QIODevice::writeData is not triggered when working with with 64 channels QAuioFormat

[Thinium]

I want to use a custom QIODevice to process audio data received from Soundflower (64 channel). I noticed the QIODevice::writeData function is only triggered if I manually change QAudioFormat's channelCount to anything less than 16. How can I make it work with soundflower's original audioformat, which is 64 channel? Or is this a bug in the QIODevice? Here is the sample codes:

void MyWidget::deviceChanged (const QAudioDeviceInfo &device)
{
    if (m_audioInput)
    {
        m_audioInput->stop();
        delete m_audioInput;
    }

    if (m_streamReader)
    {
        m_streamReader->stop();
        delete m_streamReader;
    }

    QAudioFormat format = device.preferredFormat();

    // this line shows the original format: QAudioFormat(44100Hz, 32bit, channelCount=64, sampleType=SignedInt, byteOrder=BigEndian, codec="audio/pcm")
    qDebug() << "original format:" << format;

    // in order to make QIODevice::writeData(const char *data, qint64 len) being triggered, I need to manually set the channel count to anything less than 16, if the channel count is more than 15, the QIODevice:writeData won't be triggered
    format.setChannelCount(8);
    m_audioInput = new QAudioInput(device, format);
    // StreamReader is a subclass of QIODevice
    m_streamReader = new StreamReader(m_audioInput->format(), this);
    m_streamReader->start();
    m_audioInput->start(m_streamReader);
    qDebug() << m_audioInput->state();
}

// here is the function of QIODevice subclass that doesn't work as expected
qint64 StreamReader::writeData(const char *data, qint64 len)
{
    // this function is called only if audioFormat.channelCount is <=16
}

When the channelCount is set to 64, the AudioInput state is StoppedState, but if I hard-code the channelCount to anything <= 16, the state is IdleState

[Christian Ehrlicher]

@Thinium said in QIODevice::writeData is not triggered when working with with 64 channels QAuioFormat:

MyQIODevice* streamReader = new StreamReader(m_audioInput->format(), this);

What do you expect from this line (except maybe a memleak)?

[Thinium]

@Christian-Ehrlicher thanks for the reply! I just updated the question to reflect the actual code. StreamReader is a member of myWidget and it has a function that receives audio signal from the selected audio device (in my case Soundflower 64 channels)

qint64 StreamReader::writeData(const char *data, qint64 len)

[Christian Ehrlicher]

But why would a reader and an input would call write()?

btw: still leaks when deviceChanged() more than once.

[Thinium]

@Christian-Ehrlicher yes I am aware this would leak. I didn't add the delete codes just to show the core of the issue.

According to the documentation, AudioInput "Writes" to the QIODevice. For more info:
https://doc.qt.io/qt-5/qaudioinput.html#start
Starts transferring audio data from the system's audio input to the device. The device must have been opened in the WriteOnly, Append or ReadWrite modes.

[Bonnie]

@Thinium
Wow, I think it is hard to know bacause I don't have a 64-channel device on Windows. :(
BTW, what's the bufferSize after start?

@Christian-Ehrlicher
The internal process of an audio input is that, it reads data from the audio device, then write that data to the user provided device.

[Thinium]

@Bonnie thanks for the reply! It fails when you manually set the channelCount to anything more than 16 channels. You don't really need a real device to reproduce that.

The buffer size after start is 4096, no matter what my channel settings are. Here are the buffer size for 2channels and 64channels:

"Soundflower (2ch)"
QAudioFormat(44100Hz, 32bit, channelCount=2, sampleType=SignedInt, byteOrder=BigEndian, codec="audio/pcm")
IdleState
buffersize 4096

"Soundflower (64ch)"
QAudioFormat(44100Hz, 32bit, channelCount=64, sampleType=SignedInt, byteOrder=BigEndian, codec="audio/pcm")
StoppedState
buffersize 4096

As you can see, when AudioFormat is set to 64ch, the QAudioInput is in stopped state. Which I don't understand why.

[Christian Ehrlicher]

@Bonnie said in QIODevice::writeData is not triggered when working with with 64 channels QAuioFormat:

then write that data to the user provided device.

So at least the class name is wrong (it's a StreamWriter, not StreamReader).

[Bonnie]

@Thinium Seems there's an open error. You can check the m_audioInput->error() result.
Does device.isFormatSupported(m_audioInput->format()) returns true in 64ch mode?

[Thinium]

@Bonnie indeed the format is not supported! That's surely why the audioInput can't be started.

[Bonnie]

@Thinium I found this in the source code (coreaudiodeviceinfo.mm)

QList<int> CoreAudioDeviceInfo::supportedChannelCounts()
    {
        static QList<int> supportedChannels;
    
        if (supportedChannels.isEmpty()) {
            // If the number of channels is not supported by an audio device, Core Audio will
            // automatically convert the audio data.
            for (int i = 1; i <= 16; ++i)
                supportedChannels.append(i);
        }
    
        return supportedChannels;
    }

So Qt will only allow 1~16 channels.