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MP3 to SWF converter in Actionscript 3

February 13th, 2009

A while ago I was playing with SWF generation within Flash itself.
I read the Adobe SWF specs and now I'm able to generate SWF's from FLV's and MP3's or both.

A simple Flash 10 example, haven't tested all possible MP3 rates so if someone encounters a bug, please let me know.
(Just found out that some MP3's with embedded art-work won't work... added to my TODO's).

Code snippet which writes the MP3 data to a SWF bytearray:

while(mp3.audioFrames.length > currentAudioFrame)
	var needSamples:int = 0.001 * ((1000/(__frameRate))*i) * audioRate;
	var startSamples:int = currentSamples;
	var audioData:ByteArray = new ByteArray();
	while(mp3.audioFrames.length > currentAudioFrame && currentSamples< needSamples)
		var aframe:AudioFrame = AudioFrame(mp3.audioFrames[currentAudioFrame]);
		currentSamples = (currentAudioFrame+1) * (576*2);
	var samples:int = currentSamples - startSamples;
	var seek:int = startSamples ? needSamples - startSamples : 0;
	var af:AudioFrame = mp3.audioFrames[i] as AudioFrame;
	_swfBytes.writeShort(createHeader(SWFTags.PLACE_OBJECT2,3)); //Place frame data
	_swfBytes.writeShort(1); //depth

This way I write all SWF headers the correct way and not like most other code out there which uses a precompiled SWF and replace the sound object.

Bernard Actionscript, Flash

  1. March 6th, 2009 at 03:49 | #1

    Very cool. With this technique can you load the swf and access the data as a Sound object? How would you play the mp3? Is there a way to generate a Sound instance and load the data into it outside of a URLRequest?

    I was using the precompiled SWF / Sound object technique you mentioned above until I decided to use fvorbis instead for my audio needs.

    But I’m interested in how you accomplish accessing the mp3 (AudioFrame in particular) and writing it out to generate a proper swf for playback.

    I guess I’ll have to look link the SWF specs sometime.

    Thanks for the post. I hope to see more on this!

    -Don Q.

  2. March 6th, 2009 at 21:25 | #2

    Well the audioframe is my own class. It loads a mp3 into a bytearray. Then I analyze the mp3 and pick out all frames. When you create a SWF you can add mp3 frames to a sound object like it’s an embedded sound.
    When I tidy up some of the code I will post some more.

  3. March 7th, 2009 at 00:42 | #3

    Cool. Thanks for the reply.

    -Don Q.

  4. Jérémie
    March 17th, 2009 at 11:45 | #4

    Do you know if there is a way to do the opposite : to create a mp3 file from a ByteArray, build with the SoundMixer class ? My goal is to generate a FLV file, from several external ressources (videos, sounds, graphics…).

  5. March 24th, 2009 at 16:37 | #5

    Haha! I found a bug :) My MP3 Encoder creates MP3’s that can’t be loaded this way. I’ll have to take a look at it.

  6. July 24th, 2009 at 06:23 | #6

    very cool!
    i know you can get an flv in a byteArray when you load using fileReference.data so you recon its possible to do the same with an flv?

  7. July 24th, 2009 at 08:09 | #7

    I know it’s possible with a FLV, because I did it already.
    The main thing is you have to parse the FLV to single FLV frames and put them in a SWF. Both specifications are open and downloadable at the adobe site.

  8. Atanas
    August 16th, 2009 at 04:18 | #8


    Hi Jérémie,

    I was wandering if you have found a way to create an MP3 from a ByteArray? I’ve been trying for more a day now but with little success.



  9. July 15th, 2010 at 09:19 | #9

    Nice work man you should develop this in to a full featured library

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