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Sound Activated Recorder with Spectrogram in C#

, 27 Jan 2008 GPL3
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Audio event processing with visual display


This project demonstrates an implementation of the waterfall spectrogram and use of statistical data to trigger events in near real-time. This code is an elaboration of my previous submission (SoundViewer). This demonstration utilizes the Wave classes developed by Ianier Munoz.

Using the Code

Audio is supplied by the default input device which is typically the microphone. Events are triggered when audio amplitude exceeds the desired threshold value, which can be set under Options on the menu bar. To make this more useful, I've added functionality to save the stream to disk which results in a nice sound activated recorder.

Points of Interest

In order to draw the spectrogram fast enough to allow for near real-time operation, I needed to write directly to memory using unsafe code.

// lock image
PixelFormat format = canvas.PixelFormat;
BitmapData data = 
    canvas.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadOnly, format);
int stride = data.Stride;
int offset = stride - width * 4;

// draw image
    byte* pixel = (byte*)data.Scan0.ToPointer();
    // for each column
    for (int y = 0; y <= height; y++)
      if (y < _fftLeftSpect.Count)
        // for each row
        for (int x = 0; x < width; x++, pixel += 4)
          double amplitude = ((double[])_fftLeftSpect[_fftLeftSpect.Count - y - 1])
                [(int)(((double)(_fftLeft.Length) / (double)(width)) * x)];
          double color = GetColor(min, max, range, amplitude);
          pixel[0] = (byte)0;
          pixel[1] = (byte)color;
          pixel[2] = (byte)0;
          pixel[3] = (byte)255;
        pixel += offset;
catch (Exception ex)

// unlock image

I noticed that the results vary wildly depending on the hardware and associated drivers being used.

Some things I'd like to experiment with further when I get the time:

  1. Use of frequency domain to produce "motion" detector equivalent
  2. Use of spectrogram in sound identification
  3. Improving performance/robustness


  • 01/16/2008: Created


This article, along with any associated source code and files, is licensed under The GNU General Public License (GPLv3)


About the Author

Jeff Morton
Systems / Hardware Administrator
United States United States
No Biography provided

Comments and Discussions

GeneralThrows exception when using Freq &lt; 11025 Hz Pinmemberk^s10-Jun-09 8:07 
QuestionHow do I get it to work? PinmemberMaiden Computer Company22-Apr-09 11:48 
AnswerRe: How do I get it to work? PinmemberJeff Morton22-Apr-09 11:51 
QuestionError in Frequency domain? PinmemberElysium8-Mar-09 12:17 
AnswerRe: Error in Frequency domain? PinmemberJeff Morton8-Mar-09 12:41 
GeneralRe: Error in Frequency domain? PinmemberElysium8-Mar-09 23:02 
QuestionQuestion on how to get the frequencies Pinmemberkyanmark_johnwill2-Mar-09 14:52 
AnswerRe: Question on how to get the frequencies Pinmemberhazeljane13-Mar-09 4:49 
GeneralRe: Question on how to get the frequencies PinmemberJeff Morton13-Mar-09 11:34 
GeneralRe: Question on how to get the frequencies Pinmemberafmf23-Oct-09 2:54 
GeneralRe: Question on how to get the frequencies Pinmemberafmf3-Nov-09 0:38 
GeneralRe: Question on how to get the frequencies Pinmember Muammar©1-Jan-10 1:56 
Questionspectrogram of a wav file Pinmemberplouvou12-Jan-09 10:59 
AnswerRe: spectrogram of a wav file PinmemberTae-Sung18-Jan-09 10:07 
I am interested in processing wave file for spectrogram too. But I don't think it would be simple task to change input to wave file by changing a couple of code lines because (1) wave file can be different in sample rate, mono or stereo, and so on and (2) microphone input signal would be relatively very short but wave file can be huge. Nevertheless, I think it is still great reference for spectrogram in c#. Thanks the author for that.
I think it would be as simple as following.
1. read wave file - read header of the file in riff format and get info about samples. I believe there are several good articles showing a way to handle wave file.
2. pass samples from #1 into Process method in AudioFrame. I think it will additionally require more code changes according to the wave file header info, e.g., mono or stereo, sample rate, etc. Also you might have to change drawing routine as program might need to draw huge data.
GeneralRe: spectrogram of a wav file Pinmembertortexy2-Feb-09 13:50 
GeneralRe: spectrogram of a wav file PinmemberGeoNav31-Oct-09 2:52 
GeneralDraw waveform for already existing wave file Pinmemberjugal_piet@indiatimes.com5-Jan-09 21:26 
GeneralVery great app PinmemberMr. Yi26-Nov-08 5:31 
GeneralBug in event detection - left channel only is used PinmemberAtis Straujums9-Oct-08 11:43 
GeneralRe: Bug in event detection - left channel only is used PinmemberJeff Morton9-Oct-08 13:16 
GeneralAnother hi! Pinmemberstano24-Sep-08 19:16 
GeneralAnger detection software Pinmember100,000 Cold Calls12-Aug-08 12:44 
GeneralRe: Anger detection software Pinmemberflowerchild128-Feb-09 13:48 
Generalthe file is not saving ...!! PinmemberAshok Nalam9-Jul-08 2:05 
QuestionDisplay Of Frequencies PinmemberRonD25-May-08 5:17 
AnswerRe: Display Of Frequencies PinmemberMember 47088011-Aug-08 16:24 
GeneralRe: Display Of Frequencies PinmemberMember 47088011-Aug-08 16:25 
QuestionHow can i make it faster? Pinmemberpauloricca16-May-08 7:20 
AnswerRe: How can i make it faster? PinmemberJeff Morton16-May-08 8:05 
GeneralRe: How can i make it faster? Pinmemberpauloricca19-May-08 0:42 
GeneralTwo Questions Pinmemberrodriguesd23-Mar-08 1:45 
GeneralThis is great stuff Pinmemberpeter.roca6-Mar-08 10:34 
GeneralHi Pinmembercircass4-Mar-08 7:32 
GeneralJust a thought... PinmemberEd.Poore29-Jan-08 14:53 
QuestionProblem with Solution file? PinmemberEitsopGmail29-Jan-08 0:41 
AnswerRe: Problem with Solution file? PinmemberJeff Morton29-Jan-08 1:16 
GeneralRe: Problem with Solution file? Pinmembercircass4-Mar-08 7:34 

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