QMediaRecorder::record() delay on Windows

szumial

Hi! I have a program which uses the QMediaRecorder class. Everything is configured as required (including my input devices and the QMediaCaptureSession). The program triggers audio recording at a press of a push button and stops the recording procedure when the button is released. On Windows, I have noticed that there is significant delay between the moment when the button triggers the QMediaRecorder::record() method and the time when the QMediaRecorder state is changed to RecordingState. This results in a glitch, where you think the recording has already started, but in fact you need keep the pushbutton pressed for long enough so it actually starts the recording process.

The same program works without issues on MacOS. This is using Qt 6.5.0 with both platforms utilizing their native multimedia backend (I am not using ffmpeg).

What could be the cause of such delay?

Don't know if this is related, but when launching the program in debug mode, I caught the following exception and the app crashed after that:
Exception at 0x7ffda9c1cf19, code: 0xe06d7363: C++ exception, flags=0x1 (execution cannot be continued) (first chance) in igd10um64xe!OpenAdapter_D3D11On12

SGaist

Hi,

Do you have the same issue if you run in release mode ? Debug mode can sometime incur performance hit that can explain such behaviour.

szumial

@SGaist In Release mode I can observe the delay when recording audio. In Debug mode I reached the exception posted in the original message.

SGaist

Do you have the same issue if you trigger the recoding using the button's clicked signal ?

szumial

@szumial Yes, it happens also when I trigger the recording using the "clicked" signal.

Whatever I do, there is always a noticeable delay. Before the recorder goes to RecordingState it is around 3 seconds when I trigger recording for the first time. Consecutive times have a slightly smaller delay time.

Here is a sample of my code:

// QMediaRecorder settings
m_recorder.setMediaFormat({QMediaFormat::MP3});
m_recorder.setAudioChannelCount(1);
m_recorder.setQuality(QMediaRecorder::Quality::VeryHighQuality);
m_recorder.setEncodingMode(QMediaRecorder::ConstantQualityEncoding);

// Pushbutton connections
connect(recordButton, &QToolButton::pressed, this, [this]
{
    record(true);
});
connect(recordButton, &QToolButton::released, this, [this]
{
    record(false);
});

// Record method
void MyWidget::record(bool on)
{
    if (on)
    {
        recordButton->setChecked(true);
	QDir{}.mkpath(m_outputPath); 
        m_recorder.setOutputLocation(QUrl::fromLocalFile(m_recordedFile));
	m_audio->recorder().record();
	recordButton->setText(tr(" Recording..."));
    }
    else
    {
        recordButton->setChecked(false);
        m_recorder.stop();
	recordButton->setText(tr(" Start recording"));
    }
}

SGaist

Might be a silly question but where are you starting the recording ? Is it locally ? On a network drive ?

szumial

@SGaist I am recording the microphone input to a local drive.

I was wondering if moving the recorder logic to a separate thread could improve it, but initial trials did not bring any benefits.

Another idea I have is a silly workaround to start recording at program start in order for any QMediaRecorder resources to be initialized before I trigger the recording with that pushbutton. This would not solve the root cause of the problem, but maybe improve user experience at least.

SGaist

Do you encounter the same issue if you use your application on a different machine ?

szumial

@SGaist Yes, that is on Windows PCs. I don't see such issue when running my program on MacOS.

SGaist

Just to be sure I understand this correctly, you have the same issue in several different Windows machines ?

szumial

@SGaist That is correct. The same application tested on different Windows machines presents exactly the same behavior.