How to implement simple audio loopback in QT - qt

Question

I need to loopback audio recorded from the microphone straight to the speaker using QT (qt 5.7 on Windows...) - assume I can't use window's internal mic->speaker loopback (enable "listen to this device" on the microphone panel).
Any direction how to do that?

Answer 1

Based on the discussion on https://forum.qt.io/topic/19960/qaudioinput-redirect-to-qaudiooutput/3
#include <iostream>
#include <QCoreApplication>
#include <QAudioInput>
#include <QAudioOutput>
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
const auto decideAudioFormat = [](const QAudioDeviceInfo& devInfo)
{
QAudioFormat format;
format.setSampleRate(8000);
format.setChannelCount(1);
format.setSampleSize(16);
format.setCodec("audio/pcm");
format.setByteOrder(QAudioFormat::LittleEndian);
format.setSampleType(QAudioFormat::SignedInt);
if (devInfo.isFormatSupported(format))
{
return format;
}
else
{
std::cerr << "Raw audio format not supported by backend, cannot play audio.\n";
throw 0;
}
};
QAudioInput audioInput(decideAudioFormat(QAudioDeviceInfo::defaultInputDevice()));
QAudioOutput audioOutput(decideAudioFormat(QAudioDeviceInfo::defaultOutputDevice()));
audioOutput.start(audioInput.start());
return app.exec();
}
Adding a buffer in-between
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
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
So, with separate buffers:
#include <iostream>
#include <cassert>
#include <QCoreApplication>
#include <QAudioInput>
#include <QAudioOutput>
#include <QBuffer>
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
QBuffer rdBuff;
QBuffer wrBuff;
wrBuff.open(QBuffer::WriteOnly);
rdBuff.open(QBuffer::ReadOnly);
QObject::connect(&wrBuff, &QIODevice::bytesWritten, [&wrBuff, &rdBuff](qint64)
{
// remove all data that was already read
rdBuff.buffer().remove(0, rdBuff.pos());
// set pointer to the beginning of the unread data
const auto res = rdBuff.seek(0);
assert(res);
// write new data
rdBuff.buffer().append(wrBuff.buffer());
// remove all data that was already written
wrBuff.buffer().clear();
wrBuff.seek(0);
});
const auto decideAudioFormat = [](const QAudioDeviceInfo& devInfo)
{
QAudioFormat format;
format.setSampleRate(8000);
format.setChannelCount(1);
format.setSampleSize(16);
format.setCodec("audio/pcm");
format.setByteOrder(QAudioFormat::LittleEndian);
format.setSampleType(QAudioFormat::SignedInt);
if (devInfo.isFormatSupported(format))
{
return format;
}
else
{
std::cerr << "Raw audio format not supported by backend, cannot play audio.\n";
throw 0;
}
};
QAudioInput audioInput(decideAudioFormat(QAudioDeviceInfo::defaultInputDevice()));
QAudioOutput audioOutput(decideAudioFormat(QAudioDeviceInfo::defaultOutputDevice()));
audioInput.start(&wrBuff);
audioOutput.start(&rdBuff);
return app.exec();
}