or parameter-ID lookup
- no collection growth
- one parameter-cache snapshot per block
- fixed sixteen voices and fixed-size DSP state

Envelope coefficient work remains event/segment-based. Drive dB conversion occurs once per voice-render segment. Audio-rate Filter Envelope modulation requires one `exp2()` and the ladder pole requires one `exp()` per active voice/sample. This work is bounded and directly serves smooth cutoff modulation.

No limiter, compressor, hard output clip, or hidden global gain correction was added.

## 13. Builds and Artifacts

Fresh configuration:

```powershell
cmake --fresh -S . -B build -G "Visual Studio 18 2026" -A x64
```

Final builds:

```powershell
cmake --build build --config Debug --parallel
cmake --build build --config Release --parallel
```

Results:

- Debug build: PASS
- Release build: PASS
- no source compiler errors in final builds
- local artifact-copy targets: PASS

Artifacts verified:

- `Artifacts/Debug/DeepPluck.vst3`
- `Artifacts/Debug/DeepPluck.exe` (27,084,288 bytes)
- `Artifacts/Release/DeepPluck.vst3`
- `Artifacts/Release/DeepPluck.exe` (8,948,224 bytes)

VST3 binary sizes:

- Debug: 24,698,368 bytes
- Release: 7,766,528 bytes

No system plugin directory was modified.

## 14. Automated Tests

Final Debug CTest: **9/9 passed**, 0.76 s total.

Final Release CTest: **9/9 passed**, 0.33 s total.

Suites:

1. `DeepPluck.parameters` - PASS
2. `DeepPluck.oscillator` - PASS
3. `DeepPluck.envelope` - PASS
4. `DeepPluck.filter` - PASS
5. `DeepPluck.filter_envelope` - PASS
6. `DeepPluck.synthesiser` - PASS
7. `DeepPluck.sample_accuracy` - PASS
8. `DeepPluck.matrix` - PASS
9. `DeepPluck.smoothing_stability` - PASS

Key measured/verified results:

- stopband slope: 24.2165 dB/octave
- isolated maximum Filter stress peak: 1.23405
- immediate Filter Attack/Decay Note-Off: PASS
- live shorter/longer Filter Release: PASS
- 100% Key Tracking octave relation: PASS
- positive/zero/negative amount contracts: PASS
- dry bypass identity: PASS
- 10 ms bypass and 20 ms cutoff smoothing: PASS
- 44.1/48/96 kHz and 1/32/64/127/512-sample matrix: PASS
- repeated 1/4/8/16-voice lifecycle stress: PASS

## 15. Listening Acceptance

The session did not monitor audio. The following remain open:

- Default First Impression: **MANUAL VERIFICATION REQUIRED**
- Cutoff sweep: **MANUAL VERIFICATION REQUIRED**
- Resonance sweep: **MANUAL VERIFICATION REQUIRED**
- Drive character: **MANUAL VERIFICATION REQUIRED**
- positive Filter Envelope: **MANUAL VERIFICATION REQUIRED**
- negative Filter Envelope: **MANUAL VERIFICATION REQUIRED**
- Note-Off during Filter Attack: **MANUAL VERIFICATION REQUIRED**
- Note-Off during Filter Decay: **MANUAL VERIFICATION REQUIRED**
- live Filter Release shorter: **MANUAL VERIFICATION REQUIRED**
- live Filter Release longer: **MANUAL VERIFICATION REQUIRED**
- Filter bypass A/B: **MANUAL VERIFICATION REQUIRED**
- Key Tracking across registers: **MANUAL VERIFICATION REQUIRED**
- polyphony and voice stealing: **MANUAL VERIFICATION REQUIRED**
- all factory presets: **MANUAL VERIFICATION REQUIRED**
- complete Sound Manifest listening judgment: **MANUAL VERIFICATION REQUIRED**

The corresponding DSP/event contracts pass automated tests, but that is not presented as a listening result.

## 16. Cubase Validation

No plugin was installed into a system directory and Cubase was not launched.

- Plugin discovery and same-ID recognition: **MANUAL VERIFICATION REQUIRED**
- V0.2 project load and filter-default migration: **MANUAL VERIFICATION REQUIRED**
- V0.3 save/close/reopen: **MANUAL VERIFICATION REQUIRED**
- automation of all filter controls: **MANUAL VERIFICATION REQUIRED**
- MIDI timing and short-note Filter Release: **MANUAL VERIFICATION REQUIRED**
- factory presets and Custom: **MANUAL VERIFICATION REQUIRED**
- visible GUI/DPI inspection: **MANUAL VERIFICATION REQUIRED**
- realtime/offline audio: **MANUAL VERIFICATION REQUIRED**
- multiple instances and lifecycle: **MANUAL VERIFICATION REQUIRED**

## 17. Headroom and Performance

Measured Release peaks at 48 kHz:

- default sustained 16-voice test: **0.435055**
- Deep Pluck preset, 16 voices: **0.222464**
- Soft Pluck preset, 16 voices: **0.157052**
- Tight Pluck preset, 16 voices: **0.413761**
- Long Pluck preset, 16 voices: **0.157656**
- 16-voice maximum resonance/Drive/modulation stress: **0.294181**

Every factory result remains below the required 0.95 peak without limiting.

Functional and stability renders cover one, four, eight, and sixteen voices; 44.1, 48, and 96 kHz; and block sizes 1, 32, 64, 127, and 512. Release CTest completed in 0.33 s on the confirmed local machine. This is a realtime/CPU plausibility result, not a controlled DAW CPU benchmark.

A reproducible host CPU percentage benchmark across audio devices was not performed: **MANUAL VERIFICATION REQUIRED**

## 18. Sound Manifest Review

The implementation is intentionally one focused filter rather than a filter collection. Its defaults and presets emphasize reduced brightness, midrange presence, controlled resonance, soft Drive, natural envelope motion, dynamics, and conservative headroom. The dry signal remains available for honest click-free A/B comparison. No effect, compressor, limiter, or feature-count expansion hides the source.

Automated evidence supports stability, smooth movement, tonal attenuation, useful modulation polarity, and dynamics. Final judgments of warm, soft, rounded, elegant, emotional, and organic remain listening questions: **MANUAL VERIFICATION REQUIRED**

Mandatory Development Question:

> Does this make DeepPluck a better instrument for emotional Deep House plucks?

Engineering answer: **Yes**. It adds the focused subtractive stage explicitly required to soften, animate, and focus the dry Pluck while preserving dynamics and simplicity. Final subjective confirmation remains pending listening acceptance.

## 19. Regression Review

Automated PASS:

- unchanged plugin identity fields in CMake/module metadata
- VST3 and Standalone build in Debug and Release
- fixed 16 voices and one sound
- sample-accurate MIDI dispatch
- PolyBLEP oscillator regression suite
- Amp Envelope timing/live updates/immediate Note-Off
- shorter and longer live Amp Release
- deterministic quiet-release-first voice stealing
- linear Velocity scaling
- Tune smoothing without phase reset
- Output smoothing after voice sum
- state migration and roundtrip
- four presets and Custom matching
- deterministic silence without voices
- existing seven suites plus two new suites
- local artifact convention

Filter Off preserves the dry V0.2.0b voice path within tight numerical tolerance after the internal crossfade reaches dry.

Host-level regression remains part of Cubase manual validation.

## 20. Known Limitations and Technical Debt

- Listening, visible GUI, and Cubase acceptance are still manual.
- Host parameters are loaded once per block rather than from sample-accurate host automation queues.
- Audio-rate modulation uses bounded transcendental math per active voice/sample; a controlled DAW CPU benchmark is still needed.
- PolyBLEP reduces aliasing but is not brick-wall band limiting.
- Drive has no oversampling; the deliberately bounded range and warm default reduce but do not mathematically eliminate nonlinear aliasing.
- The fixed development GUI is functional, calm, and grouped, but not final product artwork.
- Exactly one low-pass model is intentional, not missing scope.

## 21. Git

The implementation and report are intended as focused local commits after the final automatable verification. No push is performed. `CubaseTest/` and both old untracked Rev1 prompt files remain excluded.

Final commit hashes are added to the user-facing completion summary after commits are created.

## 22. Design Review

**Did the implementation improve musical quality?**

By design and measurable tonal behavior, yes: the instrument now has a smooth per-note subtractive contour, musical tracking, bounded resonance, and warm Drive. Subjective confirmation remains manual.

**Did complexity increase, and was it justified?**

Yes. One focused DSP class, one reused envelope instance, five parameter smoothers, and ten parameters implement a named musical contract. The growth is local, bounded, documented, and tested.

**Does it follow the Sound Manifest?**

Yes by architecture: one strong sweet-spot-oriented filter, dry quality, dynamics, headroom, calm GUI grouping, no effects, and no general-purpose feature expansion.

**Would a musician immediately hear or feel the improvement?**

The filter contour, darker default balance, Drive, and tracking are designed to be immediately audible. Actual listening confirmation is **MANUAL VERIFICATION REQUIRED**.

**Keep, refactor, or revert?**

Keep pending manual listening/Cubase acceptance. Automated results identify no architectural or stability reason to revert.

## 23. Next Milestone Recommendation

After V0.3.0 manual acceptance, make the next milestone a focused **Velocity-to-Filter Expression** refinement: one carefully bounded velocity influence on filter-envelope amount or cutoff, without a modulation matrix, new oscillator, unison, noise, or effects. This directly strengthens playable emotional expression while keeping the instrument specialized.
