Detect a specific frequency/tone from raw wave-data

Question

I am reading a raw wave stream coming from the microphone.
(This part works as I can send it to the speaker and get a nice echo.)

For simplicity lets say I want to detect a DTMF-tone in the wave data. In reality I want to detect any frequency, not just those in DTMF. But I always know which frequency I am looking for.

I have tried running it through FFT, but it doesn't seem very efficient if I want high accuracy in the detection (say it is there for only 20 ms). I can detect it down to an accuracy of around 200 ms.

What are my options with regards to algorithms? Are there any .Net libs for it?

Answer 1

You may want to look at the Goertzel algorithm if you're trying to detect specific frequencies such as DTMF input. There is a C# DTMF generator/detector library on Sourceforge based on this algorithm.

Answer 2

Very nice implementation of Goertzel is there. C# modification:

private double GoertzelFilter(float[] samples, double freq, int start, int end)
    {
        double sPrev = 0.0;
        double sPrev2 = 0.0;
        int i;
        double normalizedfreq = freq / SIGNAL_SAMPLE_RATE;
        double coeff = 2 * Math.Cos(2 * Math.PI * normalizedfreq);
        for (i = start; i < end; i++)
        {
            double s = samples[i] + coeff * sPrev - sPrev2;
            sPrev2 = sPrev;
            sPrev = s;
        }
        double power = sPrev2 * sPrev2 + sPrev * sPrev - coeff * sPrev * sPrev2;
        return power;
    }

Works great for me.