How to use two channels in QAudioOutput?

kitfox

I'm investigating using QAudioOutput for generating sound. I've figured out how to generate mono sound on a single channel, and am now trying to expand my program to handle stereo output.

For my current test, I want to send a 440Hz wave to one speaker and a 220Hz wave to the other, but the same mixed sound is being sent to both. How do I fix this?

I'm calculating the buffer I play in notifyPlayPressed():

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QAudioOutput>
#include <QByteArray>
#include <QBuffer>
#include <QMutex>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

    QAudioOutput* _audioOut = nullptr;
    QIODevice* _audioDevice = nullptr;
    QByteArray _audioBuf;

    int sampleRate = 44100;
    QMutex _bufMutex;

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    void queueAudio(char* data, int size);

public slots:
    void notifyPlayPressed();
    void audioNotify();
    void handleStateChanged(QAudio::State);

private:
    void writeToDevice();

    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QAudioFormat>
#include <QDebug>
#include "math.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    connect (ui->playBn, &QPushButton::clicked, this, &MainWindow::notifyPlayPressed);


    QAudioFormat format;
    // Set up the format, eg.
    format.setSampleRate(sampleRate);
    format.setChannelCount(2);
    format.setSampleSize(8 * sizeof(float));
    format.setCodec("audio/pcm");
    format.setByteOrder(QAudioFormat::LittleEndian);
    format.setSampleType(QAudioFormat::Float);

    QAudioDeviceInfo info(QAudioDeviceInfo::defaultOutputDevice());
    if (!info.isFormatSupported(format)) {
        qWarning() << "Raw audio format not supported by backend, cannot play audio.";
        return;
    }

    _audioOut = new QAudioOutput(format, this);
    _audioOut->setNotifyInterval(128);

    connect(_audioOut, &QAudioOutput::notify, this, &MainWindow::audioNotify);
    connect(_audioOut, &QAudioOutput::stateChanged, this, &MainWindow::handleStateChanged);
    _audioDevice = _audioOut->start();
}

MainWindow::~MainWindow()
{
    delete ui;

    _audioOut->stop();
    delete _audioOut;
}

void MainWindow::queueAudio(char* data, int size)
{
    _bufMutex.lock();
    _audioBuf.append(data, size);
    writeToDevice();
    _bufMutex.unlock();
}


void MainWindow::audioNotify()
{
    _bufMutex.lock();
    writeToDevice();
    _bufMutex.unlock();
}


void MainWindow::writeToDevice()
{
    int numBytes = std::min(_audioOut->bytesFree(), _audioBuf.size());
    if (numBytes > 0)
    {
        _audioDevice->write(_audioBuf.data(), numBytes);
        _audioBuf.remove(0, numBytes);
    }
}


void MainWindow::notifyPlayPressed()
{
    int bufferSize = sampleRate * sizeof(float) * 2;
    QByteArray _dataBuf(bufferSize, 0);

    {
        //Create 1 second of audio
        float* floatBuf = (float*)_dataBuf.data();

        for (int i = 0; i < sampleRate; ++i)
        {
            float seconds = i / (float)sampleRate;
            floatBuf[i * 2] = (float)cos(2 * M_PI * seconds * 220);
            floatBuf[i * 2 + 1] = (float)cos(2 * M_PI * seconds * 440);
        }
    }

    queueAudio(_dataBuf.data(), bufferSize);
}

mrjj

Hi
As far as i know, you can have 2 mono channels or one stereo .
2 stereo channels is not supported as far as i have seen.
They will always be mixed.
It been suggested to use
http://www.portaudio.com/
instead but i can't say if good solution or not.

kitfox

This post is deleted!

mrjj

Hi
Yes as far as i know it should be possible.
http://fredericgoset.ovh/dungeon/en/part32.html
https://stackoverflow.com/questions/41417778/qt-qaudiooutput-only-playing-through-left-ear-while-in-stereo

kitfox

Actually, it looks like the above code is working. I just wasn't able to hear the difference clearly with the samples I as playing on my speakers.