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Sound, tone and DTMF generation by using managed DirectSound and C# and sine tone detection with pure managed Goertzel algorithm implementation

Well, today, we’ll speak about math. A lot of math. We have a number of challenges today.

  • Generate sine, square or dual sine tone (DTMF – sounds, that your phone keyboard produces)
  • Playing it in managed code
  • Detection of base frequencies by using Goertzel (Fast Fourier) algorithm

Let’s start with generation of tone. All we need is DirectSound. First of all, we should create format of wave sound

using DXS = Microsoft.DirectX.DirectSound;

Now, let’s create our Device

Microsoft.DirectX.DirectSound.Device m_device = new Microsoft.DirectX.DirectSound.Device();
SourceHost = App.Current.MainWindow.Content as FrameworkElement;

m_device.SetCooperativeLevel(((HwndSource)PresentationSource.FromVisual(SourceHost)).Handle, DXS.CooperativeLevel.Priority);

As you can see, it, actually Control, thus we have to give him handle to sit on. 90%, that when you’ll run it, you’ll get strange exception: "DLL ‘C:\Windows\assembly\GAC\Microsoft.DirectX\1.0.2902.0__31bf3856ad364e35\Microsoft.DirectX.dll’ is attempting managed execution inside OS Loader lock. Do not attempt to run managed code inside a DllMain or image initialization function since doing so can cause the application to hang.". In order to get rid of it, just disable handling of LoaderLock exceptions under Managed Debugging Assistants section in Visual Studio Debug menu. Not very clear why it happens, but it is. After doing it, we can continue with in memory wave generation.

DXS.WaveFormat format = new DXS.WaveFormat();
            format.BitsPerSample = BitsPerSample;
            format.Channels = Channels;
            format.BlockAlign = BlockAlign;
            format.FormatTag = DXS.WaveFormatTag.Pcm;
            format.SamplesPerSecond = SampleRate;
            format.AverageBytesPerSecond = format.SamplesPerSecond * format.BlockAlign;

            DXS.BufferDescription desc = new DXS.BufferDescription(format);
            desc.DeferLocation = true;
            desc.BufferBytes = sample.Length;         

Now, when we’ll create SecondaryBuffer to play with the Device

CurrentSample = sample;
CurrentBuffer = new DXS.SecondaryBuffer(desc, m_device);
CurrentBuffer.Write(0, sample, DXS.LockFlag.EntireBuffer);
CurrentBuffer.Play(0, DXS.BufferPlayFlags.Default);           

We done. Now let’sw create buffer to play. We’ll start from the simplest sine wave. This type of wave looks like this


Not too hard to understand how to generate it

int length = (int)(SampleRate * BlockAlign * duration);
byte[] buffer = new byte[length];

double A = frequency * 2 * Math.PI / (double)SampleRate;
for (int i = 0; i < length; i++)
     //buffer[i] = (byte)(Math.Sin(i*A));

However, phone tones (DTMF) have double wave format. It looks like this


As you can see, we take two waves and create it’s composition. Here the table of tones to be used for DTMF creation.




    1209 Hz 1336 Hz 1477 Hz 1633 Hz


697 Hz






770 Hz






852 Hz





941 Hz






There are also events in telephony. They build on two frequencies as well

  Lower Band Upper Band
Busy signal 480 Hz 620 Hz
Dial tone 350 Hz 440 Hz
Flash (ringback) 440 Hz 480 Hz

So, in order to create such waves, we’ll have to use following function (the simple one)

(128 + 63 * Math.Sin(n * 2 * Math.PI * LowFreq / rate) + 63 * Math.Sin(n * 2 * Math.PI * HighFreq / rate))

Actually, dual (or triple) sines should be generated by using something like this

int length = (int)(SampleRate * BlockAlign * duration);
            byte[] buffer = new byte[length];

            double A = frequency * 2 * Math.PI / (double)SampleRate;
            for (int i = 0; i < length; i++)
                if (i > 1) buffer[i] = (byte)(2 * Math.Cos(A) * buffer[i – 1] – buffer[i – 2]);
                else if (i > 0) buffer[i] = (byte)(2 * Math.Cos(A) * buffer[i – 1] – (Math.Cos(A)));
                else buffer[i] = (byte)(2 * Math.Cos(A) * Math.Cos(A) – Math.Cos(2 * A));

Done. We can generate and play sounds, tones and DTMF. How to detect what has been played? For this purpose, we should use Fourier transform, but it’s very slow, thus Dr’ Gerald Goertzel developed fast Fourrier Transform algorithm, that used almost in every DSP related product. A little theory about it can be found in Wikipedia (implementation there rather bad).

I simplified it, due to fact, that we actually know source possible frequencies, thus all we have to do is to check the result with well-known values

public double CalculateGoertzel(byte[] sample, double frequency, int samplerate)
    double Skn, Skn1, Skn2;
    Skn = Skn1 = Skn2 = 0;
    for (int i = 0; i < sample.Length; i++)
        Skn2 = Skn1;
        Skn1 = Skn;
        Skn = 2 * Math.Cos(2 * Math.PI * frequency / samplerate) * Skn1 – Skn2 + sample[i];

    double WNk = Math.Exp(-2 * Math.PI * frequency / samplerate);

    return 20* Math.Log10(Math.Abs((Skn – WNk * Skn1)));

This method checks it will steps of 5

public int TestGoertzel(int frequency, byte[] sample)
           int stepsize = frequency / 5;
           Dictionary<int,double> res = new Dictionary<int,double>();
           for (int i = 0; i < 10; i++)
               int freq = stepsize * i;

And the actual result is rather clear

1200 – -272.273842954695
2400 – -198.538262992751
3600 – -214.476846236253
4800 – -224.242925995697
6000 – 87.1653837206957 <- here result
7200 – -225.52220053475
8400 – -222.836254116698
9600 – -230.526410368965
10800 – -220.587695185849

Have a fun with sines, waves and math and be good people. BTW, you can implement full telephone keypad, that actually working with one of my previous posts. To test it, just call any service, that required you to press any of phone buttons and sound the phone you just generated – it will accept it :)

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17 Responses to “Sound, tone and DTMF generation by using managed DirectSound and C# and sine tone detection with pure managed Goertzel algorithm implementation”

  1. Capturing and streaming sound by using DirectSound with C# | Tamir Khason - Just code Says:

    Pingback from  Capturing and streaming sound by using DirectSound with C# | Tamir Khason – Just code

  2. Reading and decoding RDS (Radio Data System) in C# | Tamir Khason - Just code Says:

    Pingback from  Reading and decoding RDS (Radio Data System) in C# | Tamir Khason – Just code

  3. Just code - Tamir Khason Says:

    I already wrote a little about managed way to use DirectX DirectSound . Today we’ll speak about how to

  4. Just code - Tamir Khason Says:

    RDS or Radio Data System is very common in US and many European countries. It is communication protocol

  5. Godwin Says:

    How do you finally get the digits from it??

    I don’t understand what you mean by this…

    1200 – -272.273842954695

    2400 – -198.538262992751

    3600 – -214.476846236253

    4800 – -224.242925995697

    6000 – 87.1653837206957 <- here result

    7200 – -225.52220053475

    8400 – -222.836254116698

    9600 – -230.526410368965

    10800 – -220.587695185849


    How do I relate these to 1,2 and 3??

    I’m confused.

    Please help me if possible.


  6. Ways to play beep (tones) by programming « Everyone Has Problems Says:

    Pingback from  Ways to play beep (tones) by programming &laquo; Everyone Has Problems

  7. sagar Kadam Says:

    Can you explain me which frequency you are using here in above example?

    Because each dtmf signal has two frequecies associated with it. one lowerband freq and another is upperband frequency.

    Thanks and regards,

    Sagar Kadam

  8. zubair Says:

    hi,i m makin a projecton dtmf detection via mic.i m not cleared  how the gortezel work.i have used a 50 ms sound sample of dtmf tone of key 1 i.e 1906 hz.using sampling rate of 8000 i got the result on calculation as 39.0567….. .this is the result after passing the parameters in to calculate gortezel function .Help me out how can i recognise the exact dtmf on run time using gortezel algorithim.

    Waiting for your response .

    Thanks and regards


  9. Tamir Khason Says:

    Teo. code parts here were taken from big project. I cannot attach it to the post. However, thatnk you for interesting my blog.

  10. teo Says:

    You say you have tested that piece of code ?

    If you are so kind to attach the c# file with all the code, THAT WORKS.


  11. louisa Says:

    im abit comfused im only ten so im not sure what age group this website is for

  12. Steven Says:

    Essentially the result of the Goertzel algorithm shows a peak at 6kHz
    (not one of the frequencies from DTMF)

    This is actually a harmonic of the combined DTMF signal,
    The values of freq coming in need to be shown, and the correct data needs to be displayed

  13. william Says:

    What are the references mate ?

  14. steff Says:

    Hi could u please attach the c# code? :$

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  16. Mike Says:

    This is great stuff- I was searching high and low for understandable info on DTMF and C# then stumbled across your blog. This is just what I needed, Thanks for sharing!

  17. Alon Says:

    as the frequecny is growing the result that the Goertzel return is smaller and smaller. Is there any way to calibrate that?

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