I would like to see how certain frequencies, specifically low bass at 20 - 60hz are present in a piece of audio. I have the audio as a byte array, I convert it to array of shorts, then into a complex number by (short[i]/(double)short.MaxValue, 0). Then i pass this to the FFT from Aforge.
The audio is mono and sample rate of 44100. I understand I can only put chucks through the FFT at ^2. So 4096 for example. I don't understand what frequencies be in the output bins.
if I am taking 4096 samples from the audio that is at 44100 sample rate. Does this mean I am taking milliseconds worth of audio? or only getting some of the frequencies that will be present?
I add the output of the FFT to a array, my understanding is that as I am taking 4096 then bin 0 would contain 0*44100/4096 = 0hz, bin 1 would hold 1*44100/4096 = 10.7666015625hz and so on. Is this correct? or im I doing something fundamentally wrong here?
My goal would be to average the frequencies between say 20 - 60 hz, so for a song with very low, heavy bass then this number would be higher than say a soft piano piece with very little bass.
Here is my code.
OpenFileDialog file = new OpenFileDialog();
file.ShowDialog();
WaveFileReader reader = new WaveFileReader(file.FileName);
byte[] data = new byte[reader.Length];
reader.Read(data, 0, data.Length);
samepleRate = reader.WaveFormat.SampleRate;
bitDepth = reader.WaveFormat.BitsPerSample;
channels = reader.WaveFormat.Channels;
Console.WriteLine("audio has " + channels + " channels, a sample rate of " + samepleRate + " and bitdepth of " + bitDepth + ".");
short[] shorts = data.Select(b => (short)b).ToArray();
int size = 4096;
int window = 44100 * 10;
int y = 0;
Complex[] complexData = new Complex[size];
for (int i = window; i < window + size; i++)
{
Complex tmp = new Complex(shorts[i]/(double)short.MaxValue, 0);
complexData[y] = tmp;
y++;
}
FourierTransform.FFT(complexData, FourierTransform.Direction.Forward);
double[] arr = new double[complexData.Length];
//print out sample of conversion
for (int i = 0; i < complexData.Length; i++)
{
arr[i] = complexData[i].Magnitude;
}
Console.Write("complete, ");
return arr;
edit : changed to FFT fro DFT
Here's a modified version of your code. Note the comments starting with "***".
Once you get the results, and assuming a 44100 Hz sample rate and size = 4096, elements 2 - 4 should be the values you are looking for. There's a way to convert them to dB, but I don't remember it offhand.
Good luck!