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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

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Comments and Discussions

Questionspectrogram of a wav file Pin
plouvou12-Jan-09 10:59
memberplouvou12-Jan-09 10:59 
AnswerRe: spectrogram of a wav file Pin
Tae-Sung18-Jan-09 10:07
memberTae-Sung18-Jan-09 10:07 
GeneralRe: spectrogram of a wav file Pin
tortexy2-Feb-09 13:50
membertortexy2-Feb-09 13:50 
GeneralRe: spectrogram of a wav file Pin
GeoNav31-Oct-09 2:52
memberGeoNav31-Oct-09 2:52 
I am getting the spectrogram of wav files using the same classes. Only thing is that I am playing the wav file using DirectSound or SoundPlayer and then collect the audio from lineout using wavout class. It works, but there is a great amount of noise and the frequency domain display stops after about five minutes.
GeneralDraw waveform for already existing wave file Pin
jugal_piet@indiatimes.com5-Jan-09 21:26
memberjugal_piet@indiatimes.com5-Jan-09 21:26 
GeneralVery great app Pin
Mr. Yi26-Nov-08 5:31
memberMr. Yi26-Nov-08 5:31 
GeneralBug in event detection - left channel only is used Pin
Atis Straujums9-Oct-08 11:43
memberAtis Straujums9-Oct-08 11:43 
GeneralRe: Bug in event detection - left channel only is used Pin
Jeff Morton9-Oct-08 13:16
memberJeff Morton9-Oct-08 13:16 
GeneralAnother hi! Pin
stano24-Sep-08 19:16
memberstano24-Sep-08 19:16 
GeneralAnger detection software Pin
100,000 Cold Calls12-Aug-08 12:44
member100,000 Cold Calls12-Aug-08 12:44 

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