FFT in the real world

  • Thread starter btb4198
  • Start date
  • #1
431
5

Main Question or Discussion Point

Hi
so I have a working FFT. I tested it but inputting sin waves in to it.
will not i am inputting sound waves from a Mic and it is not looking to do...
example:
I am using to youtube vid
it outputs B7 (Musical note) b′′′′ Four-lined 3951.066 Frequency
and I run my program this is what I am getting 3951.066

I am getting more action around 41951, 42112, 20062, and 24035

then
3951.066

so is there anything I am add to my FFT that would help ?

oh I am sampling at 44100 Hz
and my N = 88200

I also tried this one
http://www.youtube.com/watch?v=tgMQOAWeVs0&list=PLE558EB4A35F28F6A

and I got a higher magnitude from 424.5 and 435.5
then
from 528
do you think there too much noise coming from the computer speakers ?
 
Last edited by a moderator:

Answers and Replies

  • #2
DrClaude
Mentor
7,259
3,413
As I wrote in your other thread, you most probably made an error in your program.
 
  • #3
jim mcnamara
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3,869
2,252
If you want meaningful help please consider posting your code.
 
  • #4
431
5
public polar1[] FFT(polar1[] x)
{
int N2 = x.Length;
polar1[] X = new polar1[N2];
if (N2 == 1)
{
return x;
}
polar1[] odd = new polar1[N2 / 2];
polar1[] even = new polar1[N2 / 2];
polar1[] Y_Odd = new polar1[N2 / 2];
polar1[] Y_Even = new polar1[N2 / 2];


for (int t = 0; t < N2 / 2; t++)
{
even[t].img = x[t * 2].img;
even[t].real = x[t * 2].real;
odd[t].img = x[(t * 2) + 1].img;
odd[t].real = x[(t * 2) + 1].real;
}
Y_Even = FFT(even);
Y_Odd = FFT(odd);
polar1 temp4;

for (int k = 0; k < (N2 / 2); k++)
{
temp4 = Complex1(k, N2);
X[k].real = Y_Even[k].real + (Y_Odd[k].real * temp4.real);
X[k + (N2 / 2)].real = Y_Even[k].real - (Y_Odd[k].real * temp4.real);
X[k].img = Y_Even[k].img + (Y_Odd[k].real * temp4.img);
X[k + (N2 / 2)].img = Y_Even[k].img - (Y_Odd[k].real * temp4.img);
}

return X;
}


public polar1 Complex1(int K, int N3)
{
polar1 temp;
double temp1;
temp1 = (2D * Math.PI *K) / N3;
temp.real = Math.Cos(temp1);
temp.img = Math.Sin(temp1);
return temp;

}


public struct polar1
{
public double real;
public double img;

};
 
  • #5
431
5
DrClaude

have you had time to look at the code ?
what do you think?
that is the same code i use in my other thread
 
  • #6
DrClaude
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7,259
3,413
I'm sorry, but I don't have the patience to read through an implementation of a FT. If you just need the result, then use one of the many wonderful implementations available (GSL, FFTW, etc.). If you are doing it to learn about numerical methods, then roll up your sleves and debug the code as if there was no tomorrow. You will probably not learn as much if someone else just points out the error.
 
  • #7
431
5
that is cool.. but I do think it is working
 

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