Minimal audio synthesis library for non-FPU architectures

Architecture

isopod provides a set of simple and efficient DSP primitives which can be chained, added and multiplied together. They are grouped into three categories, oscillators (osc), effects (fx), and envelopes (env). The central component in isopod is the Synth structure which implements the Iterator trait and connects the signal chain to the audio stream handler. Depending on the hardware setup it can also implement the rodio::source::Source trait. The next() function in Synth returns the next sample and updates the internal state. It also contains the definition of the signal chain:

// Sawtooth signal with frequency modulated low pass filter
fn next() -> Option<i16> {
    self.sin.set_freq(Hertz(5));
    let f = Hertz(100) + Hertz(30) * self.sin.next();
    self.low_pass.set_freq(f);
    Some(self.low_pass.next(self.sawtooth.next()))
}

Primitives

The chain can consist of the following primitives (unchecked is not yet implemented).

• Oscillators (Osc)
  • Wavetable oscillators
    • Sine
    • Arbitrary
  • Algorithmic
    • Saw
    • Square
    • Chaos
  • Noise
    • WhiteNoise
    • PinkNoise
    • BitFlipNoise
    • CrackleNoise
• Effects (FX)
  • Linear filters (small buffer sizes)
    • LPF (part of the state variable filter)
    • BPF (part of the state variable filter)
    • HPF (part of the state variable filter)
    • Notch (part of the state variable filter)
  • Delays and Reverbs (requires large buffer )
    • Delay
    • Reverb
• Envelops (Env)
  • Envelops
    • LinExp (parametrized piecewise linear exponential decay)
    • AD
    • ADSR
    • Arbitrary

Signal formats

There are three integer and one float signal format in isopod.

i32

Most common data type on MCUs. Common and efficient also on CPUs. Dynamic range of 192 dB is more than enough for most purposes.

i16

Dynamic range of 96 dB is adequate for most purposes.

i8

Constrained dynamic range of 48 dB is only relevant for lo-fi applications on 8-bit microcontrollers.

f32

Only usable if your architecture has an FPU. Dynamic range extends to 1529 dB which is far above audio requirements. Some computations are more efficient with f32 and FPU, others are less efficient as compared to integer operations.

Other formats

frequency

Frequencies are represented in Hz oder mHz and implemented as u32. For some internal operations frequencies have to be multiplied and hence some 64 bit operations are necessary to avoid overflow.

Representing the frequency as u16 for increased performance would reduce the frequency resolution to almost 1 Hz and is therefore not acceptable.

Project status