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Polyphonic Synthesizer

This project is intended to be a polyphonic synthesizer for use in embedded microcontrollers. It features multi-voice synthesis for up to 16 channels, with channels outputting either a fixed DC offset, sinusoidal output or whitenoise.

The phase or amplitude of one channel may be modulated by the output of another channel, allowing for various effects.

Configuration

There are two ways to configure this library:

Using C preprocessor constants

The following C preprocessor definitions should be defined in your project Makefile.

  • _POLY_NUM_CHANNELS: The number of polyphonic channels (voices) that you wish to instantiate. Each channel occupies 16 bytes.
  • _POLY_FREQ: The output sample rate for the polyphonic synthesizer in Hz.

Using linker symbols

Alternatively, these things can be decided at run-time. Your application should export the following symbols:

  • const uint8_t poly_num_channels: The number of polyphonic channels currently configured.
  • const uint16_t poly_freq: The output sample rate for the polyphonic synthesizer in Hz.
  • const uint16_t poly_freq_max: The maximum output frequency for the polyphonic synthesizer. This should be set to poly_freq/2 (the nyquist frequency).
  • struct poly_voice_t poly_voice[]: The array of voice channels.

You may declare functions using these symbols, or you may use linker aliasing to expose variables/structures with alternate names.

Usage

Global structures

A global counter, poly_remain, counts down the number of audio samples remaining before the next set of events are due to be loaded. When this variable reaches 0, you should start calling poly_load with new data or call poly_reset to stop playback.

Functions

poly_reset clears the state of the polyphonic synthesizer, cancelling any in-progress playback. This should be done as part of your program initialisation.

poly_load is used to load in the next event. Events are covered below.

poly_next returns the next audio sample, or 0 if there is no more audio left to be played.

Events

The structure of the event system is loosely based on ideas from the MIDI standard. Essentially, an "event" is a parameter change for a given channel.

Each event is represented by a 4-byte data structure, a struct poly_evt_t which has two fields, flags and value. poly.h covers each of the events. See the comments up the top of that file for detail.