# Phase 2: Audio Filters

**Status:** 🔨 In Progress
**Parent plan:** [PLAN.md](./PLAN.md)
**Domain:** backend

## Objective
Implement all 11 audio filters and register them with the AudioFilterRegistry.

## Tasks

- [x] Task 1: **Channel Extract** filter (`core/audio/filters/channel_extract.py`)
  - Options: `channel` (select: mono | left | right)
  - Stateful: No
  - Behavior: Replaces main rms/spectrum with selected channel data. If "mono", averages L+R. If "left"/"right", copies that channel's data to the main fields.
- [x] Task 2: **Band Extract** filter (`core/audio/filters/band_extract.py`)
  - Options: `band` (select: bass | mid | treble | custom), `freq_low` (float, 20-20000), `freq_high` (float, 20-20000)
  - Stateful: No
  - Behavior: Computes a band mask for the 64 log-spaced bins, applies it to spectrum, recomputes RMS from in-band data. Reuse logic from existing `core/audio/band_filter.py`.
  - Presets: bass=20-250Hz, mid=250-4000Hz, treble=4000-20000Hz
- [x] Task 3: **Peak Hold** filter (`core/audio/filters/peak_hold.py`)
  - Options: `decay_rate` (float, 0.1-50.0, dB/s), `per_bin` (bool, default true)
  - Stateful: Yes
  - Behavior: For each spectrum bin (if per_bin) or for rms/peak, retains the maximum value seen and decays it over time. Outputs the max of current value and held peak.
- [x] Task 4: **Gain** filter (`core/audio/filters/gain.py`)
  - Options: `factor` (float, 0.1-10.0, default 1.0)
  - Stateful: No
  - Behavior: Multiplies rms, peak, spectrum, and per-channel values by factor. Clamps to [0, 1] for rms/peak.
- [x] Task 5: **Noise Gate** filter (`core/audio/filters/noise_gate.py`)
  - Options: `threshold` (float, 0.0-1.0), `hysteresis` (float, 0.0-0.2, default 0.05)
  - Stateful: No (hysteresis is stateless — it's a secondary threshold, not temporal)
  - Behavior: If rms < threshold, zeros out all levels and spectrum. Hysteresis means: if gate was open and rms drops below (threshold - hysteresis), close it; if gate was closed and rms rises above threshold, open it.
  - Actually stateful for hysteresis tracking: needs to remember gate open/closed state.
- [x] Task 6: **Envelope Follower** filter (`core/audio/filters/envelope_follower.py`)
  - Options: `attack_ms` (float, 1-500, default 10), `release_ms` (float, 10-2000, default 200)
  - Stateful: Yes
  - Behavior: Smooths rms and peak with asymmetric time constants. When signal rises, uses attack rate. When signal falls, uses release rate. Applied per-bin to spectrum optionally.
  - Fast attack + slow release = punchy transients that fade smoothly.
- [x] Task 7: **Spectral Smoothing** filter (`core/audio/filters/spectral_smoothing.py`)
  - Options: `factor` (float, 0.0-0.99, default 0.5)
  - Stateful: Yes (maintains previous spectrum state)
  - Behavior: Applies exponential moving average per-bin: `smoothed[i] = factor * prev[i] + (1-factor) * current[i]`. Higher factor = smoother/slower.
- [x] Task 8: **Compressor** filter (`core/audio/filters/compressor.py`)
  - Options: `threshold` (float, 0.0-1.0, default 0.5), `ratio` (float, 1.0-20.0, default 4.0), `makeup_gain` (float, 0.0-2.0, default 1.0)
  - Stateful: Yes (envelope tracking for gain reduction)
  - Behavior: When signal exceeds threshold, reduces by ratio. `output = threshold + (input - threshold) / ratio`. Apply makeup_gain after. Applied to rms, peak, and spectrum.
- [x] Task 9: **Inverter** filter (`core/audio/filters/inverter.py`)
  - Options: none (or `invert_spectrum` bool, default true)
  - Stateful: No
  - Behavior: `rms = 1.0 - rms`, `peak = 1.0 - peak`, spectrum bins inverted if option set. Beat fields unchanged.
- [x] Task 10: **Beat Gate** filter (`core/audio/filters/beat_gate.py`)
  - Options: `hold_ms` (float, 10-500, default 50) — how long to hold signal after beat
  - Stateful: Yes (tracks last beat timestamp)
  - Behavior: When beat detected, passes signal through for `hold_ms` milliseconds. Between beats, zeros out rms/peak/spectrum. Beat fields themselves always pass through.
- [x] Task 11: **Delay** filter (`core/audio/filters/delay.py`)
  - Options: `delay_ms` (float, 10-2000, default 100)
  - Stateful: Yes (ring buffer of AudioAnalysis snapshots)
  - Behavior: Buffers incoming AudioAnalysis snapshots and outputs the one from `delay_ms` ago. Ring buffer sized based on ~30Hz update rate.
- [x] Task 12: Register all 11 filters in `core/audio/filters/__init__.py`
- [x] Task 13: Update Noise Gate to be stateful (hysteresis requires gate state tracking)

## Files to Modify/Create
- `core/audio/filters/channel_extract.py` — **create**
- `core/audio/filters/band_extract.py` — **create**
- `core/audio/filters/peak_hold.py` — **create**
- `core/audio/filters/gain.py` — **create**
- `core/audio/filters/noise_gate.py` — **create**
- `core/audio/filters/envelope_follower.py` — **create**
- `core/audio/filters/spectral_smoothing.py` — **create**
- `core/audio/filters/compressor.py` — **create**
- `core/audio/filters/inverter.py` — **create**
- `core/audio/filters/beat_gate.py` — **create**
- `core/audio/filters/delay.py` — **create**
- `core/audio/filters/__init__.py` — **modify** — register all filters

## Acceptance Criteria
- All 11 filters are implemented and registered
- Each filter correctly transforms AudioAnalysis according to its specification
- Stateful filters (peak hold, envelope follower, spectral smoothing, compressor, beat gate, delay, noise gate) properly maintain and reset state
- Filter option schemas are complete and accurate
- All filters are accessible via `GET /api/v1/audio-filters`

## Notes
- 6 stateful filters: peak hold, envelope follower, spectral smoothing, compressor, beat gate, delay. Noise gate is also stateful due to hysteresis.
- Band extract can reuse math from existing `core/audio/band_filter.py` — `compute_band_mask()` and `apply_band_filter()`
- Filters must produce a NEW AudioAnalysis (immutability principle), not mutate the input
- For delay filter, ring buffer size = `delay_ms / (1000 / update_rate)`. At 30Hz, 2000ms delay = 60 slots.

## Review Checklist
- [ ] All tasks completed
- [ ] Code follows project conventions
- [ ] No unintended side effects
- [ ] Build passes
- [ ] Tests pass (new + existing)

## Handoff to Next Phase

### What was built
- All 11 audio filters implemented, each in its own file under `core/audio/filters/`
- 7 stateful filters (peak_hold, noise_gate, envelope_follower, spectral_smoothing, compressor, beat_gate, delay) with proper `is_stateful` and `reset()` implementations
- 4 stateless filters (channel_extract, band_extract, gain, inverter)
- All filters registered in `__init__.py` via import-triggered `@AudioFilterRegistry.register`
- All filters produce NEW AudioAnalysis via `dataclasses.replace()` (immutability preserved)
- Band extract reuses existing `compute_band_mask()` and `apply_band_filter()` from `core/audio/band_filter.py`

### What Phase 3 needs to know
- All 11 filters + the `audio_filter_template` meta-filter are now registered in the AudioFilterRegistry (12 total)
- `GET /api/v1/audio-filters` will return all filters with their option schemas
- Filters are instantiated via `AudioFilterRegistry.create_instance(filter_id, options)`
- Stateful filters need per-stream instances (not shared) due to internal state
- The `process()` method signature is `process(analysis: AudioAnalysis) -> AudioAnalysis`

### Known deviations from plan
- None. All 11 filters implemented exactly as specified plus Task 13 (noise gate stateful).