87 lines
2.5 KiB
Markdown
87 lines
2.5 KiB
Markdown
# Signalsmith Stretch: pitch/time library
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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_.
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## How to use it
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```cpp
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#include "signalsmith-stretch.h"
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signalsmith::stretch::SignalsmithStretch<float> stretch;
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```
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### Configuring
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The easiest way to configure is `.presetDefault()`:
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```cpp
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stretch.presetDefault(channels, sampleRate);
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```
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If you want to test out different block-sizes etc. then you can use `.configure()` manually, and even change `.freqWeight`/`.timeWeight`/`.channelWeight`.
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### Processing (and resetting)
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```cpp
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// Clears internal buffers
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stretch.reset();
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float **inputBuffers, **outputBuffers;
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int inputSamples, outputSamples;
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stretch.process(inputBuffers, inputSamples, outputBuffers, outputSamples);
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// Inspect latency
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int totalLatency = stretch.inputLatency() + stretch.outputLatency();
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```
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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.
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To get a time-stretch, just hand it differently-sized input/output buffers.
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### Pitch-shifting
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```cpp
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stretch.setTransposeFactor(2); // up one octave
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stretch.setTransposeSemitones(12); // also one octave
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```
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You can set a "tonality limit", which uses a non-linear frequency map to preserve a bit more of the timbre:
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```cpp
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stretch.setTransposeSemitones(4, 8000/sampleRate);
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```
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### Custom pitch map
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This stretcher does (fairly rough) peak-detection, and creates a non-linear frequency map based on that.
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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:
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```cpp
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stretch.setMap([&](int channel) {
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for (auto &peak : stretch.peaks) {
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peak.output = peak.input*2; // up one octave
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}
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});
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```
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The input/output frequencies are relative to Nyquist. It's not currently-tested what happens if your map is non-monotonic.
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## Compiling
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Just include `signalsmith-stretch.h` in your build.
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It's pretty slow if optimisation is disabled though, so you might want to enable optimisation just where it's used.
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### DSP Library
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This uses the Signalsmith DSP library for FFTs and other bits and bobs.
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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.
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## License
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[MIT License](LICENSE.txt) for now - get in touch if you need anything else.
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