Flash: Sound

Sound is one of the things the flash got right, and is one of the reasons you want to pick flash for your project. HTML5, as good as it has become in last few years does not have the capabilities that matches flash when it comes to sound.

Sound related APIs in flash a very pleasently designed, and barring a few points can be almost called pythonic.

Embed MP3 File In SWF And Play It

Playing sound or mp3 is very easy in flash using "flash.media.Sound" class. There are two ways to create instances of this class. We can either use the constructor of flash.media.Sound class, as we will see later, or we can flash's ability to embed media in swf file.

Embedding mp3 in swf increases the size of swf file, it takes longer to download the file, but it makes file management easier, all we have to worry about is one swf file.

Flash provides a common way to embed images, mp3, flv etc in the swf file, by using a construct like this:

package
{
    import flash.display.Sprite;

    public class EmbedSoundTest extends Sprite
    {
        [Embed(source="sound.mp3")] private static const MySound:Class;
    }
}

What this [Embed] construct does is, it verifies that file indeed exists, then it makes sure file is something flash can handle, flash can handle few image formats, few sound formats and some video formats. You will see compiler errors if the file does not exist or if it is a format flash can not handle.

After all verification, flash compiler embeds the referenced file sound.mp3 into generated swf file, and at runtime flash runtime creates a class for us, the class is then stored as a private class attribute by name MySound, of type Class.

So we have created EmbedSoundText class, a subclass of Sprite, and EmbedSoundText.MySound, a class that can be used to create instances of Sound class. Why are we not storing it as const MySound:Sound? Because that would imply MySound is an instance of Sound class, which it is not. MySound is an instance of class Class, just like in python where every class is instance of class "type".

So lets take our MySound class and create an instance of Sound class and play our sound:

package
{
    import flash.display.Sprite;
    import flash.media.Sound;

    public class EmbedSoundTest extends Sprite
    {
        [Embed(source="sound.mp3")] private static const MySound:Class;

        public function EmbedSoundTest()
        {
            var mysound:Sound = new MySound() as Sound;
            mysound.play();
        }
    }
}

We created an instance of MySound class, which happens to be subclass of Sound, so we casted it as such. All that is left to do is call .play() on it to start playing the sound.

Load A MP3 From Server And Play It

Embedding mp3 in swf is good for a set of predefined sounds, but other times we want to load sound from net. Either because we are creating a mp3 player, or because we want to keep a few mp3 files out of swf to reduce the swf file size, and loading the mp3 dynamically when needed, eg at the end of game, or beginning of next stage etc.

So how do we download sound files from net? flash.media.Sound class has two way to load mp3 from net, it can either take an instance of flash.net.URLRequest class as constructor argument, or we can pass URLRequest instance to sound class instance's .load() method.

In either case flash will start downloading the mp3 file right away, and will play it when .play() is called.

URLRequest takes the path of mp3 file as a string constructor parameter. The path can be relative to swf file, or it can be absolute URL. Flash will check flash policy file in case the URL points to a different domain.

Lets take a look at how it all looks like:

package
{
    import flash.display.Sprite;
    import flash.media.Sound;
    import flash.net.URLRequest;

    public class ExternalSoundTest extends Sprite
    {
        public function ExternalSoundTest()
        {
            var sound:Sound = new Sound(new URLRequest("song.mp3"));
            sound.play();
        }
    }
}

As soon as sound object is created, flash starts downloading and buffering the mp3. By default it waits for 1 second buffer, and starts playing as soon as 1 second worth of mp3 is downloaded, so playback will start after a small delay.

Playing At Offset And Repeating

The .play() method on Sound class instance can optionally take two parameters: .play(start_offset_in_milliseconds=0, repeat_count=1), with default values 0 and 1. Repeat cound has to be an integer, if you want to keep repeating a song, pass a large integer value, eg int.MAX_VALUE.

Stoppping, Pausing And Resuming Playback

Stopping playback is trivial. Sound.play() method return an instance of SoundChannel, and this instance has a method .stop()

There is no native support of pause and resume in Sound API, but we can implement pause by storing the current position of sound when stoppping and implement resume by passing the stored position to .play() method.

The current position can be obtained as soundchannel.position attribute. This attribute contains time in milliseconds since the beginning of sound.

Detecting End Of Playback

Before we implement pause and resume we have to track if sound has already finished playing.

One way to do that is by using Sound.leght property to find length of sound in milliseconds and compare it with SoundChannel.position when play/pause button is clicked.

Another option is to monitor Event.SOUND_COMPLETE fired by SoundChannel object.

Lets combine all this to make a mp3 player.

We will have two buttons on it, "Play" and "Stop". "Stop" button is initially disabled. Once we hit play the sound will start playing, and the text of "Play" button with change to "Pause", and "Stop" button gets enabled. We monitor the SOUND_COMPLETE event, when it is fired we change the text of "Pause" back to "Play" and disable "Stop" button.

While the sound is playing, if one hits the "Pause" button, we store the SoundChannel.current in a variable, call SoundChannel.stop() and change the "Pause" button text to "Resume". If the user hits "Resume" button, we call Sound.play() with stored position to resume sound from old position, and change the text of "Resume" button to "Pause" again.

While the sound if playing we also listen to ENTER_FRAME event and draw a bar showing total sound length and another bar indicating the current position in playback.

...

Buffering, Tracking Buffering and Stopping Download

  1. mp3 and ID3 tags
  2. Sound levels and spectrum for visualization
  3. Controlling volume
  4. SoundMixer class for global sound management

Back to Flash For Python Developers.

 
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