Qt6 linux: no audio output via QAudioSink

Audetto

Hi, I am porting my app to Qt6 and struggling to make QAudioSink produce any sound.
I am on Ubuntu 24.04, using Qt 6.4.2

After various checks I think the issue is in the default choice of the audio device.

Qt5's default is: alsa_output.pci-0000_00_1b.0.analog-stereo

On Qt6 I can only get one of these

Built-in Audio Analog Stereo
Built-in Audio Digital Stereo (HDMI)

These are the real hardware devices which are already managed by alsa / pulse / pipewire, and cannot be used directly.
Qt5 plays a virtual alsa device which is mixed by the audio system.

How do I do the same in Qt6? I've read about QT_MEDIA_BACKEND=gstreamer or ffmpeg but it makes no difference.

Andrea

SGaist

Hi,

How did you set that environment variable ?

Audetto

Finally found a solution.

It is a change in behaviour between Qt5 and Qt6.

When the QIODevice produces 0 bytes, in Qt6, the QAudioSink goes to IdleState and effectively stops, even if more data is available.

https://github.com/qt/qtmultimedia/blob/c0c276b9013acac9214026c9645eee353d67f00e/src/multimedia/pulseaudio/qpulseaudiosink.cpp#L468

One needs to emit readyRead(); when more data is available.

In Qt5, nothing of the sort happens.

https://github.com/qt/qtmultimedia/blob/5dc0ed6f5059b6ab81c3009cf72797a36ec5b211/src/plugins/pulseaudio/qaudiooutput_pulse.cpp#L477

These are links to particular implementations of the output plugin, I have no way to know if this is the intended general behaviour, which anyway is not documented anywhere in https://doc.qt.io/qt-6/qaudiosink.html

SGaist

The multimedia module has been fully rewritten for Qt 6.

That said, the current behavior makes sens to me and I would say the Qt 5 implementation was wrong in that regard. How is the system supposed to know you are making new data available if you don't tell it ? The other option is to have an active polling which would just waste resources.

Audetto

@SGaist Don't disagree, but without looking into the source code, I would have never figured it out. A little mention of this in the linked page would help.

SGaist

I am realizing I might be seeing this from the wrong perspective. Are you providing your own custom QIODevice ?

Audetto

Yes, Qt5 would never go in Idle, while Qt6 does. I know now how to wake it up.