Signalsmith Stretch: C++ 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.

It can handle a wide-range of pitch-shifts (multiple octaves) but time-stretching sounds best for more modest changes (between 0.75x and 1.5x). There are some audio examples on the main project page.

How to use it

#include "signalsmith-stretch.h"

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

Configuring

The easiest way to configure is a .preset???() method:

stretch.presetDefault(channels, sampleRate);
stretch.presetCheaper(channels, sampleRate);

If you want to test out different block-sizes etc. then you can use .configure() manually.

stretch.configure(channels, blockSamples, intervalSamples);

You can query the current configuration:

int block = stretch.blockSamples();
int interval = stretch.intervalSamples();
int inputLatency = stretch.inputLatency();
int outputLatency = stretch.outputLatency();

Processing (and resetting)

// Clears internal buffers
stretch.reset();

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

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.

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);

Alternatively, you can set a custom frequency map, mapping input frequencies to output frequencies (both normalised against the sample-rate):

stretch.setFreqMap([](float inputFreq) {
	return inputFreq*2; // up one octave
});

Time-stretching

To get a time-stretch, hand differently-sized input/output buffers to .process(). There's no maximum block size for either input or output.

Since the buffer lengths (inputSamples and outputSamples above) are integers, it's up to you to make sure that the block lengths average out to the ratio you want over time.

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 library 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.