Qt 5.11 | Qt Multimedia | C++ Classes | QAudioOutput

Qt 5.11.3 Reference Documentation

# QAudioOutput Class

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

Header: | #include <QAudioOutput>
qmake: | QT += multimedia

- List of all members, including inherited members

## Public Functions

| QAudioOutput(const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)
| QAudioOutput(const QAudioDeviceInfo &audioDevice, const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)
virtual | ~QAudioOutput()
int | bufferSize() const
int | bytesFree() const
QString | category() 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 | setCategory(const QString &category)
void | setNotifyInterval(int ms)
void | setVolume(qreal volume)
void | start(QIODevice *device)
QIODevice * | start()
QAudio::State | state() const
void | stop()
void | suspend()
qreal | volume() const

## Signals

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

## 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 sourceFile;   // class member.
      QAudioOutput* audio; // class member.
      {
          sourceFile.setFileName("/tmp/test.raw");
          sourceFile.open(QIODevice::ReadOnly);

          QAudioFormat format;
          // Set up the format, eg.
          format.setSampleRate(8000);
          format.setChannelCount(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)), this, SLOT(handleStateChanged(QAudio::State)));
          audio->start(&sourceFile);
      }

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 AudioOutputExample::handleStateChanged(QAudio::State newState)
      {
          switch (newState) {
              case QAudio::IdleState:
                  // Finished playing (no more data)
                  audio->stop();
                  sourceFile.close();
                  delete audio;
                  break;

              case QAudio::StoppedState:
                  // Stopped for other reasons
                  if (audio->error() != QAudio::NoError) {
                      // Error handling
                  }
                  break;

              default:
                  // ... other cases as appropriate
                  break;
          }
      }

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 AudioOutputExample::handleStateChanged(QAudio::State newState)
      {
          switch (newState) {
              case QAudio::IdleState:
                  // Finished playing (no more data)
                  audio->stop();
                  sourceFile.close();
                  delete audio;
                  break;

              case QAudio::StoppedState:
                  // Stopped for other reasons
                  if (audio->error() != QAudio::NoError) {
                      // Error handling
                  }
                  break;

              default:
                  // ... other cases as appropriate
                  break;
          }
      }

See also QAudioInput and QAudioDeviceInfo.

## Member Function Documentation

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

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 = nullptr)

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

### `[virtual]` QAudioOutput::~QAudioOutput()

Destroys this audio output.

This will release any system resources used and free any buffers.

### 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 number of free bytes available in the audio buffer.

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

### QString QAudioOutput::category() const

Returns the audio category of this audio stream.

Some platforms can group audio streams into categories and manage their volumes independently, or display them in a system mixer control. You can set this property to allow the platform to distinguish the purpose of your streams.

See also setCategory().

### 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.

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

This signal is emitted when a certain interval of milliseconds of audio data has been processed. The interval is set by setNotifyInterval().

### int QAudioOutput::notifyInterval() const

Returns the notify interval in milliseconds.

See also setNotifyInterval().

### int QAudioOutput::periodSize() const

Returns the period size in bytes. This is the amount of data required each period to prevent buffer underrun, and to ensure uninterrupted playback.

Note: It is recommended to provide at least enough data for a full period with each write operation.

### qint64 QAudioOutput::processedUSecs() const

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

### 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.

### 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 - call bufferSize() anytime after start() to return the actual buffer size being used.

See also bufferSize().

### void QAudioOutput::setCategory(const QString &category)

Sets the audio category of this audio stream to category.

Some platforms can group audio streams into categories and manage their volumes independently, or display them in a system mixer control. You can set this property to allow the platform to distinguish the purpose of your streams.

Not all platforms support audio stream categorization. In this case, the function call will be ignored.

Changing an audio output stream's category while it is opened will not take effect until it is reopened.

See also category().

### 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 wall clock time. The minimum resolution of the timer is platform specific and values should be checked with notifyInterval() to confirm the actual value being used.

See also notifyInterval().

### void QAudioOutput::setVolume(qreal volume)

Sets the output volume to volume.

The volume is scaled linearly from `0.0` (silence) to `1.0` (full volume). Values outside this range will be clamped.

The default volume is `1.0`.

Note: Adjustments to the volume will change the volume of this audio stream, not the global volume.

UI volume controls should usually be scaled nonlinearly. For example, using a logarithmic scale will produce linear changes in perceived loudness, which is what a user would normally expect from a volume control. See QAudio::convertVolume() for more details.

See also volume().

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

Starts transferring audio data from the device to the system's audio output. The device must have been opened in the ReadOnly or ReadWrite modes.

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

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

See also QIODevice.

### QIODevice *QAudioOutput::start()

Returns a pointer to the internal QIODevice being used to transfer data to the system's audio output. The device will already be open and write() can write data directly to it.

Note: The pointer will become invalid after the stream is stopped or if you start another stream.

If the QAudioOutput is able to access the system's audio device, state() returns QAudio::IdleState, error() returns QAudio::NoError and the stateChanged() signal is emitted.

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

See also QIODevice.

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

Returns the state of audio processing.

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

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 emits stateChanged() signal.

### qreal QAudioOutput::volume() const

Returns the volume between 0.0 and 1.0 inclusive.

See also setVolume().