Click here to Skip to main content
Click here to Skip to main content

Sound Activated Recorder with Spectrogram in C#

, 27 Jan 2008 GPL3
Rate this:
Please Sign up or sign in to vote.
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

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 
GeneralBug in event detection - left channel only is used PinmemberAtis Straujums9-Oct-08 11:43 
GeneralRe: Bug in event detection - left channel only is used PinmemberJeff Morton9-Oct-08 13:16 
GeneralAnother hi! Pinmemberstano24-Sep-08 19:16 
GeneralAnger detection software Pinmember100,000 Cold Calls12-Aug-08 12:44 
GeneralRe: Anger detection software Pinmemberflowerchild128-Feb-09 13:48 
Refer to
to learn the facts about
Stewart L. Rogers of Olympia, Wa, formerly of
Raleigh, NC.....
more about this so-called telemarketing expert,
his criminal arrests and civil lawsuits related to
his past telemarketing businesses from unhappy and ripped off

No wonder he wants to detect anger first on a cold call...
Stewart Rogers has a lot of people who don't like him or
his business practices!

is this legal? Or an invasion of the privacy of
the person receiving a call?
Beware! you were warned! better to get your money up front, he did not respect written contracts in his past business dealings and has declared bankruptcy numerous times, AND is currently in bankruptcy. Ask the business owners he made deals with in the past, they got paid nothing.
Generalthe file is not saving ...!! PinmemberAshok Nalam9-Jul-08 2:05 
QuestionDisplay Of Frequencies PinmemberRonD25-May-08 5:17 
AnswerRe: Display Of Frequencies PinmemberMember 47088011-Aug-08 16:24 
GeneralRe: Display Of Frequencies PinmemberMember 47088011-Aug-08 16:25 
QuestionHow can i make it faster? Pinmemberpauloricca16-May-08 7:20 
AnswerRe: How can i make it faster? PinmemberJeff Morton16-May-08 8:05 
GeneralRe: How can i make it faster? Pinmemberpauloricca19-May-08 0:42 
GeneralTwo Questions Pinmemberrodriguesd23-Mar-08 1:45 
GeneralThis is great stuff Pinmemberpeter.roca6-Mar-08 10:34 
GeneralHi Pinmembercircass4-Mar-08 7:32 
GeneralJust a thought... PinmemberEd.Poore29-Jan-08 14:53 
QuestionProblem with Solution file? PinmemberEitsopGmail29-Jan-08 0:41 
AnswerRe: Problem with Solution file? PinmemberJeff Morton29-Jan-08 1:16 
GeneralRe: Problem with Solution file? Pinmembercircass4-Mar-08 7:34 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

| Advertise | Privacy | Terms of Use | Mobile
Web03 | 2.8.150414.1 | Last Updated 28 Jan 2008
Article Copyright 2008 by Jeff Morton
Everything else Copyright © CodeProject, 1999-2015
Layout: fixed | fluid