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+# Phase 5: Waveform Types and Bank Decoupling - Context
+
+**Gathered:** 2026-03-26
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+Extend the synthesis oscillator to support four waveform types (sine, square, sawtooth, triangle) using bandlimited additive synthesis, and decouple the OscillatorBank from the hardcoded `ClassFreqConfigs` global and `classify.AllClasses()` iteration — making it accept an injected config map instead.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### Waveform Presets
+- **D-01:** Use bandlimited additive synthesis with 8-12 partials per waveform type. Square wave uses odd harmonics (1,3,5,...,11), sawtooth uses all harmonics (1-12), triangle uses odd harmonics with 1/n^2 amplitude rolloff. This is the standard approach for aliasing-free waveform generation.
+- **D-02:** Add a `WaveformType` enum to `FreqConfig` (`Sine`, `Square`, `Sawtooth`, `Triangle`). When waveform is set, generate the `[]HarmonicDef` from the preset formula. When waveform is unset/custom, use the existing hand-tuned `Harmonics` array.
+
+### Built-in Harmonics Migration
+- **D-03:** (Claude's Discretion) Decide whether built-in classes keep their hand-tuned HarmonicDef arrays or migrate to waveform presets. Recommended approach: keep existing harmonics as-is for v1.0 classes (preserves sound character), default them to `WaveformType = ""` (custom). Waveform presets only take effect when explicitly set via config in Phase 6.
+
+### GainPerLayer Scaling
+- **D-04:** Fix GainPerLayer now in Phase 5 — compute dynamically as `1.0 / float64(len(layers))` inside `NewBank` instead of using the hardcoded `NumLayers=14` constant. This establishes the correct foundation before Phase 7 adds dynamic class counts.
+
+### Bank Decoupling
+- **D-05:** (Claude's Discretion) Change `NewBank` to accept a `map[classify.TrafficClass]FreqConfig` parameter instead of reading the `ClassFreqConfigs` global. This is the injection seam that Phase 6 will use to pass merged config. The existing `ClassFreqConfigs` var remains as the default map.
+
+</decisions>
+
+<canonical_refs>
+## Canonical References
+
+**Downstream agents MUST read these before planning or implementing.**
+
+### Synthesis Architecture
+- `synth/oscillator.go` — Current sine-only Oscillator with phase accumulator and `Advance([]HarmonicDef)`
+- `synth/config.go` — `FreqConfig`, `HarmonicDef`, `ClassFreqConfigs` global, constants (`SampleRate`, `NumLayers`, `GainPerLayer`)
+- `synth/bank.go` — `NewBank(tau)` iterates `classify.AllClasses()` and reads `ClassFreqConfigs` global
+- `synth/layer.go` — `Layer` with EMA smoothing, uses `FreqConfig` from config.go
+
+### Research
+- `.planning/research/ARCHITECTURE.md` — Integration points and build order for v1.1
+- `.planning/research/PITFALLS.md` — Pitfall A3 (aliasing) and A6 (bank class mismatch)
+
+No external specs — requirements fully captured in decisions above.
+
+</canonical_refs>
+
+<code_context>
+## Existing Code Insights
+
+### Reusable Assets
+- `Oscillator.Advance([]HarmonicDef)` — Already supports additive synthesis via harmonic series. Waveform presets just need different `[]HarmonicDef` arrays, not a new oscillator type.
+- `FreqConfig` struct — Has `BaseHz`, `Harmonics`, `Pan`. Adding `WaveformType` field is backward-compatible.
+
+### Established Patterns
+- Phase accumulator in `Oscillator` wraps at 1.0 — all harmonic ratios are integer multiples of the fundamental.
+- `Layer` delegates to `Oscillator.Advance()` — waveform change is transparent to the layer.
+- `ClassFreqConfigs` is a package-level `var` (not `const`) — can be replaced by parameter injection without breaking existing tests.
+
+### Integration Points
+- `NewBank(tau)` → `NewBank(tau, configs map[TrafficClass]FreqConfig)` — single signature change
+- `bank.RenderWindow()` iterates `classify.AllClasses()` — must iterate `b.layers` map keys instead
+- `encode.RunSynthesis` calls `NewBank(1.0)` — will need to pass config map (Phase 6 concern, but seam established here)
+
+</code_context>
+
+<specifics>
+## Specific Ideas
+
+No specific requirements — standard bandlimited synthesis approach with 8-12 partials as user requested.
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+None — discussion stayed within phase scope.
+
+</deferred>
+
+---
+
+*Phase: 05-waveform-types-and-bank-decoupling*
+*Context gathered: 2026-03-26*
diff --git a/.planning/phases/05-waveform-types-and-bank-decoupling/05-DISCUSSION-LOG.md b/.planning/phases/05-waveform-types-and-bank-decoupling/05-DISCUSSION-LOG.md
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+# Phase 5: Waveform Types and Bank Decoupling - Discussion Log
+
+> **Audit trail only.** Do not use as input to planning, research, or execution agents.
+> Decisions are captured in CONTEXT.md — this log preserves the alternatives considered.
+
+**Date:** 2026-03-26
+**Phase:** 05-waveform-types-and-bank-decoupling
+**Areas discussed:** Waveform presets, GainPerLayer scaling
+
+---
+
+## Waveform presets
+
+### Harmonic richness
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| Bandlimited (8-12 partials) | Accurate waveform shapes, no aliasing. Standard for quality synthesis. | ✓ |
+| Lightweight (4-6 partials) | Recognizably different but softer/rounder. Less CPU. | |
+| You decide | Claude picks based on Nyquist and ambient use case | |
+
+**User's choice:** Bandlimited (8-12 partials)
+**Notes:** None
+
+### Built-in class harmonics
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| Keep current harmonics | Built-in classes retain hand-tuned arrays. Waveform presets only via config. | |
+| Migrate to sine preset | Switch to pure fundamental. Simpler but loses v1.0 character. | |
+| You decide | Claude picks best approach for preserving v1.0 sound | ✓ |
+
+**User's choice:** You decide (Claude's Discretion)
+**Notes:** None
+
+---
+
+## GainPerLayer scaling
+
+| Option | Description | Selected |
+|--------|-------------|----------|
+| Fix now in Phase 5 | Compute dynamically as 1/len(layers). Clean foundation for Phase 7. | ✓ |
+| Defer to Phase 7 | Keep NumLayers=14 constant. Fix when user classes land. | |
+| You decide | Claude picks timing based on complexity | |
+
+**User's choice:** Fix now in Phase 5
+**Notes:** None
+
+---
+
+## Claude's Discretion
+
+- Built-in class harmonics migration strategy (D-03)
+- Bank config injection API design (D-05)
+
+## Deferred Ideas
+
+None