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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 Pinmemberchronogeo19-Oct-14 23:27 
AnswerRe: Shutdown and stereo/mono fixes Pinmemberchronogeo19-Oct-14 23:28 
GeneralRe: Shutdown and stereo/mono fixes Pinmemberchronogeo19-Oct-14 23:29 
GeneralRe: Shutdown and stereo/mono fixes Pinmemberchronogeo19-Oct-14 23:31 
QuestionUI Thread locks up on close PinmemberMember 1107721311-Sep-14 23:07 
QuestionHelp please: Nasalance ratio [modified] PinmemberMember 1070843925-Jun-14 7:16 
GeneralThanks a lot! Pinmemberlopopoo21-Nov-13 5:25 
QuestionExcellent article; quick question: would it be possible to "listen" to speaker output? PinmemberMember 1029319528-Sep-13 8:17 
QuestionDelay/Latency Pinmemberhieronymusde13-Aug-13 12:21 
GeneralMy vote of 5 Pinmemberhieronymusde13-Aug-13 11:04 
QuestionHelp for operation on a modulated signal Pinmembergalbandrea14-Apr-13 23:13 
Bug"В экземпляре объекта не задана ссылка на объект." [modified] PinmemberMember 99607234-Apr-13 2:54 
Questionis there a way to select the iinput device (directx name ?) Pinmemberpatrick zuili17-Mar-13 19:34 
Generalamazing one PinmemberMember 810718412-Jan-13 6:07 
GeneralMy vote of 5 Pinmemberjavcastaalm23-Nov-12 10:08 
GeneralThanks PinmemberLeviButler3-Nov-12 14:27 
GeneralMy vote of 5 PinmemberMember 806726522-May-12 1:47 
Questiondraw spectrogram with a wav file. Pinmemberjacky_zz4-Feb-12 19:27 
QuestionLocking up when closing if there is activity [modified] Pinmemberprogrammerdon17-Jan-12 6:46 
QuestionObject reference not set to an instance of an object Pinmemberdeepak thomas5-Dec-11 5:01 
AnswerRe: Object reference not set to an instance of an object Pinmemberdeepak thomas5-Dec-11 7:57 
GeneralRe: Object reference not set to an instance of an object PinmemberMember 1029319526-Sep-13 19:08 
QuestionSound Catcher freezes on Exit in Win 7 32-bit PinmemberMember 836994911-Nov-11 12:13 
AnswerRe: Sound Catcher freezes on Exit in Win 7 32-bit PinmemberTimothy Ayodele14-Jan-12 2:54 
QuestionPlease how can i make the recording every 1 hour PinmemberTimothy Ayodele2-Nov-11 8:26 

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