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Posted 16 Jan 2008

Sound Activated Recorder with Spectrogram in C#

, 27 Jan 2008
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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

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Comments and Discussions

AnswerRe: Object reference not set to an instance of an object Pin
deepak thomas5-Dec-11 6:57
memberdeepak thomas5-Dec-11 6:57 
GeneralRe: Object reference not set to an instance of an object Pin
Member 1029319526-Sep-13 18:08
memberMember 1029319526-Sep-13 18:08 
QuestionSound Catcher freezes on Exit in Win 7 32-bit Pin
Member 836994911-Nov-11 11:13
memberMember 836994911-Nov-11 11:13 
AnswerRe: Sound Catcher freezes on Exit in Win 7 32-bit Pin
Timothy Ayodele14-Jan-12 1:54
memberTimothy Ayodele14-Jan-12 1:54 
QuestionPlease how can i make the recording every 1 hour Pin
Timothy Ayodele2-Nov-11 7:26
memberTimothy Ayodele2-Nov-11 7:26 
QuestionVote 5 Pin
serega4675-Aug-11 0:41
memberserega4675-Aug-11 0:41 
GeneralIn VB.NET Pin
emicroxxx23-May-11 2:55
memberemicroxxx23-May-11 2:55 
GeneralMy vote of 5 Pin
abdillahi26-Apr-11 22:23
groupabdillahi26-Apr-11 22:23 
very helpful ! Thanks.
QuestionHow can i save output to .txt ? Pin
stefan_brato27-Nov-10 16:48
memberstefan_brato27-Nov-10 16:48 
QuestionSpectrogram Disappears Pin
GeoNav12-Jul-10 18:26
memberGeoNav12-Jul-10 18:26 
QuestionGPLv2 Pin
Member 42771197-Jul-10 10:00
memberMember 42771197-Jul-10 10:00 
QuestionHow to generate wave-form if we have the byte-array with us. Pin
Mark0211-Mar-10 23:06
memberMark0211-Mar-10 23:06 
GeneralIntegration Time Pin
GeoNav17-Feb-10 4:17
memberGeoNav17-Feb-10 4:17 
GeneralSound Equalizer Pin
BazKhan25-Jan-10 19:34
memberBazKhan25-Jan-10 19:34 
GeneralAmplitude -sound loudness Pin
ginwithlime9-Jan-10 4:10
memberginwithlime9-Jan-10 4:10 
GeneralRe: Amplitude -sound loudness Pin
Jeff Morton9-Jan-10 5:36
memberJeff Morton9-Jan-10 5:36 
QuestionHow to stop the thread if a invoke methode gets called in the DataArrived Event Pin
Michael Muehllehner7-Dec-09 3:57
memberMichael Muehllehner7-Dec-09 3:57 
AnswerRe: How to stop the thread if a invoke methode gets called in the DataArrived Event Pin
kelibox6-Aug-15 13:09
memberkelibox6-Aug-15 13:09 
QuestionHow to make the IFFT? Pin
afmf3-Nov-09 5:33
memberafmf3-Nov-09 5:33 
AnswerRe: How to make the IFFT? Pin
Rahdu23-Sep-10 9:35
memberRahdu23-Sep-10 9:35 
QuestionHow to reduce the noise? Pin
GeoNav31-Oct-09 1:55
memberGeoNav31-Oct-09 1:55 
Generallineout audio Pin
GeoNav28-Oct-09 23:10
memberGeoNav28-Oct-09 23:10 
QuestionChange Source to wav file Pin
GeoNav21-Oct-09 21:36
memberGeoNav21-Oct-09 21:36 
AnswerRe: Change Source to wav file Pin
Jeff Morton22-Oct-09 11:23
memberJeff Morton22-Oct-09 11:23 
GeneralRe: Change Source to wav file Pin
GeoNav26-Oct-09 18:35
memberGeoNav26-Oct-09 18:35 

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