Silverlight 4 - encoding PCM data from the microphone

Question

I've written a basic SL4 application to capture audio data from the microphone using CaptureSource. The trouble is, it's raw PCM output - which means huge and uncompressed.

Given that I need this application to run purely within a SL4 environment, how can I compress the PCM audio data into something that can be delivered to a remote server more easily?

Essentially I need a solution that I can also deploy/include in a Windows Phone Series 7 application as well as one that will work in the browser environment - so managed code solutions only, I think?

In conversation, people have suggested Speex and WMA for instance, but I haven't found any libraries or examples that work without requiring reference to DLL's that won't work in a SL4 project.

Answer 1

Please see the WavFileHelper class in Silverlight 4 Rough Notes: Camera and Microphone Support on Mike Taulty's blog (a bit lower than the middle of the page, but the full article is worthwhile) in which he compresses the PCM file to WAV.

Here's another example of when writing to WAV you can change values such as Mono/Stereo, which will directly change the size of the WAV file: Audio recorder Silverlight 4 sample. And one more that gives more details about writing to WAV: Creating Sound using MediaStreamSource in Silverlight 3 Beta

Answer 2

Just a small addition to Jason's post:

There is another port of Speex to .Net and Silverlight 4 called NSpeex.

Answer 3

Take a look at this. It looks like he has ported the Speex encoder to C# for the exact problem you are trying to solve. It is available here. Speex is designed for speech and should perform better than wma, mp3, or other audio codecs that are designed to handle music if you are just encoding speech, which I assume since you are grabbing from the mic.

Answer 4

This article http://alvas.net/alvas.audio,articles.aspx#how-to-save-audio-to-mp3-on-silverlight about save audio on client. To send audio data to a server you can use WebClient, for example.

Answer 5

You can do encoding thru the server, by send all stream to WCF service and do your encoding thru Microsoft Expression Encoding SDK API.

Please, see this url that i have asked before: http://forums.silverlight.net/forums/t/181141.aspx

Regards