QAudioOutput Class Reference
[QtMultimedia module]

Detailed Description

The QAudioOutput class provides an interface for sending audio data to an audio output device.

You can construct an audio output with the system's default audio output device. It is also possible to create QAudioOutput with a specific QAudioDeviceInfo. When you create the audio output, you should also send in the QAudioFormat to be used for the playback (see the QAudioFormat class description for details).

To play a file:

Starting to play an audio stream is simply a matter of calling start() with a QIODevice. QAudioOutput will then fetch the data it needs from the io device. So playing back an audio file is as simple as:

 QFile inputFile;     // class member.
 QAudioOutput* audio; // class member.

 inputFile.setFileName("/tmp/test.raw");
 inputFile.open(QIODevice::ReadOnly);

 QAudioFormat format;
 // Set up the format, eg.
 format.setFrequency(8000);
 format.setChannels(1);
 format.setSampleSize(8);
 format.setCodec("audio/pcm");
 format.setByteOrder(QAudioFormat::LittleEndian);
 format.setSampleType(QAudioFormat::UnSignedInt);

 QAudioDeviceInfo info(QAudioDeviceInfo::defaultOutputDevice());
 if (!info.isFormatSupported(format)) {
     qWarning()<<"raw audio format not supported by backend, cannot play audio.";
     return;
 }

 audio = new QAudioOutput(format, this);
 connect(audio,SIGNAL(stateChanged(QAudio::State)),SLOT(finishedPlaying(QAudio::State)));
 audio->start(&inputFile);

The file will start playing assuming that the audio system and output device support it. If you run out of luck, check what's up with the error() function.

After the file has finished playing, we need to stop the device:

 void finishedPlaying(QAudio::State state)
 {
   if(state == QAudio::IdleState) {
     audio->stop();
     inputFile.close();
     delete audio;
   }
 }

At any given time, the QAudioOutput will be in one of four states: active, suspended, stopped, or idle. These states are described by the QAudio::State enum. State changes are reported through the stateChanged() signal. You can use this signal to, for instance, update the GUI of the application; the mundane example here being changing the state of a play/pause button. You request a state change directly with suspend(), stop(), reset(), resume(), and start().

While the stream is playing, you can set a notify interval in milliseconds with setNotifyInterval(). This interval specifies the time between two emissions of the notify() signal. This is relative to the position in the stream, i.e., if the QAudioOutput is in the SuspendedState or the IdleState, the notify() signal is not emitted. A typical use-case would be to update a slider that allows seeking in the stream. If you want the time since playback started regardless of which states the audio output has been in, elapsedUSecs() is the function for you.

If an error occurs, you can fetch the error type with the error() function. Please see the QAudio::Error enum for a description of the possible errors that are reported. When an error is encountered, the state changes to QAudio::StoppedState. You can check for errors by connecting to the stateChanged() signal:

     void stateChanged(QAudio::State newState)
     {
         switch (newState) {
             case QAudio::StopState:
                 if (output->error() != QAudio::NoError) {
                     // Perform error handling
                 } else {
                     // Normal stop
                 }
                 break;

See also QAudioInput and QAudioDeviceInfo.

Member Function Documentation

 qint64 bytesInBuffer = bufferSize() - bytesFree();
 qint64 usInBuffer = (qint64)(1000000) * bytesInBuffer / ( channels() * sampleSize() / 8 ) / frequency();
 qint64 usPlayed = processedUSecs() - usInBuffer;