People who tried to create a casual game in Silverlight, encountered problems when they added sound effects to the game. The time between starting a sound effect and actually hearing it, is unacceptably long. This is called “audio latency”.
Short effects, typically less than a second, sound good the first time, while they sound bad the second time when they are played. The source of this problem is not clear, however long sounds don’t have this problem. This is called the “short sound problem”.
Also, it is very hard in Silverlight to play music in an idle loop. An idle loop with the out-of-the-box MediaElement, plays a silence every time the music is repeated. The silence at the beginning is actually caused by the same audio latency described above.
The above problems exist in Silverlight 4 and lower versions.
The audio gap
The music compression format MP3, is standard encoded with a silence at the beginning – the audio gap. This is because the decompression code needs some time to get ready, like an engine which needs time to get started. Tools like WinLame may encode an MP3 without an audio gap at the beginning, but this does no good to Silverlight’s decompression code. An MP3 created with WinLame will sound bad.
The audio buffer
Silverlight uses an audio buffer internally, which needs to be filled with audio data before any sound is played. The size of the audio buffer is measured in time, and it costs exactly that time to fill the audio buffer, before any sound is heard.
Reducing audio latency
The audio latency is caused by the audio gap from the music format and because of the audio buffer. So if we want to decrease the time between calling the Play function and actually hearing sound, we need to handle both the audio gap and audio buffer. Now obviously, a latency of 0.0 is impossible, otherwise Microsoft would have provided this with Silverlight.
The solution: VorbisPlayer
The solution is to choose for an audio format which is gapless. Second, the default length of the audio buffer must be decreased. And for a seamless idle loop, we must stream the next music part into the audio buffer, right after the previous music part has finished.
Ogg Vorbis is a music format which can be encoded without audio gap, however it is not supported by Silverlight. However, Silverlight does allow developers to implement their own audio decoder.
The VorbisPlayer provides exactly the solutions described above. More important, the VorbisPlayer is easy to use, for not so technical people.
The VorbisPlayer allows low latency for sound effects, much lower than Silverlight provides out of the box. Because of the used technique, the short sound problem is also solved. Also, the VorbisPlayer has an easy function to invoke a playlist of music, which will be played seamless without gaps in between.
The Silverlight VorbisPlayer can be found on Codeplex: http://vorbisplayer.codeplex.com/
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