Qt 4.8: QAudioOutput Class Reference

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# QAudioOutput Class Reference

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

     #include <QAudioOutput>

Inherits: QObject.

This class was introduced in Qt 4.6.

- List of all members, including inherited members

## Public Functions

| QAudioOutput ( const QAudioFormat & format = QAudioFormat(), QObject * parent = 0 )
| QAudioOutput ( const QAudioDeviceInfo & audioDevice, const QAudioFormat & format = QAudioFormat(), QObject * parent = 0 )
| ~QAudioOutput ()
int | bufferSize () const
int | bytesFree () const
qint64 | elapsedUSecs () const
QAudio::Error | error () const
QAudioFormat | format () const
int | notifyInterval () const
int | periodSize () const
qint64 | processedUSecs () const
void | reset ()
void | resume ()
void | setBufferSize ( int value )
void | setNotifyInterval ( int ms )
void | start ( QIODevice * device )
QIODevice * | start ()
QAudio::State | state () const
void | stop ()
void | suspend ()

- 29 public functions inherited from QObject

## Signals

void | notify ()
void | stateChanged ( QAudio::State state )

- 1 signal inherited from QObject

### Additional Inherited Members

- 1 property inherited from QObject
- 1 public slot inherited from QObject
- 7 static public members inherited from QObject
- 8 protected functions inherited from QObject

## 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 *audioOutput; // class member.
         ...
         void startPlaying()
         {
             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;
             }

             audioOutput = new QAudioOutput(format, this);
             connect(audioOutput,SIGNAL(stateChanged(QAudio::State)),SLOT(finishedPlaying(QAudio::State)));
             audioOutput->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) {
                 audioOutput->stop();
                 inputFile.close();
                 delete audioOutput;
             }
         }

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::StoppedState:
                     if (audioOutput->error() != QAudio::NoError) {
                         // Perform error handling
                     } else {
                         // Normal stop
                     }
                     break;

See also QAudioInput and QAudioDeviceInfo.

## Member Function Documentation

### QAudioOutput::QAudioOutput ( const QAudioFormat & format = QAudioFormat(), QObject * parent = 0 )

Construct a new audio output and attach it to parent. The default audio output device is used with the output format parameters.

### QAudioOutput::QAudioOutput ( const QAudioDeviceInfo & audioDevice, const QAudioFormat & format = QAudioFormat(), QObject * parent = 0 )

Construct a new audio output and attach it to parent. The device referenced by audioDevice is used with the output format parameters.

### QAudioOutput::~QAudioOutput ()

Destroys this audio output.

### int QAudioOutput::bufferSize () const

Returns the audio buffer size in bytes.

If called before start(), returns platform default value. If called before start() but setBufferSize() was called prior, returns value set by setBufferSize(). If called after start(), returns the actual buffer size being used. This may not be what was set previously by setBufferSize().

See also setBufferSize().

### int QAudioOutput::bytesFree () const

Returns the free space available in bytes in the audio buffer.

NOTE: returned value is only valid while in QAudio::ActiveState or QAudio::IdleState state, otherwise returns zero.

### qint64 QAudioOutput::elapsedUSecs () const

Returns the microseconds since start() was called, including time in Idle and Suspend states.

### QAudio::Error QAudioOutput::error () const

Returns the error state.

### QAudioFormat QAudioOutput::format () const

Returns the QAudioFormat being used.

### void QAudioOutput::notify ()`[signal]`

This signal is emitted when x ms of audio data has been processed the interval set by setNotifyInterval(x).

### int QAudioOutput::notifyInterval () const

Returns the notify interval in milliseconds.

See also setNotifyInterval().

### int QAudioOutput::periodSize () const

Returns the period size in bytes.

Note: This is the recommended write size in bytes.

### qint64 QAudioOutput::processedUSecs () const

Returns the amount of audio data processed by the class since start() was called in microseconds.

Note: The amount of audio data played can be determined by subtracting the microseconds of audio data still in the systems audio buffer.

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

### void QAudioOutput::reset ()

Drops all audio data in the buffers, resets buffers to zero.

### void QAudioOutput::resume ()

Resumes processing audio data after a suspend().

Sets error() to QAudio::NoError. Sets state() to QAudio::ActiveState if you previously called start(QIODevice*). Sets state() to QAudio::IdleState if you previously called start(). emits stateChanged() signal.

Note: signal will always be emitted during execution of the resume() function.

### void QAudioOutput::setBufferSize ( int value )

Sets the audio buffer size to value in bytes.

Note: This function can be called anytime before start(), calls to this are ignored after start(). It should not be assumed that the buffer size set is the actual buffer size used, calling bufferSize() anytime after start() will return the actual buffer size being used.

See also bufferSize().

### void QAudioOutput::setNotifyInterval ( int ms )

Sets the interval for notify() signal to be emitted. This is based on the ms of audio data processed not on actual real-time. The minimum resolution of the timer is platform specific and values should be checked with notifyInterval() to confirm actual value being used.

See also notifyInterval().

### void QAudioOutput::start ( QIODevice * device )

Uses the device as the QIODevice to transfer data. Passing a QIODevice allows the data to be transferred without any extra code. All that is required is to open the QIODevice.

If able to successfully output audio data to the systems audio device the state() is set to QAudio::ActiveState, error() is set to QAudio::NoError and the stateChanged() signal is emitted.

If a problem occurs during this process the error() is set to QAudio::OpenError, state() is set to QAudio::StoppedState and stateChanged() signal is emitted.

In either case, the stateChanged() signal may be emitted either synchronously during execution of the start() function or asynchronously after start() has returned to the caller.

See also QIODevice.

### QIODevice * QAudioOutput::start ()

Returns a pointer to the QIODevice being used to handle the data transfer. This QIODevice can be used to write() audio data directly.

If able to access the systems audio device the state() is set to QAudio::IdleState, error() is set to QAudio::NoError and the stateChanged() signal is emitted.

If a problem occurs during this process the error() is set to QAudio::OpenError, state() is set to QAudio::StoppedState and stateChanged() signal is emitted.

In either case, the stateChanged() signal may be emitted either synchronously during execution of the start() function or asynchronously after start() has returned to the caller.

See also QIODevice.

### QAudio::State QAudioOutput::state () const

Returns the state of audio processing.

### void QAudioOutput::stateChanged ( QAudio::State state )`[signal]`

This signal is emitted when the device state has changed. This is the current state of the audio output.

### void QAudioOutput::stop ()

Stops the audio output, detaching from the system resource.

Sets error() to QAudio::NoError, state() to QAudio::StoppedState and emit stateChanged() signal.

### void QAudioOutput::suspend ()

Stops processing audio data, preserving buffered audio data.

Sets error() to QAudio::NoError, state() to QAudio::SuspendedState and emit stateChanged() signal.