QAudioSink Class

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

Header: #include <QAudioSink>
CMake: find_package(Qt6 COMPONENTS Multimedia REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Multimedia)
qmake: QT += multimedia
Inherits: QObject

Public Functions

QAudioSink(const QAudioDevice &audioDevice, const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)
QAudioSink(const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)
virtual ~QAudioSink()
qsizetype bufferSize() const
qsizetype bytesFree() const
qint64 elapsedUSecs() const
QAudio::Error error() const
QAudioFormat format() const
qint64 processedUSecs() const
void reset()
void resume()
void setBufferSize(qsizetype value)
void setVolume(qreal volume)
void start(QIODevice *device)
QIODevice *start()
QAudio::State state() const
void stop()
void suspend()
qreal volume() const

Signals

void stateChanged(QAudio::State state)

Detailed Description

You can construct an audio output with the system's default audio output device. It is also possible to create QAudioSink with a specific QAudioDevice. 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. QAudioSink 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.
QAudioSink* 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.setSampleFormat(QAudioFormat::UInt8);

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

    audio = new QAudioSink(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 QAudioSink 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().

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 QAudioSource and QAudioDevice.

Member Function Documentation

QAudioSink::QAudioSink(const QAudioDevice &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.

QAudioSink::QAudioSink(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.

[signal] void QAudioSink::stateChanged(QAudio::State state)

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

[virtual] QAudioSink::~QAudioSink()

Destroys this audio output.

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

qsizetype QAudioSink::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().

qsizetype QAudioSink::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.

qint64 QAudioSink::elapsedUSecs() const

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

QAudio::Error QAudioSink::error() const

Returns the error state.

QAudioFormat QAudioSink::format() const

Returns the QAudioFormat being used.

qint64 QAudioSink::processedUSecs() const

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

void QAudioSink::reset()

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

void QAudioSink::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 QAudioSink::setBufferSize(qsizetype 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 QAudioSink::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 non-linearly. 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 QAudioSink::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 QAudioSink 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 *QAudioSink::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 QAudioSink 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 QAudioSink::state() const

Returns the state of audio processing.

void QAudioSink::stop()

Stops the audio output, detaching from the system resource.

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

void QAudioSink::suspend()

Stops processing audio data, preserving buffered audio data.

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

qreal QAudioSink::volume() const

Returns the volume between 0.0 and 1.0 inclusive.

See also setVolume().

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