QAudioInput redirect to QAudioOutput

goldrake

Hi all,

we are trying to do some data processing between audio input and audio output.
We have written a simple application that does redirect input to output:

@AudioTest::AudioTest(QObject *parent) :
QObject(parent)
{
QAudioFormat format;
format.setSampleSize(16);
format.setSampleRate(8000);
format.setChannelCount(1);
format.setCodec("audio/pcm");
format.setSampleType(QAudioFormat::SignedInt);
format.setByteOrder(QAudioFormat::LittleEndian);

m_AudioInput = createAudioInput(format);
m_AudioOutput = createAudioOutput(format);
m_AudioOutput->start(m_AudioInput->start());

}

QAudioInput* AudioTest::createAudioInput(QAudioFormat &format)
{
QAudioDeviceInfo info = QAudioDeviceInfo::defaultInputDevice();
if (!info.isFormatSupported(format))
{
qDebug() << "Format not supported";
return NULL;
}

return new QAudioInput(format, this);

}

QAudioOutput* AudioTest::createAudioOutput(QAudioFormat &format)
{
QAudioDeviceInfo info(QAudioDeviceInfo::defaultOutputDevice());
if (!info.isFormatSupported(format))
{
qDebug() << "Format not supported";
return NULL;
}

return new QAudioOutput(format, this);

}@

This works like a charm.
The next step that we were planning was to implement our custom IODevice.
So we tried to change the line:

@m_AudioOutput->start(m_AudioInput->start());@

with:

@QBuffer* buffer = new QBuffer();
buffer->open(QIODevice::ReadWrite);
m_AudioInput->start(buffer);
m_AudioOutput->start(buffer);@

In this way nothing works!!
Can anybody explain us what we are doing wrong??

Thanks

twsimpson

Try seeking back to the beginning of the buffer before passing it to anything that reads from it
@buffer->seek(0);@

goldrake

Hi Terence,
thanks for your reply. To solve the issue that you pointed out we have tried to implement a custom IODevice: IntermediateBuffer. Than we change the code like this:

@IntermediateBuffer* buffer = new IntermediateBuffer();
buffer->open(QIODevice::ReadWrite);
m_AudioInput->start(buffer);
m_AudioOutput->start(buffer);@

Here the read and write implementation:

@qint64 IntermediateBuffer::readData(char* data, qint64 maxlen)
{
int length = qMin(m_Buffer.length(), (int) maxlen);
qstrncpy(data, m_Buffer.constData(), length);
m_Buffer.remove(0, length);

return length;
}

qint64 IntermediateBuffer::writeData(const char* data, qint64 maxlen)
{
m_Buffer.append(data, maxlen);
return maxlen;
}@

As you can image nothing changed :( :(

P.S.: m_Buffer is a QByteArray

JKSH

I've encountered this issue before. If I'm not mistaken, the read() and write() functions share the same pos() tracker. This is what happens:
@
QBuffer buffer;

buffer.open(QBuffer::ReadWrite); // buffer.pos() == 0
buffer.write(someData, 5); // Writes in positions 0 -- 4, buffer.pos() == 5;
buffer.write(someData, 5); // Writes in positions 5 -- 9, buffer.pos() == 10;
buffer.read(otherBuffer.data(), 10); // Tries to read from position 10 onwards. No data found
@

So, when your QAudioOutput tries to read from your buffer, the buffer ignores the positions that your QAudioInput wrote to.

I can think of 2 solutions:

Implement your own, separate read-position and write-position trackers. Then, call seek() in readData() and writeData() to set pos() -- point to the end of the data before writing, but point to the middle of the data before reading

Use 2 separate buffers, and copy bytes from the input buffer into the output buffer

JKSH

P.S. You probably want to implement a Circular Buffer instead of a 'normal' buffer -- a QBuffer doesn't automatically remove old data, and will eventually run out of memory (and crash)

P.P.S. Intead of
@
qstrncpy(data, m_Buffer.constData(), length);
m_Buffer.remove(0, length);
@

You could write
@
memcpy(data, m_Buffer.constData(), length);
@

goldrake

Thanks.

It was so simple that didn't come up in my mind ;)