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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

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 
GeneralRe: Change Source to wav file Pin
afmf3-Nov-09 5:38
memberafmf3-Nov-09 5:38 
To make a band pass the filter, simply put the FFT with the value zero on the frequencies that you don't wish. For example, if you want the human voice (it's more and less 130-1100Hz). You can convert to Hz using the scaleHz on the values of the FFT array.

To get the audio frames you will have to do the inverse of the FFT calculated (IFFT).
I want this as well :P If he answers me or you find out, please post here.

aF was here -.-

AnswerRe: Change Source to wav file Pin
ravitzur18-Dec-10 23:07
memberravitzur18-Dec-10 23:07 
GeneralError in Program Pin
Piligrim200720-Oct-09 11:04
memberPiligrim200720-Oct-09 11:04 
GeneralRe: Error in Program Pin
Jeff Morton22-Oct-09 11:23
memberJeff Morton22-Oct-09 11:23 
GeneralRe: Error in Program Pin
Piligrim200724-Oct-09 0:52
memberPiligrim200724-Oct-09 0:52 
QuestionCan't run the program in Vista 64bit Pin
Yasir Salih Osman7-Aug-09 22:36
memberYasir Salih Osman7-Aug-09 22:36 
AnswerRe: Can't run the program in Vista 64bit Pin
Fullmetal9901211-Sep-09 7:20
memberFullmetal9901211-Sep-09 7:20 
AnswerRe: Can't run the program in Vista 64bit Pin
Rahdu18-Oct-10 22:09
memberRahdu18-Oct-10 22:09 
AnswerRe: Can't run the program in Vista 64bit Pin
Member 320938312-Apr-11 12:10
memberMember 320938312-Apr-11 12:10 
AnswerRe: Can't run the program in Vista 64bit Pin
Rebmem1529-Sep-11 18:33
memberRebmem1529-Sep-11 18:33 
Questionhow can i do it reverse? [modified] Pin
mojimoj2-Aug-09 21:05
membermojimoj2-Aug-09 21:05 
GeneralExcellent piece of work Jeff Pin
maxima1204-Jul-09 12:17
membermaxima1204-Jul-09 12:17 
GeneralRe: Excellent piece of work Jeff Pin
Jeff Morton5-Jul-09 5:46
memberJeff Morton5-Jul-09 5:46 
GeneralThrows exception when using Freq &lt; 11025 Hz Pin
k^s10-Jun-09 7:07
memberk^s10-Jun-09 7:07 
QuestionHow do I get it to work? Pin
Maiden Computer Company22-Apr-09 10:48
memberMaiden Computer Company22-Apr-09 10:48 
AnswerRe: How do I get it to work? Pin
Jeff Morton22-Apr-09 10:51
memberJeff Morton22-Apr-09 10:51 
QuestionError in Frequency domain? Pin
Elysium8-Mar-09 11:17
memberElysium8-Mar-09 11:17 
AnswerRe: Error in Frequency domain? Pin
Jeff Morton8-Mar-09 11:41
memberJeff Morton8-Mar-09 11:41 

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