PicoSynth

A lightweight software synthesizer designed for resource-constrained environments.

Features

• Q15 fixed-point arithmetic (no floating point required)
• Modular node-based architecture
• Multiple voice polyphony
• ADSR envelope generators
• Waveform generators: sine, saw, square, triangle, noise
• Low-pass and high-pass filters
• Soft clipper for output limiting

Architecture

Each voice contains up to 8 nodes that can be:

• Oscillators (with waveform function pointer)
• ADSR envelopes
• LP/HP filters
• Mixers (up to 3 inputs)

Nodes are wired together via pointers, allowing flexible signal routing.

Usage

Building and Running

make           # Build the example program
make run       # Build and run (produces output.wav)
make check     # Run unit tests
make clean     # Remove generated files

Example Program

The example program (tests/example.c) plays "Happy Birthday" with a piano-like timbre and writes output.wav (11025Hz, 16-bit mono):

make run
# Produces: output.wav (piano melody)

You can select different melodies:

make MELODY=twinkle run    # Play "Twinkle Twinkle"
make list-melodies         # Show available melodies

Unit Tests

Run the test suite to verify the synthesizer is working correctly:

make check
# Output: === Test Summary ===
#         Passed: 626
#         Failed: 0

Tests cover Q15 arithmetic, waveform generators, envelope processing, and synthesizer core functionality.

Integrating the Synth

The PicoSynth provides a C API for integration into your projects:

#include "picosynth.h"

/* Create synthesizer with 2 voices, 8 nodes per voice */
picosynth_t *s = picosynth_create(2, 8);

/* Get voice and nodes */
picosynth_voice_t *v = picosynth_get_voice(s, 0);
picosynth_node_t *env = picosynth_voice_get_node(v, 0);
picosynth_node_t *osc = picosynth_voice_get_node(v, 1);

/* Initialize nodes */
picosynth_init_env(env, NULL,
    &(picosynth_env_params_t){.attack=5000, .decay=500, .sustain=Q15_MAX/2,
                              .release=500,
                             });
picosynth_init_osc(osc, &env->out, picosynth_voice_freq_ptr(v),
                   picosynth_wave_sine);
picosynth_voice_set_out(v, 1);

/* Play notes */
picosynth_note_on(s, 0, 60);   /* Middle C */
q15_t sample = picosynth_process(s);
picosynth_note_off(s, 0);

/* Cleanup */
picosynth_destroy(s);

Key Functions

• picosynth_create(voices, nodes): Create synthesizer
• picosynth_destroy(s): Free resources
• picosynth_note_on(s, voice, midi_note): Trigger note
• picosynth_note_off(s, voice): Release note
• picosynth_process(s): Generate one sample
• picosynth_init_osc(node, gain, freq, wave): Initialize oscillator
• picosynth_init_env(node, gain, atk, dec, sus, rel): Initialize envelope
• picosynth_init_lp(node, gain, input, coeff): Initialize low-pass filter
• picosynth_init_hp(node, gain, input, coeff): Initialize high-pass filter
• picosynth_init_mix(node, gain, in1, in2, in3): Initialize mixer

Waveform Generators

• picosynth_wave_sine - Sine wave
• picosynth_wave_saw - Rising sawtooth
• picosynth_wave_square - Square wave
• picosynth_wave_triangle - Triangle wave
• picosynth_wave_falling - Falling ramp
• picosynth_wave_exp - Exponential decay
• picosynth_wave_noise - White noise

For a complete example, see tests/example.c which demonstrates a piano-like timbre.

License

picosynth is available under a permissive MIT-style license. Use of this source code is governed by a MIT license that can be found in the LICENSE file.