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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

AnswerRe: Can't run the program in Vista 64bit PinmemberFullmetal9901211-Sep-09 8:20 
AnswerRe: Can't run the program in Vista 64bit PinmemberRahdu18-Oct-10 23:09 
AnswerRe: Can't run the program in Vista 64bit PinmemberMember 320938312-Apr-11 13:10 
AnswerRe: Can't run the program in Vista 64bit PinmemberRebmem1529-Sep-11 19:33 
Questionhow can i do it reverse? [modified] Pinmembermojimoj2-Aug-09 22:05 
GeneralExcellent piece of work Jeff Pinmembermaxima1204-Jul-09 13:17 
GeneralRe: Excellent piece of work Jeff PinmemberJeff Morton5-Jul-09 6:46 
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 
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 

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