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Audio recorder Silverlight 4 sample

by ondrejsv 12. December 2009 14:48

[UPDATE 2010-08-28: I updated the sample to work with the final release of Silverlight 4. I also improved the design a little :-)]

Today I want to share with you my Audio Record sample using some new cool features coming in Silverlight 4. The sample demonstrates:

  • recording audio using the AudioCaptureDevice and CaptureSource classes, and saving it using an AudioSink object,
  • access to local folders on the computer,
  • simple commanding model using the new Command properties and ICommand interface,
  • changing appearance of a standard control (button) using its visual states.

Of course, full source code is attached at the end of the article.

The user interface is really simple:


What we want to achieve:

  1. three state audio recorder with three buttons “Record”, “Stop” and “Save”,
  2. the user first selects the “Record” buttons, the recorder will begin recording; the user clicks the “Stop” button when he is done with recording and finally selects the “Save” button to be prompted by a classic Save file dialog to save the record into a standard WAV file on his disc,
  3. command buttons should have a correct enabled/disabled logic,
  4. a status bar with text helping the user what to do or what is the recorder doing,

Disclaimer: Obviously I’m not the one gifted with graphical skills so don’t expect nifty animations here and there. It’s just a sample. :-)

Recording audio

An AudioCaptureDevice object represents a hardware device cable capturing sound. There may be more than one present and the user may select a default one in the new Silverlight 4 configuration tab:


You can get the default one through the CaptureDeviceConfiguration class:

var audioDevice = CaptureDeviceConfiguration.GetDefaultAudioCaptureDevice();

Then you use a CaptureSource object to capture any input from this source:

_captureSource = new CaptureSource() { AudioCaptureDevice = audioDevice };

Lastly, we need to connect a sink to the capture source. Sinks allow us to process incoming input from the source in any way we want – for example to store in a memory stream. For this we derive a new AudioSink (our is called MemoryAudioSink) and override the most important method – OnSamples:

protected override void OnSamples(long sampleTime, long sampleDuration, byte[] sampleData) { // New audio data arrived, write them to the stream. _stream.Write(sampleData, 0, sampleData.Length); }

To connect and start capturing the sound:

_sink = new MemoryAudioSink(); _sink.CaptureSource = _captureSource; _captureSource.Start()

Writing a WAV file

CaptureSource sends the captured sound to its sinks as raw PCM data. We need to add a standard WAV header to be usable for a user. This is done by the SavePcmToWav method:

public static void SavePcmToWav(Stream rawData, Stream output, AudioFormat audioFormat) { if (audioFormat.WaveFormat != WaveFormatType.Pcm) throw new ArgumentException("Only PCM coding is supported."); BinaryWriter bwOutput = new BinaryWriter(output); // Write down the WAV header. // Refer to http://technology.niagarac.on.ca/courses/ctec1631/WavFileFormat.html // for details on the format. // Note that we use ToCharArray() when writing fixed strings // to force using the char[] overload because // Write(string) writes the string prefixed by its length. // -- RIFF chunk bwOutput.Write("RIFF".ToCharArray()); // Total Length Of Package To Follow // Computed as data length plus the header length without the data // we have written so far and this data (44 - 4 ("RIFF") - 4 (this data)) bwOutput.Write((uint)(rawData.Length + 36)); bwOutput.Write("WAVE".ToCharArray()); // -- FORMAT chunk bwOutput.Write("fmt ".ToCharArray()); // Length Of FORMAT Chunk (Binary, always 0x10) bwOutput.Write((uint)0x10); // Always 0x01 bwOutput.Write((ushort)0x01); // Channel Numbers (Always 0x01=Mono, 0x02=Stereo) bwOutput.Write((ushort)audioFormat.Channels); // Sample Rate (Binary, in Hz) bwOutput.Write((uint)audioFormat.SamplesPerSecond); // Bytes Per Second bwOutput.Write((uint)(audioFormat.BitsPerSample * audioFormat.SamplesPerSecond * audioFormat.Channels / 8)); // Bytes Per Sample: 1=8 bit Mono, 2=8 bit Stereo or 16 bit Mono, 4=16 bit Stereo bwOutput.Write((ushort)(audioFormat.BitsPerSample * audioFormat.Channels / 8)); // Bits Per Sample bwOutput.Write((ushort)audioFormat.BitsPerSample); // -- DATA chunk bwOutput.Write("data".ToCharArray()); // Length Of Data To Follow bwOutput.Write((uint)rawData.Length); // Raw PCM data follows... // Reset position in rawData and remember its origin position // to restore at the end. long originalRawDataStreamPosition = rawData.Position; rawData.Seek(0, SeekOrigin.Begin); // Append all data from rawData stream into output stream. byte[] buffer = new byte[4096]; int read; // number of bytes read in one iteration while ((read = rawData.Read(buffer, 0, 4096)) > 0) { bwOutput.Write(buffer, 0, read); } rawData.Seek(originalRawDataStreamPosition, SeekOrigin.Begin); }

Simple commanding with a simple view model

I’m a fan of the new Model-View-ViewModel pattern as it allows you further properly structure your presentation layer while taking advantage of powerful WPF/Silverlight databinding. It wasn’t so simple to use it in Silverlight projects because it did not support commands as the full WPF stack did so you needed to resort to Prism or other frameworks.

However, with Silverlight 4 comes the ICommand interface:

public interface ICommand { event EventHandler CanExecuteChanged; bool CanExecute(object parameter); void Execute(object parameter); }

and new Command properties on ButtonBase-derived controls and the HyperLink control.

With this you can create your own viewmodel wrapping all your code interacting with the user interface and application logic in a single unit-testable class. The user interface will hook into your viewmodel class though public ICommand and dependency properties by leveraging databinding. Your code-behind will be almost empty!

To simplify things I created a SimpleCommand because all of my commands will just call a method on the viewmodel class and I will set their executability directly through a property:

public class SimpleCommand : ICommand { public Action ExecuteAction { get; set; } private bool _canExecute; public bool MayBeExecuted { get { return _canExecute; } set { if (_canExecute != value) { _canExecute = value; if (CanExecuteChanged != null) CanExecuteChanged(this, new EventArgs()); } } } #region ICommand Members public bool CanExecute(object parameter) { return MayBeExecuted; } public event EventHandler CanExecuteChanged; public void Execute(object parameter) { if (ExecuteAction != null) ExecuteAction(); } #endregion }

Let’s dig into the viewmodel class – the heart of the application. First some declarations:

public class RecorderViewModel : DependencyObject { private SimpleCommand _recordCommand; private SimpleCommand _playPauseCommand; private SimpleCommand _saveCommand; private MemoryAudioSink _sink; private CaptureSource _captureSource; private SaveFileDialog saveFileDialog = new SaveFileDialog() { Filter = "Audio files (*.wav)|*.wav" }; public SimpleCommand RecordCommand { get { return _recordCommand; } } public SimpleCommand PlayPauseCommand { get { return _playPauseCommand; } } public SimpleCommand SaveCommand { get { return _saveCommand; } } public static readonly DependencyProperty StatusTextProperty = DependencyProperty.Register("StatusText", typeof(string), typeof(RecorderViewModel), null); public string StatusText { get { return (string)GetValue(StatusTextProperty); } set { SetValue(StatusTextProperty, value); } }

The constructor initializes the three commands to just call their respective methods on the class:

public RecorderViewModel() { _recordCommand = new SimpleCommand() { MayBeExecuted = true, ExecuteAction = () => Record() }; _saveCommand = new SimpleCommand() { MayBeExecuted = false, ExecuteAction = () => SaveFile() }; _playPauseCommand = new SimpleCommand() { MayBeExecuted = false, ExecuteAction = () => PlayOrPause() }; var audioDevice = CaptureDeviceConfiguration.GetDefaultAudioCaptureDevice(); _captureSource = new CaptureSource() { AudioCaptureDevice = audioDevice }; GoToStartState(); }

The last piece is to define the methods doing the hard work:

protected void Record() { if (!EnsureAudioAccess()) return; if (_captureSource.State != CaptureState.Stopped) return; _sink = new MemoryAudioSink(); _sink.CaptureSource = _captureSource; _captureSource.Start(); // Enable pause command, disable record command _playPauseCommand.MayBeExecuted = true; _recordCommand.MayBeExecuted = false; StatusText = "Recording..."; } protected void PlayOrPause() { if (_captureSource.State == CaptureState.Started) { _captureSource.Stop(); // Disable pause command, enable save command _playPauseCommand.MayBeExecuted = false; _saveCommand.MayBeExecuted = true; StatusText = "Recording finished. You may save your record."; } } protected void SaveFile() { if (saveFileDialog.ShowDialog() == false) { return; } StatusText = "Saving..."; Stream stream = saveFileDialog.OpenFile(); WavManager.SavePcmToWav(_sink.BackingStream, stream, _sink.CurrentFormat); stream.Close(); MessageBox.Show("Your record is saved."); GoToStartState(); }

Putting it all together

I put three buttons and one textblock into the default MainPage.xaml:

<Button Command="{Binding RecordCommand}" Grid.Column="0" HorizontalAlignment="Left" Name="button1" Template="{StaticResource RecordCommandStyle}"/>
<Button Content="Stop" Command="{Binding PlayPauseCommand}" Grid.Column="1" HorizontalAlignment="Left" Name="button3" Width="75" Template="{StaticResource StopCommandStyle}"/>
<Button Content="Save" Command="{Binding SaveCommand}" Grid.Column="2" HorizontalAlignment="Left" Name="button2" Width="57" Margin="0,0,0,-1" Template="{StaticResource SaveCommandStyle}"/>
<TextBlock Text="{Binding StatusText}" Name="textBlock1" Margin="8,5,-32,8" Grid.Row="1" Grid.ColumnSpan="4" Foreground="#FF13E3BC" FontSize="12" />

Note the bindings to the Command properties of the buttons and Text property of the textblock element (but this is nothing special). The whole viewmodel is then set to the page’s DataContext which is the only thing we do inside the code-behind:

RecorderViewModel ViewModel; public MainPage() { InitializeComponent(); ViewModel = new RecorderViewModel(); DataContext = ViewModel; }

That’s it. I didn’t show changing the appearance of the standard button controls using visual states here because this possibility has been with us long before the fourth version but it’s included in the project.

The floppy disk icon comes from the wonderful Crystal project.

Download full sources of the sample.

kick it on DotNetKicks.com [digg]


Comments (20) -

4/5/2010 8:50:49 AM #


Carp coarse fishing has to be understood by knowing what it is not.

carpfishing United States |

5/25/2010 8:51:11 PM #


This was build on beta version of silvelight and this is expired now and does nothing pleaase provide code with original version. Thanks

izhar Iran |

5/27/2010 12:13:09 PM #



This is what am exactly looking for. thanks.

paniraj India |

6/9/2010 10:52:34 AM #

International Parcel

Is it possible to get that to work on silverlight 3?

International Parcel United Kingdom |

6/23/2010 8:46:33 AM #


hi ondrejsv

Thanks for this article.
Can you please tell me is there any way that it will work on silverlight3 with vs 2008.
Like after building xap file can I use it with the silverlight 3.

Your answer is really worth for me.

lakhan India |

7/5/2010 10:29:45 AM #


Is it possible to save the wav file in a lower quality?
The output file is too large that it seems.

Webtest Hong Kong S.A.R. |

7/14/2010 7:31:11 AM #


Thanks you for your post
It has been very helpful.

윤형복 Korea |

7/14/2010 7:48:53 AM #


and PCM --&amp;amp;amp;gt; Pcm changed

윤형복 Korea |

8/25/2010 12:54:39 PM #


Hey your post is very helpful to build application in silverlight4, but can you encode the PCM raw data to some other format that the silverlight4 supports? Really great work done by you. i am new with silverlight4.Please, help me.

Shrinivas India |

9/18/2010 1:14:59 PM #


Wave sound saving on client side but I want to save on Server.please provide code to wav file can save on server.

Saif India |

9/21/2010 9:18:30 PM #


I&amp;amp;#39;m getting a a null pointer at

if (audioFormat.WaveFormat != WaveFormatType.Pcm)

on WavManager.cs

Nice project though, definitely cool.

Kosko United States |

9/27/2010 10:43:36 AM #


Your project is very nice. Please tell me, whether we can change the wav file into some other format or make this wav file to consume very low memory size.

Hope you can help me.

Pitchai United States |

10/17/2010 11:20:35 PM #


The WAV files created from this are playing back at too high a rate (2-3 times faster than intended). However, downloaded a couple of other similar projects - and they are all doing the same thing. Tried ajusting the bytes per second rate - as an experiment. It worked - but had quality problems.

Any ideas on why this might be the case. I&amp;amp;#39;m testing on Windows 7 running under VM-Ware hosted on a Mac.


Ken United States |

10/26/2010 5:55:10 AM #


Thank&amp;amp;#39;s you very much

anailham Indonesia |

1/28/2011 7:27:32 PM #


The type or namespace name &amp;amp;#39;AudioFormat&amp;amp;#39; could not be found (are you missing a using directive or an assembly reference?)

The name &amp;amp;#39;WaveFormatType&amp;amp;#39; does not exist in the current context

The type or namespace name &amp;amp;#39;CaptureSource&amp;amp;#39; could not be found (are you missing a using directive or an assembly reference?)

hey.....a getting these above erors....Please help friends

karthick India |

1/28/2011 7:36:58 PM #


Also am getting error that...

&amp;amp;amp;quot;Developer runtime is not installed,Please install matching version&amp;amp;amp;quot;

Am having VS2010 with Silverlight 4.0.51204.0

karthick India |

2/15/2011 1:50:42 PM #

abdus samadh

hi thank u so much............can u help me how to make audio chat bw two or multiple systems......thank u

abdus samadh India |

2/19/2011 11:59:43 AM #


your sample is really useful thanks very much

but for playing back recorded media without saving wav file you should have readable stream

so i add this two function to WavManager Class

        public static Stream readableStream(Stream rawData, AudioFormat audioFormat)
            MemoryStream str = new MemoryStream();
            str.Write(WavManager.stringToASCIIByte(&amp;amp;amp;quot;RIFF&amp;amp;amp;quot;), 0, 4);
            str.Write(BitConverter.GetBytes((uint)(rawData.Length + 36)), 0, 4);
            str.Write( WavManager.stringToASCIIByte(&amp;amp;amp;quot;WAVE&amp;amp;amp;quot;), 0, 4);
            str.Write(WavManager.stringToASCIIByte(&amp;amp;amp;quot;fmt &amp;amp;amp;quot;), 0, 4);
            str.Write(BitConverter.GetBytes((uint)0x10), 0, 4);
            str.Write(BitConverter.GetBytes((ushort)0x01), 0, 2);
            str.Write(BitConverter.GetBytes((ushort)audioFormat.Channels), 0, 2);
            str.Write(BitConverter.GetBytes((uint)audioFormat.SamplesPerSecond), 0, 4);
            str.Write(BitConverter.GetBytes((uint)(audioFormat.BitsPerSample * audioFormat.SamplesPerSecond * audioFormat.Channels / 8)), 0, 4);
            str.Write(BitConverter.GetBytes((ushort)(audioFormat.BitsPerSample * audioFormat.Channels / 8)), 0, 2);
            str.Write(BitConverter.GetBytes((ushort)audioFormat.BitsPerSample), 0, 2);
            str.Write(WavManager.stringToASCIIByte(&amp;amp;amp;quot;data&amp;amp;amp;quot;), 0, 4);
            str.Write(BitConverter.GetBytes((uint)rawData.Length), 0, 4);

            long originalRawDataStreamPosition = rawData.Position;
            rawData.Seek(0, SeekOrigin.Begin);
            // Append all data from rawData stream into output stream.
            byte[] buffer = new byte[4096];
            int read;       // number of bytes read in one iteration
            while ((read = rawData.Read(buffer, 0, 4096)) &amp;amp;amp;gt; 0)
                str.Write(buffer, 0, read);
            rawData.Seek(originalRawDataStreamPosition, SeekOrigin.Begin);
            return str;
        private static byte[] stringToASCIIByte(string str)
            byte[] pre = Encoding.Unicode.GetBytes(str);
            List&amp;amp;amp;lt;byte&amp;amp;amp;gt; retByte=new List&amp;amp;amp;lt;byte&amp;amp;amp;gt;();
            // Convert unicode (2bytes) to ascii (1byte) in per charactor (silverlight doesn&amp;amp;#39;t suport ASCII convertion)
            for(int i=0;i&amp;amp;amp;lt;pre.Length;i+=2)
            return retByte.ToArray();

and use couple of code to play the return stream


   WaveMediaStreamSource wavMss = new WaveMediaStreamSource(Strm);

masoud Iran |

3/28/2011 1:30:01 AM #


hi masoud

thank&amp;amp;#39;s  for your cod ,it helps me a lot but
do you have any idea how to create a stream file to save the
recorded sound without using the saveFileDialog .

rabee Belgium |

3/8/2011 3:58:15 PM #

Mohammad Abuzaid

Hi, i want to send the recorded file to the server

how can i do this ?

Mohammad Abuzaid Egypt |

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