QAudioSource Class | Qt Multimedia | Qt 6.11.1
Description: The QAudioSource class provides an interface for receiving audio data from an audio input device.

- Qt 6.11
- Qt Multimedia
- C++ Classes
- QtMultimedia
- QAudioSource
- Qt 6.11.1 Reference Documentation

# QAudioSource Class

The QAudioSource class provides an interface for receiving audio data from an audio input device. More...

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

- List of all members, including inherited members

## Public Functions

| QAudioSource(const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)
| QAudioSource(const QAudioDevice &audioDevice, const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)
virtual | ~QAudioSource() override
qsizetype | bufferFrameCount() const
`(since 6.10)` qsizetype | bufferSize() const
qsizetype | bytesAvailable() const
qint64 | elapsedUSecs() const
QtAudio::Error | error() const
QAudioFormat | format() const
`(since 6.10)` qsizetype | framesAvailable() const
bool | isNull() const
qint64 | processedUSecs() const
void | reset()
void | resume()
void | setBufferFrameCount(qsizetype value)
`(since 6.10)` void | setBufferSize(qsizetype value)
void | setVolume(qreal volume)
QIODevice * | start()
`(since 6.11)` void | start(Callback &&cb)
void | start(QIODevice *device)
QtAudio::State | state() const
void | stop()
void | suspend()
qreal | volume() const

## Signals

void | stateChanged(QtAudio::State state)

## Detailed Description

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

QAudioSink can be used in two different modes:

- Using a QIODevice from an application thread
- Using a callback-based interface from the audio thread

### QIODevice interface

QAudioSource lets you record audio with an audio input device. The default constructor of this class will use the systems default audio device, but you can also specify a QAudioDevice for a specific device. You also need to pass in the QAudioFormat in which you wish to record.

Starting up the QAudioSource is simply a matter of calling start() with a QIODevice opened for writing. For instance, to record to a file, you can:

     QFile destinationFile; // Class member
     QAudioSource* audio;   // Class member
     {
         destinationFile.setFileName("/tmp/test.raw");
         destinationFile.open( QIODevice::WriteOnly | QIODevice::Truncate );

         QAudioFormat format;
         // Set up the desired format, for example:
         format.setSampleRate(44100);
         format.setChannelCount(1);
         format.setSampleFormat(QAudioFormat::Int16);

         QAudioDevice info = QMediaDevices::defaultAudioInput();
         if (!info.isFormatSupported(format)) {
             qWarning() << "Default format not supported, trying to use the nearest.";
         }

         audio = new QAudioSource(format, this);
         connect(audio, &QAudioSource::stateChanged, this, &AudioInputExample::handleStateChanged);

         QTimer::singleShot(3000, this, &AudioInputExample::stopRecording);
         audio->start(&destinationFile);
         // Records audio for 3000ms
     }

This will start recording if the format specified is supported by the input device (you can check this with QAudioDevice::isFormatSupported(). In case there are any snags, use the error() function to check what went wrong. We stop recording in the `stopRecording()` slot.

     void AudioInputExample::stopRecording()
     {
         audio->stop();
         destinationFile.close();
         delete audio;
     }

At any point in time, QAudioSource will be in one of four states: active, suspended, stopped, or idle. These states are specified by the QtAudio::State enum.

QAudioSource provides several ways of measuring the time that has passed since the start() of the recording. The `processedUSecs()` function returns the length of the stream in microseconds written, i.e., it leaves out the times the audio input was suspended or idle. The elapsedUSecs() function returns the time elapsed since start() was called regardless of which states the QAudioSource has been in.

#### Threading model and buffering

The QIODevice interface is designed to be used from the application thread. A wait-free ringbuffer is used to communicate to the audio thread. The size of this ringbuffer can be configured with setBufferSize() and defaults to 250ms. The state of this buffer can be queried with bytesFree(). If the ringbuffer is full because the application does not read from the QIODevice in time, the state will change to QtAudio::IdleState and resume to QtAudio::ActiveState once the application has read data from the QIODevice. Note that this state change will drop audio data, so you should always read from the QIODevice as fast as possible to avoid dropouts.

### Callback interface

The preferred way to achieve low audio latencies is to use the callback based interface. It allows you to read audio data directly from the audio device without having to go through a QIODevice. This is done by calling start() with a callback function that will be called from the audio thread. This callback function will be called with a QSpan<const SampleType> whenever the audio backend produces data.

     QAudioSource* audio;          // class member.
     std::atomic<float> peakLevel; // class member.
     {
         QAudioFormat format;
         // Set up the format, eg.
         format.setSampleRate(44100);
         format.setChannelCount(2);
         format.setSampleFormat(QAudioFormat::Float);

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

         audio = new QAudioSource(format, this);
         audio->start([&peakLevel] (QSpan<float> interleavedAudioBuffer) {
             float level = peakLevel.load();

             for (float sample : interleavedAudioBuffer) {
                 // Calculate the peak level from the audio samples
                 level = std::max(level, std::abs(sample));
             }

             peakLevel.store(level);
             // Note: care needs to be taken if the application thread needs to be notified, as the
             // audio callback should not use any potentially blocking system calls.
             // Good options are autoreset events (windows), eventfd (linux) or kqueue/EVFILT_USER on macos.
         });

         if (!audio->error() == QtAudio::Error::NoError) {
             // in addition to the other start() signatures, starting the audio callback will fail if
             // * the backend does not implement callback-based IO (the API is available on all major
             //   platforms)
             // * the signature of the audio callback does not match format.sampleFormat()

             qWarning() << "Error starting audio output:" << audio->errorString();
         }
     }

Unlike the QIODevice-based interface, the QAudioSource can only be in the states active, suspendend and stopped. The setBufferSize() API is not available when using the callback, the size of the callback argument is determined by the audio backend.

Note: This API is only available on platforms that support the callback API: Apple's CoreAudio (macOS, iOS, etc), Windows, Linux (using the PulseAudio or PipeWire backend) and Android.

Note: The callback will be called on a soft-realtime audio thread. It is important to ensure that the callback does not block, as this can cause audio glitches or dropouts. This includes performing blocking IO, locking mutexes, allocating memories or any other operations that may block. For best practices consult Ross Bencina's article Real-time audio programming 101: time waits for nothing. Also consider using clang's Realtime sanitizer to validate the audio callback.

### State and error handling

State changes are reported through the stateChanged() signal. You can request a state change directly through suspend(), resume(), stop(), reset(), and start().

The QAudioSource will enter the StoppedState when an error is encountered. The error type can be retrieved error() function. Please see the QtAudio::Error enum for a description of the possible errors that are reported. Calling stop() or reset() will reset the error state to NoError.

     void AudioInputExample::handleStateChanged(QtAudio::State newState)
     {
         switch (newState) {
             case QtAudio::StoppedState:
                 if (audio->error() != QtAudio::NoError) {
                     // Error handling
                 } else {
                     // Finished recording
                 }
                 break;

             case QtAudio::ActiveState:
                 // Started recording - read from IO device
                 break;

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

See also QAudioSink and QAudioDevice.

## Member Function Documentation

### `[explicit]` QAudioSource::QAudioSource(const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)

Construct a new audio input and attach it to parent. The default audio input device is used with the output format parameters. If format is default-initialized, the format will be set to the preferred format of the audio device.

### `[explicit]` QAudioSource::QAudioSource(const QAudioDevice &audioDevice, const QAudioFormat &format = QAudioFormat(), QObject *parent = nullptr)

Construct a new audio input and attach it to parent. The device referenced by audioDevice is used with the input format parameters. If format is default-initialized, the format will be set to the preferred format of audioDevice.

### `[override virtual noexcept]` QAudioSource::~QAudioSource()

Destroy this audio input.

### qsizetype QAudioSource::bufferFrameCount() const

Returns the audio buffer size in frames.

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

See also setBufferFrameCount() and bufferSize.

### `[since 6.10]` qsizetype QAudioSource::bufferSize() const

Returns the audio buffer size in bytes.

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

This function was introduced in Qt 6.10.

See also setBufferSize() and bufferFrameCount.

### qsizetype QAudioSource::bytesAvailable() const

Returns the amount of audio data available to read in bytes.

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

See also framesAvailable.

### qint64 QAudioSource::elapsedUSecs() const

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

### QtAudio::Error QAudioSource::error() const

Returns the error state.

### QAudioFormat QAudioSource::format() const

Returns the QAudioFormat being used.

### `[since 6.10]` qsizetype QAudioSource::framesAvailable() const

Returns the amount of audio data available to read in frames.

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

This function was introduced in Qt 6.10.

See also bytesAvailable.

### bool QAudioSource::isNull() const

Returns `true` if the audio source is `null`, otherwise returns `false`.

### qint64 QAudioSource::processedUSecs() const

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

### void QAudioSource::reset()

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

### void QAudioSource::resume()

Resumes processing audio data after a suspend().

Sets state() to the state the sink had when suspend() was called. This function does nothing if the audio sink's state is not QtAudio::SuspendedState.

### void QAudioSource::setBufferFrameCount(qsizetype value)

Sets the audio buffer size to value in frame count.

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 bufferFrameCount() anytime after start() to return the actual buffer size being used.

See also bufferFrameCount() and setBufferSize.

### `[since 6.10]` void QAudioSource::setBufferSize(qsizetype value)

Sets the audio buffer size to value 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.

This function was introduced in Qt 6.10.

See also bufferSize() and setBufferFrameCount.

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

Sets the input volume to volume.

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

If the device does not support adjusting the input volume then volume will be ignored and the input volume will remain at 1.0.

The default volume is `1.0`.

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

See also volume().

### QIODevice *QAudioSource::start()

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

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

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

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

See also QIODevice and QIODevice interface.

### `[since 6.11]` template <typename Callback, QtAudio::if_audio_source_callback<Callback> = true> void QAudioSource::start(Callback &&cb)

Starts the QAudioSource with a callback function that will be called on a soft-realtime audio thread. The callback is a callable that takes a QSpan<const SampleType> as an argument, SampleType has to match the QAudioFormat::SampleFormat of the QAudioSource's format. The span contains the interleaved audio data.

If the QAudioSource is able to successfully start, error() returns QtAudio::NoError.

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

Note: This API is only available on platforms that support the callback API: Apple's CoreAudio (macOS, iOS, etc), Windows, Linux (using the PulseAudio or PipeWire backend) and Android.

Note: The callback will be called on a soft-realtime audio thread. It is important to ensure that the callback does not block, as this can cause audio glitches or dropouts. This includes performing blocking IO, locking mutexes, allocating memories or any other operations that may block. For best practices consult Ross Bencina's article Real-time audio programming 101: time waits for nothing. Also consider using clang's Realtime sanitizer to validate the audio callback.

This function was introduced in Qt 6.11.

See also Callback interface.

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

Starts transferring audio data from the system's audio input to the device. The device must have been opened in the WriteOnly, Append or ReadWrite modes.

If the QAudioSource is able to successfully get audio data, state() returns either QtAudio::ActiveState or QtAudio::IdleState, error() returns QtAudio::NoError and the stateChanged() signal is emitted.

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

See also QIODevice and QIODevice interface.

### QtAudio::State QAudioSource::state() const

Returns the state of audio processing.

### `[signal]` void QAudioSource::stateChanged(QtAudio::State state)

This signal is emitted when the device state has changed.

Note: The QtAudio namespace was named QAudio up to and including Qt 6.6. String-based connections to this signal have to use `QAudio::State` as the parameter type: `connect(source, SIGNAL(stateChanged(QAudio::State)), ...);`

### void QAudioSource::stop()

Stops the audio input, detaching from the system resource.

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

### void QAudioSource::suspend()

Stops processing audio data, preserving buffered audio data.

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

### qreal QAudioSource::volume() const

Returns the input volume.

If the device does not support adjusting the input volume the returned value will be 1.0.

See also setVolume().