Audio Overview

Audio Features

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 = new QMediaPlayer;
audioOutput = new QAudioOutput;
player->setAudioOutput(audioOutput);
// ...
player->setSource(QUrl::fromLocalFile("/Users/me/Music/coolsong.mp3"));
audioOutput->setVolume(50);
player->play();

The same functionality in QML:

MediaPlayer {
    audioOutput: AudioOutput {}
    source: "file:///path/to/my/music.mp3"
    Component.onCompleted: { play() }
}

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 QMediaCaptureSession, 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++:

QMediaCaptureSession session;
QAudioInput audioInput;
session.setAudioInput(&input);
QMediaRecorder recorder;
session.setMediaRecorder(&recorder);
recorder.setQuality(QMediaRecorder::HighQuality);
recorder.setOutputLocation(QUrl::fromLocalFile("test.mp3"));
recorder.record();

In QML, the same can be achieved by:

CaptureSession {
    audioInput: AudioInput {}
    mediaRecorder: MediaRecorder {
        id: recorder
        outputLocation: "file:///path/to/test.mp3"
    }
    Component.onCompleted: { recorder.record() }
}

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:

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.

The 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 - push and pull. In pull mode, the audio device is started by giving it a QIODevice. For an output device, the QAudioSink class will pull data from the QIODevice (using QIODevice::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 QIODevice.

In 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 QIODevice instances.

Here's an example of decoding a local file:

QAudioFormat desiredFormat;
desiredFormat.setChannelCount(2);
desiredFormat.setSampleFormat(QAudioFormat::Int16);
desiredFormat.setSampleRate(48000);

QAudioDecoder *decoder = new QAudioDecoder(this);
decoder->setAudioFormat(desiredFormat);
decoder->setSource("level1.mp3");

connect(decoder, SIGNAL(bufferReady()), this, SLOT(readBuffer()));
decoder->start();

// Now wait for bufferReady() signal and call decoder->read()

Examples

There are both C++ and QML examples available.

C++ Examples

Reference Documentation

C++ Classes

QAudio

Contains enums used by the audio classes

QAudioBuffer

Represents a collection of audio samples with a specific format and sample rate

QAudioDecoder

Implements decoding audio

QAudioDevice

Information about audio devices and their functionality

QAudioFormat

Stores audio stream parameter information

QAudioInput

Represents an input channel for audio

QAudioOutput

Represents an output channel for audio

QAudioSink

Interface for sending audio data to an audio output device

QAudioSource

Interface for receiving audio data from an audio input device

QMediaCaptureSession

Allows capturing of audio and video content

QMediaRecorder

Used for encoding and recording a capture session

QSoundEffect

Way to play low latency sound effects

QML Types

AudioInput

An audio input to be used for capturing audio in a capture session

AudioOutput

An audio output to be used for playback or monitoring of a capture session

CaptureSession

Allows capturing of audio and video content

MediaPlayer

Adds media playback to a scene

MediaRecorder

For encoding and recording media generated in a CaptureSession

SoundEffect

Type provides a way to play sound effects in QML

audioDevice

Describes an audio device

mediaMetaData

Provides meta-data for media files

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