Implementing audio effects using noise gates in C++

03 Oct 2023

Noise gates are essential tools in audio processing that help control the level of unwanted noise in the audio signal. In this blog post, we’ll explore how to implement noise gates in C++ to apply audio effects and improve audio quality.

What is a Noise Gate?

A noise gate is a dynamic audio processor that helps reduce unwanted noise during silent or quiet parts of an audio signal. It works by specifying a threshold level below which the signal will be attenuated or muted. When the audio level falls below the threshold, the noise gate applies a specified amount of attenuation or mutes the audio entirely.

Implementing a Simple Noise Gate

To implement a noise gate in C++, we can follow these steps:

Read the audio input from a source, such as a file or a microphone.
Determine the audio level and compare it to the specified threshold.
If the audio level is below the threshold, apply attenuation or mute the audio.
If the audio level is above the threshold, pass the audio through without any modification.
Write the processed audio to an output file or play it through a speaker.

Here’s an example code snippet that illustrates the implementation of a noise gate in C++ using the PortAudio library:

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

constexpr double threshold = 0.1;
constexpr double attenuation = 0.5;

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

    for (unsigned long i = 0; i < framesPerBuffer; ++i)
    {
        float sample = *in++;

        if (sample < threshold)
            sample *= attenuation;

        *out++ = sample;
        *out++ = sample; // Stereo audio, duplicate the sample for both channels
    }

    return paContinue;
}

int main()
{
    PaError err = Pa_Initialize();

    if (err != paNoError)
    {
        std::cerr << "Error initializing PortAudio: " << Pa_GetErrorText(err) << std::endl;
        return 1;
    }

    PaStream* stream;
    err = Pa_OpenDefaultStream(&stream, 1, 2, paFloat32, 44100, paFramesPerBufferUnspecified, audioCallback, nullptr);

    if (err != paNoError)
    {
        std::cerr << "Error opening PortAudio stream: " << Pa_GetErrorText(err) << std::endl;
        return 1;
    }

    err = Pa_StartStream(stream);

    if (err != paNoError)
    {
        std::cerr << "Error starting PortAudio stream: " << Pa_GetErrorText(err) << std::endl;
        return 1;
    }

    std::cout << "Noise gate running, press Enter to stop..." << std::endl;
    std::cin.get();

    err = Pa_StopStream(stream);

    if (err != paNoError)
    {
        std::cerr << "Error stopping PortAudio stream: " << Pa_GetErrorText(err) << std::endl;
        return 1;
    }

    err = Pa_CloseStream(stream);

    if (err != paNoError)
    {
        std::cerr << "Error closing PortAudio stream: " << Pa_GetErrorText(err) << std::endl;
        return 1;
    }

    Pa_Terminate();

    return 0;
}

Conclusion

Noise gates are valuable audio processing tools that can enhance audio quality by reducing unwanted noise during silent or quiet parts of an audio signal. The implementation of noise gates in C++ can be achieved using frameworks or libraries like PortAudio. By applying noise gates, audio engineers can create cleaner and more professional-sounding audio recordings.

#audioeffects #noisegates #cplusplus