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docs/engine_modulation.md

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+# Modulation
+
+Modulation effects vary parameters over time using LFOs or envelopes.
+
+## Vibrato
+
+Vibrato modulates pitch with an LFO.
+
+```forth
+saw 5 vib 0.5 vibmod .       ( 5 Hz, 0.5 semitone depth )
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `vib` | Hz | LFO rate |
+| `vibmod` | semitones | Modulation depth |
+| `vibshape` | shape | LFO waveform (sine, tri, saw, square) |
+
+## Pitch Envelope
+
+The pitch envelope applies an ADSR to the oscillator frequency.
+
+```forth
+sine 100 freq 24 penv 0.001 patt 0.1 pdec .
+```
+
+| Parameter | Description |
+|-----------|-------------|
+| `penv` | Envelope depth in semitones |
+| `patt` | Attack time in seconds |
+| `pdec` | Decay time in seconds |
+| `psus` | Sustain level (0-1) |
+| `prel` | Release time in seconds |
+
+## Glide
+
+Glide interpolates between pitch changes over time.
+
+```forth
+saw c4 0.1 glide .           ( 100ms glide )
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `glide` | seconds | Glide time |
+
+## FM Synthesis
+
+FM modulates the carrier frequency with a modulator oscillator.
+
+```forth
+sine 440 freq 2 fm 2 fmh .   ( modulator at 2× carrier frequency )
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `fm` | 0+ | Modulation index |
+| `fmh` | ratio | Harmonic ratio (modulator / carrier) |
+| `fmshape` | shape | Modulator waveform |
+
+FM has its own envelope (`fme`, `fma`, `fmd`, `fms`, `fmr`).
+
+## Amplitude Modulation
+
+AM multiplies the signal by an LFO.
+
+```forth
+pad 4 am 0.5 amdepth .       ( 4 Hz tremolo )
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `am` | Hz | LFO rate |
+| `amdepth` | 0-1 | Modulation depth |
+| `amshape` | shape | LFO waveform |
+
+## Ring Modulation
+
+Ring modulation multiplies two signals, producing sum and difference frequencies.
+
+```forth
+saw 150 rm 0.8 rmdepth .     ( ring mod at 150 Hz )
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `rm` | Hz | Modulator frequency |
+| `rmdepth` | 0-1 | Modulation depth |
+| `rmshape` | shape | Modulator waveform |
+
+## Phaser
+
+Phaser sweeps notches through the frequency spectrum using allpass filters.
+
+```forth
+pad 0.5 phaser 0.6 phaserdepth .
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `phaser` | Hz | Sweep rate |
+| `phaserdepth` | 0-1 | Sweep depth |
+| `phasersweep` | cents | Sweep range |
+| `phasercenter` | Hz | Center frequency |
+
+## Flanger
+
+Flanger mixes the signal with a short modulated delay (0.5-10ms).
+
+```forth
+pad 0.3 flanger 0.7 flangerdepth 0.5 flangerfeedback .
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `flanger` | Hz | Modulation rate |
+| `flangerdepth` | 0-1 | Modulation depth |
+| `flangerfeedback` | 0-0.95 | Feedback amount |
+
+## Chorus
+
+Chorus uses multiple modulated delay lines with 120° phase offset for stereo width.
+
+```forth
+pad 1 chorus 0.4 chorusdepth 20 chorusdelay .
+```
+
+| Parameter | Range | Description |
+|-----------|-------|-------------|
+| `chorus` | Hz | Modulation rate |
+| `chorusdepth` | 0-1 | Modulation depth |
+| `chorusdelay` | ms | Base delay time |