How to choose Output Channel to play sound with QAudioOutput

[Pierreko]

Hello,

I make a tight multi-tracks audio player on Linux and I have troubles with the output selection.

I use a wave decoder to generate the buffer and I send it to QAudioOutput :

QFile track("./file.wav";
QWaveDecoder decoder = new QWaveDecoder(&m_trackFile, this);

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

QAudioOutput output = new QAudioOutput(decoder->audioFormat());
    }

But because I have an external sound card (Presonus Audiobox 1818VSL), I have to choose the channel at which my (stereo) 2 channels will be sent.

I listed the devices :

const QAudioDeviceInfo &defaultDeviceInfo = QAudioDeviceInfo::defaultOutputDevice();
    for (auto &deviceInfo: QAudioDeviceInfo::availableDevices(QAudio::AudioOutput)) {
        if (deviceInfo != defaultDeviceInfo)
            m_audiodevices.append(deviceInfo);
    }

and it returned :

PulseAudioService: pa_context_connect() failed
0 - "sysdefault:CARD=ALSA" (1, 2, 4, 6, 8)
1 - "dmix:CARD=ALSA,DEV=0" (1, 2, 4, 6, 8)
2 - "dmix:CARD=ALSA,DEV=1" (1, 2, 4, 6, 8)
ALSA lib pcm_dsnoop.c:557:(snd_pcm_dsnoop_open) The dsnoop plugin supports only capture stream
3 - "dsnoop:CARD=ALSA,DEV=0" ()
ALSA lib pcm_dsnoop.c:557:(snd_pcm_dsnoop_open) The dsnoop plugin supports only capture stream
4 - "dsnoop:CARD=ALSA,DEV=1" ()
5 - "hw:CARD=ALSA,DEV=0" (1, 2, 4, 6, 8)
6 - "hw:CARD=ALSA,DEV=1" (1, 2, 4, 6, 8)
7 - "plughw:CARD=ALSA,DEV=0" (1, 2, 4, 6, 8)
8 - "plughw:CARD=ALSA,DEV=1" (1, 2, 4, 6, 8)
9 - "front:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
10 - "surround21:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
11 - "surround40:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
12 - "surround41:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
13 - "surround50:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
14 - "surround51:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
15 - "surround71:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)
16 - "iec958:CARD=VSL,DEV=0" (1, 2, 4, 6, 8)

I can't understand how I can for example send a stereo sound to Output channels 3 & 4 instead of 1&2 of device 10 (surround21...) which looks like to be a configuration of my external sound card.

Do you have a idea ?

Thanks !

[SGaist]

Hi and welcome to devnet,

If you’d like to make more advanced audio stuff, you will have to use something other than QtMultimedia.

As a first step, you might be able to configure your ALSA device to be seen by QtMultimedia.

[Pierreko]

What are you meaning instead of using QtMultimedia ?

I'm not sure I understand what you said about ALSA device. I'm searching for a solution to route my audio buffer (mono or stereo) to one or 2 channels of my external sound card (which is "VSL" in the list above) but I have some difficulties to interpret the result because I was waiting for a list of channels, in fact...

thanks :)

[SGaist]

I mean using something like PortAudio.

As for ALSA, something like described here.