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.