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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

AnswerRe: Can't run the program in Vista 64bit Pin
Fullmetal9901211-Sep-09 8:20
memberFullmetal9901211-Sep-09 8:20 
AnswerRe: Can't run the program in Vista 64bit Pin
Rahdu18-Oct-10 23:09
memberRahdu18-Oct-10 23:09 
AnswerRe: Can't run the program in Vista 64bit Pin
Member 320938312-Apr-11 13:10
memberMember 320938312-Apr-11 13:10 
AnswerRe: Can't run the program in Vista 64bit Pin
Rebmem1529-Sep-11 19:33
memberRebmem1529-Sep-11 19:33 
Questionhow can i do it reverse? [modified] Pin
mojimoj2-Aug-09 22:05
membermojimoj2-Aug-09 22:05 
GeneralExcellent piece of work Jeff Pin
maxima1204-Jul-09 13:17
membermaxima1204-Jul-09 13:17 
GeneralRe: Excellent piece of work Jeff Pin
Jeff Morton5-Jul-09 6:46
memberJeff Morton5-Jul-09 6:46 
GeneralThrows exception when using Freq &lt; 11025 Hz Pin
k^s10-Jun-09 8:07
memberk^s10-Jun-09 8:07 
Hi, i've been testing your project and i've to say that is very nice, i've been looking for something like this sometimes.

I've been testing different "SamplesPerSecond" values, but when using values under 11025 ( minimum is 8000) it throws an exception.

The first thing i've seen is that the buffer size for AudioFrame.Process() methodmust be dividable by 4 (len % 4 = 0).

The second is that the FourierTransform.FFT() method throws an "Index out of range exception". I've been debubbing it and i found that in line 69
tr = xre[k + n2] * c + xim[k + n2] * s;

"k + n2" is absolutely out of range, about 100 positions over the limit.

Is there any bug or is not posible to plot frequencies under 11025Hz?

Note: i am plotting using:
public void init()
    oAudioFrame = new AudioFrame(false);
    oAudioFrame.IsDetectingEvents = true;
    oAudioFrame.AmplitudeThreshold = 16384;
public void Plot()
    oAudioFrame.Process(ref sound_data);
    oAudioFrame.RenderTimeDomainRight(ref pb1);

QuestionHow do I get it to work? Pin
Maiden Computer Company22-Apr-09 11:48
memberMaiden Computer Company22-Apr-09 11:48 
AnswerRe: How do I get it to work? Pin
Jeff Morton22-Apr-09 11:51
memberJeff Morton22-Apr-09 11:51 
QuestionError in Frequency domain? Pin
Elysium8-Mar-09 12:17
memberElysium8-Mar-09 12:17 
AnswerRe: Error in Frequency domain? Pin
Jeff Morton8-Mar-09 12:41
memberJeff Morton8-Mar-09 12:41 
GeneralRe: Error in Frequency domain? Pin
Elysium8-Mar-09 23:02
memberElysium8-Mar-09 23:02 
QuestionQuestion on how to get the frequencies Pin
kyanmark_johnwill2-Mar-09 14:52
memberkyanmark_johnwill2-Mar-09 14:52 
AnswerRe: Question on how to get the frequencies Pin
hazeljane13-Mar-09 4:49
memberhazeljane13-Mar-09 4:49 
GeneralRe: Question on how to get the frequencies Pin
Jeff Morton13-Mar-09 11:34
memberJeff Morton13-Mar-09 11:34 
GeneralRe: Question on how to get the frequencies Pin
afmf23-Oct-09 2:54
memberafmf23-Oct-09 2:54 
GeneralRe: Question on how to get the frequencies Pin
afmf3-Nov-09 0:38
memberafmf3-Nov-09 0:38 
GeneralRe: Question on how to get the frequencies Pin
Muammar©1-Jan-10 1:56
member Muammar©1-Jan-10 1:56 
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 
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 

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