Implementing binaural audio processing in C++

03 Oct 2023

Binaural audio processing is a technique used to create immersive and realistic audio experiences, simulating the way sounds are perceived by human ears. It involves capturing audio with two microphones placed strategically to mimic the ear position, and then processing the audio signals to recreate an accurate sound localization effect for listeners.

In this blog post, we will explore how to implement binaural audio processing in C++. We will cover the basics of capturing audio, processing the signals, and playing them back to achieve a binaural effect.

1. Audio Capture

Before we start with binaural processing, we need to capture audio using two microphones placed similarly to human ears. This can be done using a hardware setup or by using specialized libraries that can capture audio from multiple devices simultaneously.

Here’s an example of how to capture audio signals using the PortAudio library in C++:

#include <iostream>
#include <portaudio.h>

// Callback function to process audio samples
int audioCallback(const void* inputBuffer, void* outputBuffer,
                  unsigned long framesPerBuffer,const PaStreamCallbackTimeInfo* timeInfo,
                  PaStreamCallbackFlags statusFlags, void* userData)
{
    // Process audio samples here

    return paContinue;
}

int main()
{
    PaStream* stream;
    Pa_Initialize();

    // Open a stream for audio capture
    Pa_OpenDefaultStream(&stream, 2, 0, paFloat32, 44100, 256, audioCallback, nullptr);

    // Start the audio stream
    Pa_StartStream(stream);

    // Keep the program running while audio is being captured
    std::cout << "Press Enter to stop capturing audio...";
    std::cin.ignore();

    // Stop and close the audio stream
    Pa_StopStream(stream);
    Pa_CloseStream(stream);
    Pa_Terminate();

    return 0;
}

2. Binaural Processing

Once we have captured the audio signals, we can start with the binaural processing. The goal is to emulate the way sounds arrive at each ear, considering factors such as interaural time difference (ITD) and interaural level difference (ILD).

Here’s an example of how to apply ITD and ILD to the captured audio signals in C++:

#include <cmath>

// Apply ITD and ILD to a stereo audio signal
void applyBinauralProcessing(float* leftSignal, float* rightSignal,
                             unsigned long numFrames, float azimuth)
{
    float distanceSource = 1.0f; // Distance from source to ears
    float speedOfSound = 343.0f; // Speed of sound in m/s

    float headWidth = 20.0f;     // Approximate human head width
    float maxITD = headWidth / speedOfSound; // Maximum ITD based on head width

    float maxDelayTime = 1.0f / speedOfSound; // Maximum delay time based on speed of sound

    float delay = (distanceSource / speedOfSound) * maxITD;
    float crossfade = std::sin(azimuth) * 0.5f + 0.5f;

    // Delay and crossfade the left signal
    for (unsigned long i = 0; i < numFrames; i++)
    {
        unsigned long delayFrames = static_cast<unsigned long>(delay * 44100);
        unsigned long index = i + delayFrames;

        if (index < numFrames)
        {
            leftSignal[i] = crossfade * leftSignal[i] +
                            (1.0f - crossfade) * leftSignal[index];
        }
        else
        {
            leftSignal[i] = crossfade * leftSignal[i];
        }
    }

    // Crossfade the right signal
    for (unsigned long i = 0; i < numFrames; i++)
    {
        rightSignal[i] = (1.0f - crossfade) * rightSignal[i];
    }
}

3. Audio Playback

Finally, we can play back the processed audio signals to achieve a binaural audio effect. This can be done using audio libraries like PortAudio or other platform-specific libraries.

Here’s an example of how to play back the processed audio signals using PortAudio:

int audioCallback(const void* inputBuffer, void* outputBuffer,
                  unsigned long framesPerBuffer,
                  const PaStreamCallbackTimeInfo* timeInfo,
                  PaStreamCallbackFlags statusFlags, void* userData)
{
    float* output = static_cast<float*>(outputBuffer);

    // Generate binaural processed audio signals here

    return paContinue;
}

int main()
{
    PaStream* stream;
    Pa_Initialize();

    // Open a stream for audio playback
    Pa_OpenDefaultStream(&stream, 0, 2, paFloat32, 44100, 256, audioCallback, nullptr);

    // Start the audio stream
    Pa_StartStream(stream);

    // Keep the program running while audio is being played back
    std::cout << "Press Enter to stop audio playback...";
    std::cin.ignore();

    // Stop and close the audio stream
    Pa_StopStream(stream);
    Pa_CloseStream(stream);
    Pa_Terminate();

    return 0;
}

Conclusion

In this blog post, we have explored how to implement binaural audio processing in C++. We covered audio capture, binaural processing, and audio playback to achieve an immersive and realistic sound experience. Remember to fine-tune the parameters and experiment with different techniques to achieve the desired binaural effect.

#binauralaudio #cpp