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Windows Phone 7 : Sounding Out with Game Audio - Playing Music

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5/4/2012 4:26:36 PM
We have already discussed the fact that the SoundEffect class is not to be used for playing music, but that doesn't mean that we can't provide backing tracks for our games. XNA provides separate functionality for playing music, but in the context of Windows Phone 7 there are some things that we need to be aware of in order for our game to meet Microsoft's certification requirements.

Let's look at how (and when) you can play music within your games.

1. To Play or Not To Play

The certification requirement complication for Windows Phone 7 games revolves around the fact that one of the other primary uses for the device is as a media player. The operating system has a flexible media library that allows music and other audio content to be played on the device, even when the media library has been moved to the background.

As a result, it is entirely possible that, when the user launches your game, music is already playing in the background. Microsoft has decided that this existing music should take priority over your game music and that you must not stop it from playing without either directly asking the user for permission (by displaying a dialog box, for example, asking if the user wants to play the game music instead of the current audio track) or by providing a configuration option that allows it to be configured on a more permanent basis (in which case the option must default to not interrupting the existing media playback). Without observing this requirement, your game will be rejected when you submit it to the Windows Phone Marketplace.

We will look at how to perform this check (and how to pause the background music if appropriate) in a moment, but for now please bear in mind the need to do this.

Note that this check applies only to playing music with the MediaPlayer class, which is the class that provides access to the music player functionality. Sound effect playback is permitted even if the device is already playing music, so no special checking needs to be performed for sound effects. The certification requirements state that sound effect objects should not be used for playing background music, however, so this isn't a way to bypass the requirement.

2. Adding Music to your Project

Music is also added to the Content project. Unlike sound effects, music is expected to be in either MP3 or WMA format. This is a good thing because such formats produce much smaller files than WAV files due to the way they are compressed. Because music is likely to be much longer in duration than a sound effect, having compression applied is essential for keeping the size of your finished game under control.

MP3 files have taken over the world during the last decade and must surely form one of the most widely known file formats in existence. Sound files encoded using MP3 are compressed using a lossy compression algorithm. Although this means that there is some degradation of the audio when it is played back (just as there is a loss of image quality with JPG images), in most cases the quality loss is virtually or completely unnoticeable.

MP3 can compress audio data to different degrees, and the higher the compression the greater the quality loss on playback. The compression level is set when the MP3 is created by specifying a bit rate, which controls how many kilobits of data can be used to store each second of compressed audio. Compressing files at a bit rate of 128 kilobits per second will typically reduce CD quality audio to about 9 percent of its original file size—a massive saving.

Windows Media Audio (WMA) files are similar in approach to MP3s, also using a proprietary Microsoft compress to provide lossy compression (although a lossless variation is available). Microsoft claims that WMA files can be created that have the same quality level as an MP3 while using only half the storage space, though there are those who dispute this claim. Nevertheless, it is still a capable format and certainly worth considering as a format for your game music.

NOTE

Audacity is capable of exporting sounds in both MP3 and WMA format.

When you add a music file to the content, the properties show the Content Processor as Song, as shown in Figure 1.

Figure 1. The properties for an MP3 file added to an XNA Content project

"Song" is in fact the term that XNA uses to refer to a piece of music, and its class names reflect this. To load a song, we use the Song class along with the usual Content.Load call. In the game framework, we store a collection of songs within the GameHost class inside a Dictionary named Songs. Listing 1 shows the instruction within a project's LoadContent function that loads a song into the framework.

Example 1. Loading a song into the game framework from the Content project
// Load our song
        Songs.Add("2020", Content.Load<Song>("Breadcrumbs_2020"));

3. Playing the Music

Playing a song is very easy: just call the static MediaPlayer.Play function, passing in the Song object that you have loaded. Only one song can play at a time; trying to play a second song will stop the first song from playing.

However, we have the tricky issue of whether we are allowed to play music or not because, if the device is already playing background music, we must leave it alone.

This is determined using the MediaPlayer.GameHasControl property. If it returns true, we have full access to playing music; if it returns false, there is music already playing; and unless the user has explicitly confirmed that they want for our game to take control, we must allow it to continue. Listing 2 shows some code from the LoadContent function of the BackgroundMusic example project . If it detects that media are already playing, it doesn't even attempt to load the song; otherwise, the song is loaded and played.

Example 2. Checking whether the game is allowed to play music and then loading and starting playback
// Load songs
        if (MediaPlayer.GameHasControl)
        {
            // Load our song
            Songs.Add("2020", Content.Load<Song>("Breadcrumbs_2020"));

            // Play the song, repeating
            MediaPlayer.IsRepeating = true;
            MediaPlayer.Play(Songs["2020"]);
        }

Assuming that we can play our music, there is a number of other methods and properties that we can access from the MediaPlayer class to affect the way in which the music plays:

  • Pause and Stop can be used to halt playback. Just as with sound effects, Pause will remember the position at which the song was paused, allowing it to be later resumed; whereas Stop will discard the position. Either way, you can call the Resume method to start the song playing again.

  • IsMuted and Volume provide control over the playback volume. IsMuted is a boolean property that will completely silence the song without pausing it, while Volume allows the playback volume to be faded between silence (0.0) and the current device volume (1.0).

  • IsRepeating allows you to set the song to loop endlessly. This looping is often very useful for games because they tend to have background music that plays repeatedly the whole time the game is running. There is a slight issue with repeating music to be aware of, however, as we will discuss in a moment.

  • PlayPosition returns a TimeSpan object detailing the current playback time through the song. This can be used to create a playback time display, as will be demonstrated shortly.

Each loaded Song object also has a series of interesting-looking properties that might be interrogated. Unfortunately, it turns out that they aren't too useful after all. Properties are available to provide information on the song's Album, Artist, and Genre, among other things, but when songs are read from a game's Content project, none of them is populated even if the information is present inside the MP3 file. These properties are instead used to access data on songs contained within the device's media library.

One useful property that we can read from the Song is its Duration, which also returns a TimeSpan, this time containing the length of the song. We can use this alongside the MediaLibrary.PlayPosition for our playback time display. The BackgroundMusic example project displays such a timer and generates its text as shown in Listing 3. An example of the display it produces is shown in Figure 2.

Example 3. Displaying the current position and duration of a song
// Are we playing a song?
        if (MediaPlayer.GameHasControl)
        {
            // Yes, so read the position and duraction
            currentPosition = MediaPlayer.PlayPosition;
            duration = Songs["2020"].Duration;
            // Display the details in our text object
            TextObject positionText = (TextObject)GameObjects[0];
            positionText.Text = "Song position: "
                        + new DateTime(currentPosition.Ticks).ToString("mm:ss") + "/"
                        + new DateTime(duration.Ticks).ToString("mm:ss");
        }

					  

Figure 2. The display from the BackgroundMusic example project

This is nearly all you need to know about playing music, but there is one other thing that you should be aware of, and it is a rather annoying feature of XNA's media playback functionality. When you set a song to loop, there is a very slight pause between the song finishing and restarting. If your song fades to silence at the end, it will be unnoticeable, but if you try to loop the song so that it restarts seamlessly, this pause can be quite noticeable.

Although there is nothing you can do to eliminate the pause, you can make a minor adjustment to the song to slightly reduce its impact. The pause can be irritating on two levels: it enforces a brief moment of silence and it results in the music playback being thrown slightly off-beat. This second problem can be very distracting indeed.

To eliminate the timing problem, edit your music file and trim about one-tenth of a second from the end. Then save it and use this modified version in your game. The pause will then be offset by a corresponding gap in the music track, allowing the beat to continue uninterrupted.

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