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Draw into sound

, 21 Dec 2008 CPOL
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The article illustrates how to draw into a spectral representation of an audio file.


The idea is to draw pictures into sounds. In particular, draw pictures into the spectral representation of the sound. A sound can be represented in many ways. In particular, it can be drawn with Amplitude-Time representation (Waveform) and Frequency-Time representation (Spectrum). The first shows the amplitude of the sound in time:

The X-axis is the time while the Y-axis is the amplitude of the sound. The next kind of view shows the audio signal as components of different frequencies:

Here, the X-axis is time while the Y-axis is the frequency. In particular, lighter colors (white) means higher value of the component of the specific frequency, while darker colors (black) means a lower component. Here, it’s possible to see how the sound changes in time and frequency.

The Fast Fourier Transform

The Fourier Transform is an operation that transforms a signal (in our case, the audio signal) into its frequency domain representation. So, it is possible to see which frequencies are present in the input signal. The FFT is an algorithm that computes the Fourier Transform efficiently.

The inverse operation (Inverse Fast Fourier Transform) takes data from the frequency domain and gives the values in the time domain. We can use this algorithm to draw a sound. Just imagine the draw as an input like if it was the spectral representation of the signal. Then, we apply the IFFT algorithm, and we retrieve the audio waveform which is used to create the output wave file.

The Program

The blackboard is the frequency-time representation of the audio signal. It is possible to paint into it using the color of the bar on the left side of the start button. Clicking the bar, it’s possible to change the color. The best results can be obtained using darker colors (like dark gray). When you click the Start button, the IFFT algorithm begins to compute data and the output audio file is generated.

Just draw something:

We can view the results with programs like Cooledit.


The Code

The program is written in C#. We use two fundamental libraries here:

  • Wave File Library written by Garrett Hoofman. The library has been used to produce the Wav output file.
  • A library that implements the IFFT algorithm.
using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;
using WaveLibrary;

namespace Img2Wav
    class Core_Img2Wav
        private const double MAX_DATA = +50;
        private const double MIN_DATA = -50;

        private const String NoInputBitmap = "No input bitmap";

        public Bitmap InputBitmap { get; set; }
        public WaveFile OutputWav { get; set; }

        public void Start()
            int NumSamples = InputBitmap.Width * InputBitmap.Height;
            byte[] Samples = new byte[NumSamples];
            OutputWav = new WaveFile(1, 16, 44000);
            if (InputBitmap == null) throw new Exception(NoInputBitmap);
            double[] data = new double[InputBitmap.Height];

            int w = 0;
            for (int i = 0; i < InputBitmap.Width; i++) 
                for (int j = 0; j < InputBitmap.Height; j++)
                    Color C = InputBitmap.GetPixel(i,j);
                    data[j] = (C.R + C.G + C.B ) / 3;

                for (int x = 0; x < data.Length; x++)
                    Samples[w] = (byte)(MAX_DATA * data[x]);



This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Software Developer (Senior)
Italy Italy
Angelo Gattuso graduated in November 2003 from "Politecnico di Torino" in Software Engineering. He is a Software Engineer with experience in projecting and developing Businness Applications. Areas of expertise include ASP .Net design and develompemnt of three-tier applications.
He has interests in music, reading and cooking even if he prefers eating.

Comments and Discussions

NewsC# Wave Library PinmemberMember 1108725116-Sep-14 9:34 
QuestionHow to see the frequency PinmemberMember 859082926-Jun-12 17:08 
GeneralMy vote of 5 Pinmemberwinxpdll11-Sep-11 11:10 
GeneralMy vote of 5 Pingroupabdillahi27-Apr-11 0:08 
GeneralMy vote of 4 PinmemberLucian-aSterX22-Feb-11 23:29 
GeneralGreat, but has some bugs PinmemberM$VBC++27-Dec-10 5:33 
GeneralMy vote of 5 Pinmembervictorbos31-Jul-10 8:33 
GeneralSimply fascinating Pinmembervictorbos31-Jul-10 8:03 
GeneralLoss "WaveLibrary" in package Pinmemberkenj_huang010317-Jun-10 1:12 
GeneralRe: Loss "WaveLibrary" in package Pinmembervictorbos31-Jul-10 7:54 
GeneralRe: Loss "WaveLibrary" in package Pinmemberfilmee2426-Apr-14 2:22 
GeneralTerrible article dude! PinmemberTV Mogul16-Jan-10 14:10 
GeneralRe: Terrible article dude! Pinmemberevol7612-Mar-10 11:31 
GeneralRe: Terrible article dude! Pinmembervictorbos31-Jul-10 8:11 
GeneralRe: Terrible article dude! PinmemberTV Mogul31-Jul-10 13:37 
GeneralSpectrogram PinmemberPatt Rojazz5-Nov-09 8:35 
Would you please also post the code that generate spectrogram on the top of the page as well?
GeneralRe: Spectrogram Pinmemberevol7612-Mar-10 11:33 
GeneralBetter Image Tools Pinmembermkane726-Sep-09 13:59 
GeneralGood Idea! Pinmemberkmslove26-Jun-09 6:15 
GeneralVery Funny Idea! Pinmemberjohannesnestler21-Dec-08 22:58 

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