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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

QuestionShutdown and stereo/mono fixes Pin
chronogeo, 19-Oct-14 23:27
memberchronogeo19-Oct-14 23:27 
AnswerRe: Shutdown and stereo/mono fixes Pin
chronogeo, 19-Oct-14 23:28
memberchronogeo19-Oct-14 23:28 
GeneralRe: Shutdown and stereo/mono fixes Pin
chronogeo, 19-Oct-14 23:29
memberchronogeo19-Oct-14 23:29 
GeneralRe: Shutdown and stereo/mono fixes Pin
chronogeo, 19-Oct-14 23:31
memberchronogeo19-Oct-14 23:31 
QuestionUI Thread locks up on close Pin
Member 11077213, 11-Sep-14 23:07
memberMember 1107721311-Sep-14 23:07 
QuestionHelp please: Nasalance ratio [modified] Pin
Member 10708439, 25-Jun-14 7:16
memberMember 1070843925-Jun-14 7:16 
GeneralThanks a lot! Pin
lopopoo, 21-Nov-13 5:25
memberlopopoo21-Nov-13 5:25 
QuestionExcellent article; quick question: would it be possible to "listen" to speaker output? Pin
Member 10293195, 28-Sep-13 8:17
memberMember 1029319528-Sep-13 8:17 
QuestionDelay/Latency Pin
hieronymusde, 13-Aug-13 12:21
memberhieronymusde13-Aug-13 12:21 
GeneralMy vote of 5 Pin
hieronymusde, 13-Aug-13 11:04
memberhieronymusde13-Aug-13 11:04 
QuestionHelp for operation on a modulated signal Pin
galbandrea, 14-Apr-13 23:13
membergalbandrea14-Apr-13 23:13 
Bug"В экземпляре объекта не задана ссылка на объект." [modified] Pin
Member 9960723, 4-Apr-13 2:54
memberMember 99607234-Apr-13 2:54 
Questionis there a way to select the iinput device (directx name ?) Pin
patrick zuili, 17-Mar-13 19:34
memberpatrick zuili17-Mar-13 19:34 
Generalamazing one Pin
Member 8107184, 12-Jan-13 6:07
memberMember 810718412-Jan-13 6:07 
GeneralMy vote of 5 Pin
javcastaalm, 23-Nov-12 10:08
memberjavcastaalm23-Nov-12 10:08 
GeneralThanks Pin
LeviButler, 3-Nov-12 14:27
memberLeviButler3-Nov-12 14:27 
GeneralMy vote of 5 Pin
Member 8067265, 22-May-12 1:47
memberMember 806726522-May-12 1:47 
Questiondraw spectrogram with a wav file. Pin
jacky_zz, 4-Feb-12 19:27
memberjacky_zz4-Feb-12 19:27 
QuestionLocking up when closing if there is activity [modified] Pin
programmerdon, 17-Jan-12 6:46
memberprogrammerdon17-Jan-12 6:46 
QuestionObject reference not set to an instance of an object Pin
deepak thomas, 5-Dec-11 5:01
memberdeepak thomas5-Dec-11 5:01 
AnswerRe: Object reference not set to an instance of an object Pin
deepak thomas, 5-Dec-11 7:57
memberdeepak thomas5-Dec-11 7:57 
GeneralRe: Object reference not set to an instance of an object Pin
Member 10293195, 26-Sep-13 19:08
memberMember 1029319526-Sep-13 19:08 
QuestionSound Catcher freezes on Exit in Win 7 32-bit Pin
Member 8369949, 11-Nov-11 12:13
memberMember 836994911-Nov-11 12:13 
AnswerRe: Sound Catcher freezes on Exit in Win 7 32-bit Pin
Timothy Ayodele, 14-Jan-12 2:54
memberTimothy Ayodele14-Jan-12 2:54 
QuestionPlease how can i make the recording every 1 hour Pin
Timothy Ayodele, 2-Nov-11 8:26
memberTimothy Ayodele2-Nov-11 8:26 

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