Signalsmith Stretch: pitch/time library

This is a C++11 library for pitch and time stretching, using the final approach from the ADC22 presentation Four Ways To Write A Pitch-Shifter.

How to use it

#include "signalsmith-stretch.h"

signalsmith::stretch::SignalsmithStretch<float> stretch;

Configuring

The easiest way to configure is .presetDefault():

stretch.presetDefault(channels, sampleRate);

If you want to test out different block-sizes etc. then you can use .configure() manually, and even change .freqWeight/.timeWeight/.channelWeight.

Processing (and resetting)

// Clears internal buffers
stretch.reset();

float **inputBuffers, **outputBuffers;
int inputSamples, outputSamples;
stretch.process(inputBuffers, inputSamples, outputBuffers, outputSamples);

// Inspect latency
int totalLatency = stretch.inputLatency() + stretch.outputLatency();

The .process() method takes anything where buffer[channel][index] gives you a sample. This could be a float ** or a double ** or some custom object.

To get a time-stretch, just hand it differently-sized input/output buffers.

Pitch-shifting

stretch.setTransposeFactor(2); // up one octave

stretch.setTransposeSemitones(12); // also one octave

You can set a "tonality limit", which uses a non-linear frequency map to preserve a bit more of the timbre:

stretch.setTransposeSemitones(4, 8000/sampleRate);

Custom pitch map

This stretcher does (fairly rough) peak-detection, and creates a non-linear frequency map based on that.

You can hook into this to define your own pitch-map, by providing a callback which is called once per channel, for every FFT block:

stretch.setMap([&](int channel) {
	for (auto &peak : stretch.peaks) {
		peak.output = peak.input*2; // up one octave
	}
});

The input/output frequencies are relative to Nyquist. It's not currently-tested what happens if your map is non-monotonic.

Compiling

Just include signalsmith-stretch.h in your build.

It's pretty slow if optimisation is disabled though, so you might want to enable optimisation just where it's used.

DSP Library

This uses the Signalsmith DSP library for FFTs and other bits and bobs.

For convenience, a copy of the license is included (with its own LICENSE.txt) in dsp/, but if you're already using this elsewhere then you should remove this copy to avoid versioning issues.

License

MIT License for now - get in touch if you need anything else.

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