Strange output from FFT-function with NAudio

Question

I've been trying to implement FFT into my music program. I thought everything went great until I finished and saw the results...

To try it for the first time I rendered a Sine-wave in the C5 note (523.25Hz), too see if I got the correct results back from the program. But nope, I got a really weird frequency: -0.01171875Hz.

I must have done something wrong! But I don't know what.

Here are some functions related to the whole FFT-implementation:

// Initializing the whole playing-part
waveOut = new WaveOutEvent();
audioFileReader = new AudioFileReader(file);

nsp = new NotifyingSampleProvider(audioFileReader.ToSampleProvider());
nsp.Sample += nsp_Sample;
FadeInOut = new FadeInOutSampleProvider(nsp);
waveOut.Init(new SampleToWaveProvider(FadeInOut));

aggregator = new SampleAggregator();
aggregator.NotificationCount = audioFileReader.WaveFormat.SampleRate / 1000;
aggregator.PerformFFT = true;
aggregator.FftCalculated += (s, a) => OnFftCalculated(a);


PlaySource = Filetype.MP3;

I assume I don't have to post the SampleAggregator-class. But if I do, tell me!

void nsp_Sample(object sender, SampleEventArgs e)
{
    aggregator.Add(e.Left); // Only tried with Left sample first.
}

Here's where the FFT-data comes back and I start to process it.

protected virtual void OnFftCalculated(FftEventArgs e)
{
        float[] magnitudes = new float[data.Length / 2];

        for (int i = 0; i < data.Length / 2; i++)
            magnitudes[i] = ((float)Math.Sqrt((data[i].X * data[i].X) + (data[i].Y * data[i].Y)));


        float max_mag = float.MinValue;
        float max_index = -1;
        for (int i = 0; i < data.Length / 2; i++)
            if (magnitudes[i] > max_mag)
            {
                max_mag = magnitudes[i];
                max_index = i;
            }

        var frequency = max_index * CurrentSamplerate / 1024; // CurrentSamplerate gets set dynamically incase some tracks have a different samplerate.
        Dispatcher.Invoke(new Action(() =>
        {
            Plswork.Text = frequency.ToString() + "Hz";
        }));
}

Yep, that's all.

I assume I've must have made some rookie mistake, but I really don't know what. So any help would be appreciated.

Thank you.

Oh, also: Where I add samples to the aggregator, how do I feed both left and right channel at the same time?

That -0.01171875Hz value suggests that all magnitudes are either NaN/-Inf/MinValue so that `max_index==-1`. Which could be due to a problem with the input, or the `SampleAggregator`. Perhaps [this answer](http://stackoverflow.com/a/20414331/2994596) could help — SleuthEye, Sep 05 '14 at 23:24
I copied the SampleAggregator-class from the question you referred to, and added an answer. Thank you for your help. — Tokfrans, Sep 06 '14 at 13:35

score 0 · Accepted Answer · answered Sep 06 '14 at 13:37

I changed the sampleaggregator-class to another one (linked in the comments of the first post).

I'm now getting legit results when listening to the tone. So it had to have been a problem with that.

I haven't checked why it happend yet but I'll post it if I find the problems. Thank you guys.

Strange output from FFT-function with NAudio

1 Answers1