Time stretching is a common audio processing technique used to change the speed of an audio signal without affecting its pitch. This technique finds its application in music production, audio editing, and various multimedia projects. In this blog post, we will explore how to perform time stretching in C++.

Prerequisites

Before we dive into the implementation, make sure you have a basic understanding of audio signal processing, C++, and any necessary libraries to handle audio files. For this example, we will be using the libsndfile library, which provides functions for reading and writing audio files.

Implementation

To perform time stretching in C++, we will use a technique called phase vocoding. The basic idea behind phase vocoding is to divide the audio signal into overlapping chunks, adjust the duration of each chunk, and then combine them back together.

Here’s an example implementation of time stretching using phase vocoding in C++:

#include <iostream>
#include <cmath>
#include <sndfile.h>

// Constants
const int BUFFER_SIZE = 2048;
const float SPEED_FACTOR = 1.5;

// Time stretching function
void timeStretch(const char* inputFile, const char* outputFile, float speedFactor) {
    // Initialize input and output files
    SNDFILE* input = sf_open(inputFile, SFM_READ, NULL);
    SF_INFO inputInfo;
    sf_command(input, SFC_GET_CURRENT_SF_INFO, &inputInfo, sizeof(inputInfo));
    SNDFILE* output = sf_open(outputFile, SFM_WRITE, &inputInfo);

    // Buffer to hold audio data
    float buffer[BUFFER_SIZE];

    // Read input file in chunks
    sf_count_t bytesRead;
    while ((bytesRead = sf_read_float(input, buffer, BUFFER_SIZE)) > 0) {
        // Adjust chunk duration
        sf_count_t newSamples = ceil(bytesRead / speedFactor);

        // Perform time stretching
        for (sf_count_t i = 0; i < newSamples; i++) {
            float index = static_cast<float>(i) * speedFactor;
            sf_count_t indexFloor = static_cast<sf_count_t>(floor(index));
            sf_count_t indexCeil = static_cast<sf_count_t>(ceil(index));
            float fraction = index - indexFloor;
            buffer[i] = buffer[indexFloor] + fraction * (buffer[indexCeil] - buffer[indexFloor]);
        }

        // Write the stretched chunk to output file
        sf_write_float(output, buffer, newSamples);
    }

    // Cleanup
    sf_close(input);
    sf_close(output);
}

int main() {
    // Example usage
    const char* inputFile = "input.wav";
    const char* outputFile = "output.wav";
    timeStretch(inputFile, outputFile, SPEED_FACTOR);

    std::cout << "Time stretching complete!" << std::endl;

    return 0;
}

In this implementation, we define the timeStretch function that receives the input and output file paths, as well as the desired speed factor. The SPEED_FACTOR constant determines the stretching factor, where values greater than 1 increase the speed, and values less than 1 decrease the speed.

Inside the timeStretch function, we read the input file in chunks, adjust the duration of each chunk using the speedFactor, and then perform the time stretching by interpolating the samples between neighboring indices.

Finally, we write the stretched chunks to the output file until we reach the end of the input file.

Conclusion

Time stretching is a useful technique for altering the tempo of audio signals without changing their pitch. In this blog post, we explored how to implement time stretching using phase vocoding in C++. Remember to handle any necessary audio file input/output and use appropriate libraries to perform the required audio processing tasks.