A real-time polyphonic subtractive synthesizer written in C using SDL2 and SDL_ttf.

The project contains two implementations of the same synthesizer:

• subtractive_synth.c — simplified filter implementation
• true_subtractive_synth.c — more accurate biquad-based filter implementation

Both versions have identical UI and functionality, but differ in the internal filter processing algorithm.

• Polyphony (up to 8 simultaneous voices)
• Three oscillator waveforms:
  • Saw
  • Square
  • Triangle
• Up to 5 filters in series
• Filter types:
  • LPF (Low-pass)
  • HPF (High-pass)
  • BPF (Band-pass)
  • Notch
• Real-time waveform visualization
• On-screen piano keyboard
• Octave shifting
• Dynamic filter creation and removal

Uses a lightweight state-variable style filter:

• Fast and simple
• Good for learning purposes
• Not physically accurate
• Uses internal z1, z2 states

This version is easier to understand and modify.

Implements a proper biquad IIR filter using normalized coefficients:

• Computes b0, b1, b2, a1, a2
• Based on digital filter design equations
• More stable and realistic
• More accurate resonance behavior

This version provides a more correct subtractive synthesis approach.

Keyboard mapping:

Z S X C F V G B N J M K , L . /

These keys correspond to a piano layout shown on screen.

• Press key → note on
• Release key → note off
• 1 → Octave down
• 2 → Octave up

• TAB → Change waveform (Saw / Square / Triangle)

• + button or = key → Add filter
• Click filter → Select filter
• Q → Change filter type
• UP / DOWN → Adjust cutoff frequency
• LEFT / RIGHT → Adjust resonance
• BACKSPACE → Remove selected filter

• ESC → Exit

• SDL2
• SDL2_ttf
• C compiler (GCC / Clang)

brew install sdl2 sdl2_ttf

sudo apt install libsdl2-dev libsdl2-ttf-dev

The project uses a single Makefile.

Inside the Makefile, the source file is selected via the SRC variable.

Example:

# Choose one:
SRC = subtractive_synth.c #true_subtractive_synth.c

To build:

make        

This will compile the code and produce the executable. To run the program:

./subtractive_synth        

(or the name defined in the Makefile)

Or u can build and run the programm with one command:

make run

To remove compiled files and start fresh, you can use:

make clean

• Sample rate: 44100 Hz
• Mono output
• Maximum 8 voices
• Maximum 5 filters in series
• The font path is currently set to a macOS system font (/System/Library/Fonts/Supplemental/Arial.ttf). Update the path if running on another OS.