With the advancements in digital signal processing, audio effects have become an integral part of audio processing applications. One popular technique for creating audio effects is the phase vocoder. In this blog post, we will explore what a phase vocoder is and how to implement audio effects using this technique in C++.

What is a Phase Vocoder?

A phase vocoder is a digital signal processing technique used for time-scale modification and pitch shifting of audio signals. It works by decomposing an audio signal into its spectral components and manipulating their phases. The modified spectral components are then converted back to the time domain using overlap-add synthesis.

Implementing the Phase Vocoder in C++

To implement the phase vocoder in C++, we will need to perform the following steps:

1. Read the input audio signal.
2. Decompose the audio signal into overlapping frames.
3. Apply a window function to each frame.
4. Compute the magnitude and phase spectra of each frame using the Fast Fourier Transform (FFT).
5. Modify the phase spectra to introduce the desired audio effect.
6. Convert the modified spectra back to the time domain using inverse FFT.
7. Overlap and add the synthesized frames to obtain the output audio signal.

Here’s an example code snippet that demonstrates how to implement a simple audio effect using the phase vocoder in C++:

#include <iostream>
#include <cmath>
#include <vector>
#include <complex>

typedef std::complex<double> Complex;
typedef std::vector<Complex> ComplexVector;

// Function to implement the phase vocoder audio effect
void phaseVocoderEffect(const std::vector<double>& inputSignal, std::vector<double>& outputSignal, double timeStretchRatio, double pitchShiftFactor) {
    // Constants
    const int frameSize = 1024;
    const int hopSize = frameSize / 4;
    
    // Perform phase vocoder analysis and synthesis
    for (int i = 0; i < inputSignal.size() - frameSize; i += hopSize) {
        // Extract the current frame
        std::vector<double> frame(inputSignal.begin() + i, inputSignal.begin() + i + frameSize);
        
        // Apply window function to the frame
        
        // Compute the FFT of the frame
        
        // Modify the phase spectra
        
        // Perform inverse FFT to obtain the synthesized frame
        
        // Overlap and add the synthesized frame to the output signal
    }
}

int main() {
    // Read the input audio signal
    
    // Create an empty vector for the output audio signal
    
    // Apply the phase vocoder audio effect
    
    // Write the output audio signal to a file
    
    return 0;
}

In the above code snippet, the phaseVocoderEffect() function takes the input audio signal, the output audio signal (passed by reference), and the time stretch ratio and pitch shift factor as input parameters. The function performs the phase vocoder analysis and synthesis to introduce the desired audio effect.

It’s worth noting that this example code provides a basic framework for implementing audio effects using the phase vocoder technique. Depending on the specific audio effect you want to create, you may need to modify and extend the code accordingly.

Conclusion

The phase vocoder is a powerful technique for implementing audio effects such as time stretching and pitch shifting. In this blog post, we explored what a phase vocoder is and how to implement audio effects using this technique in C++. By understanding and implementing the phase vocoder, you can add fascinating audio effects to your applications and enhance the user experience.

#audioeffects #phasevocoder