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FM_DRUM - Two-Operator FM Percussion

Two-operator FM percussion synthesizer with dual mode operation and EEPROM parameter storage.

Controls

The FM_DRUM has two modes, each controlling 3 parameters. Toggle between modes with the button.

Mode 0 - Pitch/Ratio/Index (LED OFF)

Control Function Range Description
POT1 / A0 Pitch 30-1200 Hz Base frequency of carrier oscillator
POT2 / A1 Operator Ratio 0.5-8.0 Modulator frequency ratio (carrier × ratio)
POT3 / CV3 Modulation Index 1-10 FM depth/brightness

Mode 1 - Envelope (LED ON)

Control Function Range Description
POT1 / A0 Decay Time 0.5-10 Exponential decay rate
POT2 / A1 Ratio Envelope 0-1 Modulator ratio envelope depth
POT3 / CV3 Modulation Index 1-10 FM depth/brightness (same as Mode 0)

Jacks / Pins

Jack Function Type Description
OUT (GPIO1) Audio Output PWM Output 10-bit PWM audio at ~36.6 kHz
IN1 (GPIO7) Trigger Input Digital Input Rising edge trigger for drum hit
IN2 (GPIO0) Accent Input Digital Input HIGH = half volume (-6dB attenuation)
LED (GPIO5) Mode Indicator Digital Output OFF=Mode 0, ON=Mode 1
BUTTON (GPIO6) Mode Toggle Button Switch between modes, saves parameters

Button Function

  • Press: Toggle between Mode 0 and Mode 1
  • Auto-Save: All 6 parameters saved to EEPROM on mode change

Features

  • Two-Operator FM Synthesis:

    • Carrier + modulator architecture
    • Variable operator ratio (0.5x to 8x)
    • Adjustable modulation index (1-10)

  • Dual Mode Operation:

    • Mode 0: Sound design (pitch, ratio, index)
    • Mode 1: Envelope shaping (decay, ratio envelope, index)
    • LED indicates current mode

  • Parameter Pickup:

    • Prevents value jumping when switching modes
    • 5% threshold for smooth takeover
    • 5-sample pot smoothing/averaging

  • EEPROM Storage:

    • Saves all 6 parameters on mode change
    • Settings persist across power cycles
    • Automatic parameter recall on startup

  • Audio Processing:

    • 2-pole band-pass filter (cookbook formula)
    • Soft-clipping with tanh() waveshaping
    • 2% cosine fade-in, 10% fade-out
    • Fixed 0.3 second duration
    • 4,096 sample wavetable

Patch Recommendations

Classic 808 Tom

  • Mode 0: Pitch=150Hz, Ratio=1.5, Index=5
  • Mode 1: Decay=3, Ratio Env=0.6, Index=5
  • Use: Classic tom sound, tunable

Metallic Snare

  • Mode 0: Pitch=200Hz, Ratio=6.2, Index=8
  • Mode 1: Decay=2, Ratio Env=0.3, Index=8
  • Use: FM snare with metallic attack

Punchy Kick

  • Mode 0: Pitch=60Hz, Ratio=2.0, Index=7
  • Mode 1: Decay=4, Ratio Env=0.8, Index=7
  • Use: Electronic kick drum

Bell Tone

  • Mode 0: Pitch=400Hz, Ratio=3.5, Index=4
  • Mode 1: Decay=8, Ratio Env=0.2, Index=4
  • Use: Tuned percussion, melodic hits

Industrial Hit

  • Mode 0: Pitch=800Hz, Ratio=7.8, Index=10
  • Mode 1: Decay=1.5, Ratio Env=0.5, Index=10
  • Use: Harsh metallic percussion

Deep Bass Drum

  • Mode 0: Pitch=40Hz, Ratio=1.2, Index=6
  • Mode 1: Decay=5, Ratio Env=0.7, Index=6
  • Use: Sub bass kick

Version History

v1.0 (2025-01-17)

  • Initial release
  • Two-operator FM synthesis
  • Dual mode operation (6 total parameters)
  • EEPROM parameter storage and recall
  • Pickup feature prevents parameter jumping
  • 5-sample pot smoothing
  • 2-pole band-pass filter
  • Soft-clipping with tanh() waveshaping
  • Cosine fade-in/fade-out (2% / 10%)
  • Exponential decay envelope
  • ~36.6 kHz sample rate
  • Accent input for velocity sensitivity

Technical Specifications

  • Platform: Raspberry Pi RP2040/RP2350 (Pico 2)
  • Sample Rate: ~36.6 kHz (150 MHz / 4096)
  • Audio Resolution: 10-bit PWM (0-1023)
  • Wavetable Size: 4,096 samples
  • Duration: Fixed 0.3 seconds
  • Filter Type: 2-pole band-pass
  • Storage: EEPROM for parameter persistence

Notes

  • Mode 0 controls sound character (pitch, ratio, FM index)
  • Mode 1 controls envelope behavior (decay, ratio modulation depth)
  • Pickup feature prevents parameter jumps when switching modes
  • Parameters are saved to EEPROM automatically on mode change
  • Accent input provides -6dB attenuation for velocity sensitivity
  • Ratio envelope creates pitch sweep effect on modulator