Muon

Muon is Greybound's first envelope filter model. The reference direction is the Mu-Tron III family: a touch-sensitive filter known for vocal, percussive, organic sweeps rather than clocked modulation.

The implementation does not copy a factory schematic. It is a graybox model that captures the useful engineering behavior: high input impedance, an envelope detector, program-dependent sweep memory, a resonant band-pass filter, dry/wet blend, and low output impedance.

Baseline Assumptions

• Input impedance is high enough for guitar, compressor, or drive outputs.
• Output impedance is low enough to drive another pedal or the amp input.
• The detector belongs to private pedal state and follows guitar attack faster than it releases.
• The filter should open from pick attack and close musically as the note decays.
• Pre-amp placement is the reference rig because envelope filters usually react best before amp gain and time-based effects.

Controls

• sensitivity: envelope detector drive and sweep trigger threshold.
• range: low-to-high sweep span and release feel.
• resonance: band-pass emphasis around the moving center frequency.
• mix: dry/filter blend.
• bypass: routing state; detector and filter state remain owned by the pedal instance.

Current DSP Approximation

The current implementation uses:

• source/load input boundary and light input coupling,
• rectified sidechain with one-pole smoothing,
• fast envelope attack and slower release,
• exponential center-frequency sweep from low-mid to upper-mid range,
• state-variable band-pass filter with bounded resonance,
• dry body retention so the effect does not disappear at high mix settings,
• output lowpass and headroom guard.

This is not a SPICE model and it is not component-exact. It gives Greybound a dynamic touch filter while leaving room for future diode/op-amp/OTA or opto-style circuit validation.

Reference Direction

Useful future reference work:

• collect public Mu-Tron III topology notes and measured sweeps,
• compare low-pass, band-pass, and high-pass modes,
• add up/down sweep direction if the model needs the full original control behavior,
• validate envelope attack/release against rendered pick transients,
• measure output level across sensitivity and resonance settings,
• compare placement before and after compressor/drive pedals.

References

• Mu-Tron III family envelope filter topology and control behavior.
• Musitronics-era envelope filters using detector-driven resonant filter movement.
• State-variable filter models for controllable low-pass, band-pass, and high-pass responses.

Validation Gates

The model is not component-exact until:

• center-frequency sweep is measured across sensitivity and range settings,
• resonance is validated for stability and musical output level,
• attack and release are measured from rendered transient material,
• bypass and active output levels are matched within a useful range,
• ordering before/after compressor and overdrive is compared with renders,
• measured hardware or SPICE captures define a specific reference unit.