qt6 QMediaDevices::audioInputs() does not return all input devices

radiogeek381

It was rude of my not to include some code. Here is the relevant snippet from the qt6 example:

void AudioTest::updateAudioDevices()
{
    deviceBox->clear();
    const auto devices =
            m_mode == QAudioDevice::Input ? m_devices->audioInputs() : m_devices->audioOutputs();
    for (auto &deviceInfo : devices) {
        QString description = deviceInfo.description();
        description.replace(u"\n"_s, u" - "_s);
        deviceBox->addItem(description, QVariant::fromValue(deviceInfo));
    }
}

SGaist

Hi and welcome to devnet,

What is your audio software stack ?
Can you show the output your get on each of them ?

radiogeek381

Thanks for the response.

I'm not quite sure how to answer the question - I've always been a bit confused wrt Linux audio. So this might be more information than is needed...

The stack: I believe the answer is "pipewire" though pulse-audio is in there somewhere.

But the qt6 version dumps some disturbing chatter to stdout:

qt.multimedia.ffmpeg: Using Qt multimedia with FFmpeg version 7.1.2 GPL version 3 or later
Failed to open VDPAU backend libvdpau_nvidia.so: cannot open shared object file: No such file or directory
libva info: VA-API version 1.22.0
libva info: Trying to open /usr/lib64/dri-nonfree/iHD_drv_video.so
libva info: Trying to open /usr/lib64/dri-freeworld/iHD_drv_video.so
libva info: Trying to open /usr/lib64/dri/iHD_drv_video.so
libva info: Found init function __vaDriverInit_1_22
libva info: va_openDriver() returns 0
libva info: VA-API version 1.22.0
libva info: Trying to open /usr/lib64/dri-nonfree/iHD_drv_video.so
libva info: Trying to open /usr/lib64/dri-freeworld/iHD_drv_video.so
libva info: Trying to open /usr/lib64/dri/iHD_drv_video.so
libva info: Found init function __vaDriverInit_1_22
libva info: va_openDriver() returns 0

I know that pulse is involved as I set up a couple of virtual devices like this:

pactl load-module module-null-sink sink_name=RSink sink_properties=device.description="RadioSink"
pactl load-module module-null-sink sink_name=MSink sink_properties=device.description="ModemSink"

(These are to provide paths between a modem inside one app (wsjtx) and an SDR running in a separate process.)

I modified both example programs to write directly to the qInfo stream:
qt6 version:

const auto devices =
            m_mode == QAudioDevice::Input ? m_devices->audioInputs() : m_devices->audioOutputs();
    for (auto &deviceInfo : devices) {
        QString description = deviceInfo.description();
        description.replace(u"\n"_s, u" - "_s);
        deviceBox->addItem(description, QVariant::fromValue(deviceInfo));
	qInfo() << QString("audio device [%1] from audio%2()")
	  .arg(description)
	  .arg((m_mode == QAudioDevice::Input) ? "Inputs" : "Outputs")
	  ;
    }

which produces this output:

""
"audio device [Built-in Audio Analog Stereo] from audioOutputs()"
"audio device [RadioSink] from audioOutputs()"
"audio device [ModemSink] from audioOutputs()"
"audio device [NoMachine Output] from audioOutputs()"
""
"audio device [Remapped nx_voice_out] from audioInputs()"
""

The qt5 version looks like this:

const QAudio::Mode mode = idx == 0 ? QAudio::AudioInput : QAudio::AudioOutput;
   for (auto &deviceInfo: QAudioDeviceInfo::availableDevices(mode)) {
       deviceBox->addItem(deviceInfo.deviceName(), QVariant::fromValue(deviceInfo));
   qInfo() << QString("audio device [%1] from QAudioDeviceInfo::availableDevices(QAudio::Audio%2)")
     .arg(deviceInfo.deviceName())
     .arg((mode == 0) ? "Input" : "Output")
     ;
   }

and I get this for the output:

QSocketNotifier: Can only be used with threads started with QThread
"audio device [pipewire] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [default] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [sysdefault:CARD=PCH] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [front:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [nx_remapped_out] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [alsa_output.pci-0000_00_1f.3.analog-stereo.monitor] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [RSink.monitor] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [MSink.monitor] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
"audio device [nx_voice_out.monitor] from QAudioDeviceInfo::availableDevices(QAudio::AudioInput)"
qt.qpa.wayland: Wayland does not support QWindow::requestActivate()
"audio device [pipewire] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [default] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [sysdefault:CARD=PCH] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [front:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [surround21:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [surround40:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [surround41:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [surround50:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [surround51:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [surround71:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [hdmi:CARD=PCH,DEV=0] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [hdmi:CARD=PCH,DEV=1] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [hdmi:CARD=PCH,DEV=2] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [hdmi:CARD=PCH,DEV=3] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [alsa_output.pci-0000_00_1f.3.analog-stereo] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [RSink] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [MSink] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"
"audio device [nx_voice_out] from QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)"

(Sorry for the monster blobs...)

So. The qt5 operations do what I expect.

I don't believe the problem comes from the virtual MSink and RSink devices.
The complaint about the VDPAU backend may be a hint. I've been unable to figure out where it comes from. I don't have any nVidia hardware on this box.

I'm building the dev branch from the git sources now. I'll see if a new built-from-source version has the same lib...nvidia problem. That'll take a while, it may take until the weekend.

Thank you for your help.

Is there more information I can provide.? I apologize for the lack of linux audio sophistication.

radiogeek381

I repeated the experiment with a build-from-git dev branch reporting "3.31.6" as the version. ( git hash e98fab4c4edfa49debfab549a81862922c0b52e8 )

The results were identical to the earlier experiment. I did get this warning, however:

build> ./audiodevices
No QtMultimedia backends found. Only QMediaDevices, QAudioDevice, QSoundEffect, QAudioSink, and QAudioSource are available.

(I built the Qt library and the test app after installing ffmpeg-free-devel. Though it doesn't appear to be referenced by the app or any of the lib files:

build> ldd ~/exp/qt6/root/lib/lib*.so  | grep -i ffmpeg
build>

)

I built with DEBUG support on. Is there something that you'd like me to try? A particular place to poke around?

SGaist

Qt 5 uses GStreamer as backend on Linux while Qt 6 now uses ffmpeg by default. This is the main difference.
It's strange that QtMultimedia did not build the ffmpeg backend.
What are you getting from the configuration step output ?

radiogeek381

I've tried multiple schemes and configurations.
In answer to no question at all: I've created a GitHub repo with a reproducer (scripts and source and a README) for the problem I'm having.

git@github.com:kb1vc/qt_multimedia_audio_problem.git

In answer to your last question:

Building Qt6.10.1 under Fedora43 I see this mention of FFmpeg in the configure log:

CMake Warning at qtmultimedia/src/plugins/multimedia/ffmpeg/cmake/QtAddFFmpegStubs.cmake:38 (message):
  QT_FEATURE_vaapi is ON but FFmpeg includes VAAPI and dynamic symbols
  resolve is enabled.
Call Stack (most recent call first):
  qtmultimedia/src/plugins/multimedia/ffmpeg/cmake/QtAddFFmpegStubs.cmake:189 (qt_internal_multimedia_check_ffmpeg_stubs_configuration)
  qtmultimedia/src/plugins/multimedia/ffmpeg/CMakeLists.txt:107 (qt_internal_multimedia_add_ffmpeg_stubs)

I'm not sure what that means, but I'll try another run of the configure command:
Adding -DQT_FEATURE_vaapi=OFF got rid of the warning.

The configure completed without warning. And the build completed.

The result of the test program was still not right -- the inputs list was missing almost all the loopback devices, and the one it found (setup by pw-loopback) was named incorrectly.

radiogeek381

I found the difference. In Qt 6.10.1

eab27c5cbd src/multimedia/pulseaudio/qaudioengine_pulse.cpp          (Mikko Hallamaa    2024-03-22 16:47:14 +0100 257)     }
^2a34e88c1 src/plugins/pulseaudio/qpulseaudioengine.cpp              (Michael Goddard   2011-06-29 13:38:46 +1000 258) 
85e2f9712f src/multimedia/platform/pulseaudio/qaudioengine_pulse.cpp (Lars Knoll        2021-01-13 15:39:29 +0100 259)     // skip monitor channels
85e2f9712f src/multimedia/platform/pulseaudio/qaudioengine_pulse.cpp (Lars Knoll        2021-01-13 15:39:29 +0100 260)     if (info->monitor_of_sink != PA_INVALID_INDEX)
85e2f9712f src/multimedia/platform/pulseaudio/qaudioengine_pulse.cpp (Lars Knoll        2021-01-13 15:39:29 +0100 261)         return;
7b75983575 src/multimedia/pulseaudio/qaudioengine_pulse.cpp          (Artem Dyomin      2023-06-07 14:46:36 +0200 262)

Qt5 lists the monitor devices. Ignoring monitor devices means that devices created like this:

pactl load-module module-null-sink sink_name=RadioOutput sink_properties=device.description="RadioOutput"

Used to show up as "RadioOutput.monitor" from QAudioDeviceInfo::availableDevices(Audio::AudioInput).

This created a sink device called "RadioOutput" where the radio would send its output audio stream. The modem would listen on "RadioOutput.monitor."

The workaround is to use the pact remap module

pactl load-module module-remap-source master=RadioOutput.monitor source_name="FromRadio" source_properties=device.description=FromRadio

in addition to the module-null-sink step.

This still leaves the question as to why Qt6 wants to ignore monitor devices? This breaks backward compatibility at the user level.

From what I can tell, this was introduced in v6.4.0 in src/multimedia/platform/pulseaudio/qaudioengine_pulse.cpp

As a workaround, (or adapting to the new reality) I've changed my loopback creator script to:

pactl load-module module-null-sink sink_name=RadioOutput \
      sink_properties=device.description="RadioOutput"
pactl load-module module-null-sink sink_name=ModemOutput \
      sink_properties=device.description="ModemOutput"

pactl load-module module-remap-source master=RadioOutput.monitor \
      source_name="FromRadio" source_properties=device.description=FromRadio

pactl load-module module-remap-source master=ModemOutput.monitor \
      source_name="FromModem" source_properties=device.description="FromModem"

What is the Qt6 developer position on breaking backward application user level compatibility?

aha_1980

Hi @radiogeek381,

What is the Qt6 developer position on breaking backward application user level compatibility?

Note that this is a user forum. You may ask this question on the development mailing list or create a bugreport about this. You can find links to both in my link collection.

Regards

radiogeek381

aha_1980 Thank you. That will help.
I started here, rather than with a bug report, because I was pretty sure the problem was on my side. Despite my confidence in the previous note, I still don't have a solution - my experiments were against builds without PipeWire.

Thank you for the link collection. When I've got this a little more solid, I'll look at the development mailing list.

NielsMayer

@radiogeek381 said in qt6 QMediaDevices::audioInputs() does not return all input devices:

What is the Qt6 developer position on breaking backward application user level compatibility?

The position is they don't care:
https://qt-project.atlassian.net/browse/QTBUG-116015
https://qt-project.atlassian.net/browse/QTBUG-114846
https://qt-project.atlassian.net/browse/QTBUG-118766
https://qt-project.atlassian.net/browse/QTBUG-133298
https://qt-project.atlassian.net/browse/QTBUG-140658?focusedCommentId=2489187

Also on Linux , Qt6 no longer works (probably since 6.7) with QT_MEDIA_BACKEND=gstreamer environment-variable set to select the legacy media backend, in case FFMPEG isn't working for you.