音频概述

音频特征

Qt Multimedia offers a range of audio classes, covering both low and high level approaches to audio input, output and processing. In addition to traditional audio usage, the Qt Audio Engine QML types offer high level 3D positional audio for QML applications. See that documentation for more information.

音频实现细节

播放压缩音频

For playing media or audio files that are not simple, uncompressed audio, you can use the QMediaPlayer C++ class, or the Audio and MediaPlayer QML types. The QMediaPlayer class and associated QML types are also capable of playing video , if required. The compressed audio formats supported does depend on the operating system environment, and also what media plugins the user may have installed.

Here is how you play a local file using C++:

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

You can also put files (even remote URLs) into a playlist:

player = new QMediaPlayer;
playlist = new QMediaPlaylist(player);
playlist->addMedia(QUrl("http://example.com/myfile1.mp3"));
playlist->addMedia(QUrl("http://example.com/myfile2.mp3"));
// ...
playlist->setCurrentIndex(1);
player->play();
							

把音频录制到文件

For recording audio to a file, the QAudioRecorder class allows you to compress audio data from an input device and record it.

audioRecorder = new QAudioRecorder;
QAudioEncoderSettings audioSettings;
audioSettings.setCodec("audio/amr");
audioSettings.setQuality(QMultimedia::HighQuality);
audioRecorder->setEncodingSettings(audioSettings);
audioRecorder->setOutputLocation(QUrl::fromLocalFile("test.amr"));
audioRecorder->record();
							

Low Latency Sound Effects

In addition to the raw access to sound devices described above, the QSoundEffect class (and SoundEffect QML type) offers a slightly higher level way to play sounds. These classes allow you to specify a WAV format file which can then be played with low latency when necessary. Both QSoundEffect and SoundEffect have essentially the same API.

You can adjust the number of loops a sound effect is played, as well as the volume (or muting ) of the effect.

For older, Qt 4.x based applications QSound is also available. Applications are recommended to use QSoundEffect where possible.

在回放 (或录制) 期间监视音频数据

QAudioProbe class allows you to monitor audio data being played or recorded in the higher level classes like QMediaPlayer , QCamera and QAudioRecorder . After creating your high level class, you can simply set the source of the probe to your class, and receive audio buffers as they are processed. This is useful for several audio processing tasks, particularly for visualization or adjusting gain. You cannot modify the buffers, and they may arrive at a slightly different time than the media pipeline processes them.

Here's an example of installing a probe during recording:

audioRecorder = new QAudioRecorder;
QAudioEncoderSettings audioSettings;
audioSettings.setCodec("audio/amr");
audioSettings.setQuality(QMultimedia::HighQuality);
audioRecorder->setEncodingSettings(audioSettings);
audioRecorder->setOutputLocation(QUrl::fromLocalFile("test.amr"));
audioProbe = new QAudioProbe(this);
if (audioProbe->setSource(audioRecorder)) {
    // Probing succeeded, audioProbe->isValid() should be true.
    connect(audioProbe, SIGNAL(audioBufferProbed(QAudioBuffer)),
            this, SLOT(calculateLevel(QAudioBuffer)));
}
audioRecorder->record();
// Now audio buffers being recorded should be signaled
// by the probe, so we can do things like calculating the
// audio power level, or performing a frequency transform
							

Low Level Audio Playback and Recording

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.

QAudioOutput class offers raw audio data output, while QAudioInput offers raw audio data input. Both classes have adjustable buffers and latency, so they are suitable for both low latency use cases (like games or VOIP) and high latency (like music playback). 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 QAudioOutput class will pull data from the QIODevice (using QIODevice::read ()) when more audio data is required. Conversely, for pull mode with QAudioInput , 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.

把压缩音频解码到内存

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 实例。

Here's an example of decoding a local file:

QAudioFormat desiredFormat;
desiredFormat.setChannelCount(2);
desiredFormat.setCodec("audio/x-raw");
desiredFormat.setSampleType(QAudioFormat::UnSignedInt);
desiredFormat.setSampleRate(48000);
desiredFormat.setSampleSize(16);
QAudioDecoder *decoder = new QAudioDecoder(this);
decoder->setAudioFormat(desiredFormat);
decoder->setSourceFilename("level1.mp3");
connect(decoder, SIGNAL(bufferReady()), this, SLOT(readBuffer()));
decoder->start();
// Now wait for bufferReady() signal and call decoder->read()
							

范例

存在 C++ 和 QML 两种可用范例。

C++ 范例

参考文档编制

C++ 类

QAbstractAudioDeviceInfo

音频后端基类

QAudio

包含用于音频类的枚举

QAudioBuffer

表示具有特定格式和采样率的音频样本的集合

QAudioDecoder

允许解码音频

QAudioDeviceInfo

查询音频设备及其功能的接口

QAudioFormat

存储音频流参数信息

QAudioInput

从音频输入设备接收音频数据的接口

QAudioOutput

把音频数据发送到音频输出设备的接口

QAudioProbe

允许监视正播放 (或录制) 音频

QAudioSystemPlugin

音频插件抽象基

QSound

播放 .wav 声音文件的方法

QSoundEffect

播放低延迟声音效果的办法

QAudioBuffer::StereoFrame

立体声音频帧的简单包裹器

QML 类型

Audio

把音频回放添加到场景

MediaPlayer

把媒体回放添加到场景

Playlist

针对要播放的指定媒体列表

PlaylistItem

定义 Playlist 播放列表项

SoundEffect

Type provides a way to play sound effects in QML