QAudioInput::notify() isn't executed while capturing sound from mic - c++

Question

I want to use QAudioInput to capture sound from mic, process it and then play. As I understood, I need to connect to notify signal and inside handler for it user readAll() function to get raw data. But the problem is, this handler function is never executed. Here is my code:
void MainWindow::on_pushButton_clicked()
{
QList<QAudioDeviceInfo> list = QAudioDeviceInfo::availableDevices(QAudio::AudioInput);
if(list.isEmpty())
{
qDebug() << "No audio input device";
return;
}
QAudioDeviceInfo info = QAudioDeviceInfo::defaultInputDevice();
QAudioFormat format;
// Set up the desired format, for example:
format.setSampleRate(44100);
format.setChannelCount(1);
format.setSampleSize(32);
format.setCodec("audio/pcm");
format.setByteOrder(QAudioFormat::LittleEndian);
format.setSampleType(QAudioFormat::UnSignedInt);
if (!info.isFormatSupported(format)) {
qWarning() << "Default format not supported, trying to use the nearest.";
format = info.nearestFormat(format);
}
qDebug() << "input device :" << info.deviceName();
audio = new QAudioInput(info, format);
qDebug("created()");
connect(audio, SIGNAL(notify()), this, SLOT(onNotify()));
audio->setNotifyInterval(10);
bufferInput = audio->start();
}
void MainWindow::onNotify()
{
qDebug() << "onNotify()";
//QByteArray array = bufferInput->readAll();
//bufferOutput->write(array);
}
(audio is of type QAudioInput* and bufferInput is of type QIODevice*)
and when I click the button, "input device = " and "created()" messages are displayed, but "onNotify()" isn't shown.
What am I doing wrong?

Answer 1

QAudioInput seems quite broken. Or I'm utterly misunderstanding something.
The only thing that worked reliably for me was to use the readyRead() signal of the QIODevice buffer returned by start(). Unfortunately, that isn't fired very often on my system (around every 40 to 60ms on my system).
What I've found is that notify() starts firing when I either call resume() on the QAudioInput (because it's in idle state after calling start()) or do a readAll() on the QIODevice (!). But at least with PyQt this leads to a stack overflow after a minute or so.
I would suspect the platform matters as well, as the actual QAudioInput implementation depends on the platform and audio system used (PulseAudio on Fedora 32 in my case).

Answer 2

Connecting a slot to readyRead signal of QAudioInput and reading from QIODevice with readAll is the correct solution. Why do you think 40 or 60 ms interval is inappropriate? QAudioInput object must capture some audio during this interval and send signal after it.