Multimedia

Audio

The Audio Recorder example is a good introduction to the basic use of the API. We will use snippets from this example to illustrate how to use the API to quickly build functionality.

The first step is to demonstrate recording audio to a file. When recording from an audio source there are a number of things we may want to control beyond the essential user interface. We may want a particular encoding of the file, MP3 or Ogg Vorbis for instance, or select a different input source. The user may modify the bitrate, number of channels, quality and sample rate. Here the example will only modify the codec and the source device, since they are essential.

To begin, the developer sets up a source and a recorder object. A QAudioCaptureSource object is created and used to initialize a QMediaRecorder object. The output file name is then set for the QMediaRecorder object.

    audiosource = new QAudioCaptureSource;
    capture = new QMediaRecorder(audiosource);

    capture->setOutputLocation(QUrl("test.raw"));

A list of devices is needed so that an input can be selected in the user interface

    for(int i = 0; i < audiosource->deviceCount(); i++)
        deviceBox->addItem(audiosource->name(i));

and a list of the supported codecs for the user to select a codec,

    QStringList codecs = capture->supportedAudioCodecs();
    for(int i = 0; i < codecs.count(); i++)
        codecsBox->addItem(codecs.at(i));

To set the selected device or codec just use the index of the device or codec by calling the setter in audiosource or capture as appropriate, for example,

    audiosource->setSelectedDevice(i);
    ...
    capture->setAudioCodec(codecIdx);

Now start recording by using the record() function from the new QMediaRecorder object

    capture->record();

And stop recording by calling the matching function stop() in QMediaRecorder.

    capture->stop();

How then would this audio file be played? The QMediaPlayer class will be used as a generic player. Since the player can play both video and audio files the interface will be more complex, but for now the example will concentrate on the audio aspect.

Playing the file is simple: create a player object, pass in the filename, set the volume or other parameters, then play. Not forgetting that the code will need to be hooked up to the user interface.

    QMediaPlayer *player = new QMediaPlayer;
    ...
    player->setMedia(QUrl::fromLocalFile("test.raw"));
    player->setVolume(50);
    player->play();

The filename does not have to be a local file. It could be a URL to a remote resource. Also by using the QMediaPlaylist class from this API we can play a list of local or remote files. The QMediaPlaylist class supports constructing, managing and playing playlists.

    player = new QMediaPlayer;

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

To manipulate the playlist there are the usual management functions (which are in fact slots): previous, next, setCurrentPosition and shuffle. Playlists can be built, saved and loaded using the API.

    player = new QMediaPlayer;

    playlist = new QMediaPlaylist(player);
    playlist->append(QUrl("http://example.com/myclip1.mp4"));
    playlist->append(QUrl("http://example.com/myclip2.mp4"));
    ...
    widget = new QVideoWidget(player);
    widget->show();

    playlist->setCurrentPosition(1);
    player->play();

        videoWidget = new VideoWidget(this);
        player->setVideoOutput(videoWidget);

        playlistModel = new PlaylistModel(this);
        playlistModel->setPlaylist(playlist);

    QMediaService* requestService(const QByteArray &type, const QMediaServiceProviderHint &hint);

    void releaseService(QMediaService *service);

    QtMediaServices::SupportEstimate hasSupport(const QByteArray &serviceType,
                                        const QString &mimeType,
                                        const QStringList& codecs,
                                        int flags) const;

    QStringList supportedMimeTypes(const QByteArray &serviceType, int flags) const;

    QList<QByteArray> devices(const QByteArray &serviceType) const;

    QString deviceDescription(const QByteArray &serviceType, const QByteArray &device);