Playback, recording and processing of Audio.
Qt Multimedia offers a range of audio classes that cover both low and high level approaches to: audio input, output and processing.
Audio Implementation Details¶
Playing Compressed Audio¶
For playing media or audio files that are not simple, uncompressed audio, you can use the
QMediaPlayer C++ class, or the MediaPlayer QML type. The
QMediaPlayer class and associated QML types are also capable of playing video , if required. The audio formats that the player supports depends on:
The operating system environment.
Any decoder plugins the user may have installed.
The media player needs to be connected to a
QAudioOutput object (or the QML AudioOutput element) to play back audio.
Here is how you play a local file using C++:
player = QMediaPlayer audioOutput = QAudioOutput player.setAudioOutput(audioOutput) # ... player.setSource(QUrl.fromLocalFile("/Users/me/Music/coolsong.mp3")) audioOutput.setVolume(50) player.play()
The same functionality in QML:
Recording Audio to a File¶
To record audio to a file, you need to create a capture session and connect to it an audio input and a recorder. These elements are implemented with the
QAudioInput , and
QMediaRecorder classes. The default constructed
QAudioInput selects the system default audio input. The recorder controls the recording process with a simple record() and stop() functions. Additionally, you can use it to select the output location, audio encoder, or file container format.
A session recording audio from the default microphone would look as follows in C++:
session = QMediaCaptureSession() audioInput = QAudioInput() session.setAudioInput(input) recorder = QMediaRecorder() session.setMediaRecorder(recorder) recorder.setQuality(QMediaRecorder.HighQuality) recorder.setOutputLocation(QUrl.fromLocalFile("test.mp3")) recorder.record()
In QML, the same can be achieved by:
QMediaCaptureSession also provides support for more complex use cases such as image capturing or video recording.
Low Latency Sound Effects¶
In addition to raw access to sound devices, the
QSoundEffect class (and SoundEffect QML type) offers a more abstract way to play sounds. This class allows you to specify a WAV format file, which can then be played with low latency when necessary.
You can adjust the:
Number of loopsin which a sound effect is played.
Volumeof the sound effect.
Mutingof the sound effect.
Low Level Audio Playback and Recording¶
The C++ API of Qt Multimedia offers classes for raw access to audio input and output facilities, allowing applications to receive raw data from devices like microphones, and to write raw data to speakers or other devices. Generally these classes do not do any audio decoding, or other processing, but they can support different types of raw audio data.
QAudioSink class offers raw audio data output, while
QAudioSource offers raw audio data input. The available hardware determines what audio outputs and inputs are available.
Push and Pull¶
The low level audio classes can operate in two modes -
pull mode, the audio device is started by giving it a
QIODevice . For an output device, the
QAudioSink class will pull data from the
read() ) when more audio data is required. Conversely, for
pull mode with
QAudioSource , when audio data is available then the data will be written directly to the
push mode, the audio device provides a
QIODevice instance that can be written or read to as needed. Typically, this results in simpler code but more buffering, which may affect latency.
Decoding Compressed Audio to Memory¶
In some cases you may want to decode a compressed audio file and do further processing yourself. For example, mixing multiple samples or using custom digital signal processing algorithms.
QAudioDecoder supports decoding local files or data streams from
Here’s an example of decoding a local file:
desiredFormat = QAudioFormat() desiredFormat.setChannelCount(2) desiredFormat.setSampleFormat(QAudioFormat.Int16) desiredFormat.setSampleRate(48000) decoder = QAudioDecoder(self) decoder.setAudioFormat(desiredFormat) decoder.setSource("level1.mp3") connect(decoder, SIGNAL(bufferReady()), self, SLOT(readBuffer())) decoder.start() # Now wait for bufferReady() signal and call decoder->read()
There are both C++ and QML examples available.
The QAudio namespace contains enums used by the audio classes.
The QAudioBuffer class represents a collection of audio samples with a specific format and sample rate.
The QAudioDecoder class implements decoding audio.
The QAudioDevice class provides an information about audio devices and their functionality.
The QAudioFormat class stores audio stream parameter information.
Represents an input channel for audio.
Represents an output channel for audio.
The QAudioSink class provides an interface for sending audio data to an audio output device.
The QAudioSource class provides an interface for receiving audio data from an audio input device.
The QSoundEffect class provides a way to play low latency sound effects.
The QMediaCaptureSession class allows capturing of audio and video content.
The QMediaRecorder class is used for encoding and recording a capture session.
Describes an audio device.
An audio input to be used for capturing audio in a capture session.
An audio output to be used for playback or monitoring of a capture session.
The SoundEffect type provides a way to play sound effects in QML.
Adds media playback to a scene.
Provides meta-data for media files.
Allows capturing of audio and video content.
For encoding and recording media generated in a CaptureSession.
Defines an item in a Playlist.
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