REAPER JSFX Synthesis

Patterns for building synthesizers and virtual instruments in JSFX. Covers oscillators, envelopes, polyphony, and MIDI instrument handling.

Requires: [reaper-jsfx-core](../reaper-jsfx-core/SKILL.md) for language fundamentals.

Rules

Official Documentation

• JSFX Programming Reference
• JSFX MIDI Functions

External References

• Tale's JSFX Libraries - polyblep, adsr, midi_queue
• PolyBLEP Oscillators

Key Principles

1. Mark as Instrument

desc:My Synthesizer
tags:instrument synthesizer

// For instruments: no audio input, MIDI input expected
in_pin:none
out_pin:left output
out_pin:right output

2. Anti-Alias Oscillators

Naive oscillators create aliasing. Use PolyBLEP or wavetable:

// BAD: Naive saw (aliases)
phase += freq / srate;
phase >= 1 ? phase -= 1;
output = phase * 2 - 1;

// GOOD: PolyBLEP saw (anti-aliased)
phase += freq / srate;
phase >= 1 ? phase -= 1;
output = phase * 2 - 1;
// Apply PolyBLEP correction at discontinuities
t = phase / dt;  // Normalized position
t < 1 ? output -= polyblep(t);

3. Sample-Accurate MIDI

For tight timing, process MIDI at sample level:

@block
while(midirecv(offset, msg1, msg2, msg3)) (
  // Store event with sample offset
  midi_queue_add(offset, msg1, msg2, msg3);
);

@sample
// Check for events at this sample
while(midi_queue_peek(current_sample) >= 0) (
  midi_queue_get(msg1, msg2, msg3);
  process_midi(msg1, msg2, msg3);
);
current_sample += 1;

4. Efficient Polyphony

Allocate voice arrays once, manage with indexes:

@init
MAX_VOICES = 8;
voice_note = 0;
voice_gate = MAX_VOICES;
voice_env = MAX_VOICES * 2;
// ... allocate all voice state arrays

freembuf(MAX_VOICES * 10);  // Total state size

Quick Reference

Minimal Synth Template

desc:Simple Mono Synth
tags:instrument synthesizer

in_pin:none
out_pin:left output
out_pin:right output

slider1:attack_ms=10<1,1000,1:log>Attack (ms)
slider2:decay_ms=100<1,1000,1:log>Decay (ms)
slider3:sustain=0.7<0,1,0.01>Sustain
slider4:release_ms=200<1,2000,1:log>Release (ms)

@init
note = -1;
gate = 0;
env = 0;
phase = 0;

@slider
attack_coeff = exp(-1 / (attack_ms * srate / 1000));
decay_coeff = exp(-1 / (decay_ms * srate / 1000));
release_coeff = exp(-1 / (release_ms * srate / 1000));

@block
while(midirecv(offset, msg1, msg2, msg3)) (
  status = msg1 & $xF0;

  status == $x90 && msg3 > 0 ? (  // Note On
    note = msg2;
    velocity = msg3 / 127;
    gate = 1;
    freq = 440 * pow(2, (note - 69) / 12);
    dt = freq / srate;
  ) : (status == $x80 || (status == $x90 && msg3 == 0)) && msg2 == note ? (
    gate = 0;
  );
);

@sample
// Simple saw oscillator
phase += dt;
phase >= 1 ? phase -= 1;
osc = phase * 2 - 1;

// ADSR envelope
gate ? (
  env < 1 ? (
    env = attack_coeff * env + (1 - attack_coeff);
  ) : (
    env = decay_coeff * env + (1 - decay_coeff) * sustain;
  );
) : (
  env = release_coeff * env;
);

// Output
output = osc * env * velocity * 0.3;
spl0 = spl1 = output;