This content has been marked as final.
Show 19 replies

1. Re: audio files real and imaginary data for FFT
843853 May 1, 2010 4:29 PM (in response to 801494)Real data points are just complex data points with imaginary parts of zero! 
2. Re: audio files real and imaginary data for FFT
801494 May 1, 2010 5:57 PM (in response to 843853)right.
// Simple Fast Fourier Transform. class Fft { public static void complexToComplex(int sign, int n, float ar[], float ai[]) { float scale = (float)Math.sqrt(1.0f/n); int i,j; for (i=j=0; i<n; ++i) { if (j>=i) { float tempr = ar[j]*scale; float tempi = ai[j]*scale; ar[j] = ar*scale;
ai[j] = ai[i]*scale;
ar[i] = tempr;
ai[i] = tempi;
}
int m = n/2;
while (m>=1 && j>=m) {
j = m;
m /= 2;
}
j += m;
}
int mmax,istep;
for (mmax=1,istep=2*mmax; mmax<n; mmax=istep,istep=2*mmax) {
float delta = (float)sign*3.141592654f/(float)mmax;
for (int m=0; m<mmax; ++m) {
float w = (float)m*delta;
float wr = (float)Math.cos(w);
float wi = (float)Math.sin(w);
for (i=m; i<n; i+=istep) {
j = i+mmax;
float tr = wr*ar[j]wi*ai[j];
float ti = wr*ai[j]+wi*ar[j];
ar[j] = ar[i]tr;
ai[j] = ai[i]ti;
ar[i] += tr;
ai[i] += ti;
}
}
mmax = istep;
}
}
}this is the algorithm I am trying to change so it accepts input from a file. so I can set the imaginary parts array (ai) to zero? how would I get the complex data points from the sound file?

3. Re: audio files real and imaginary data for FFT
843853 May 2, 2010 1:18 AM (in response to 801494)ultiron wrote:
I don't think so. In your position I would not be be changing this code at all. I would just be trying to arrange to invoke the method complexToComplex() with the correct arguments.
this is the algorithm I am trying to change so it accepts input from a file.
>so I can set the imaginary parts array (ai) to zero?
The sound file only contains the real parts which you will read and place in a float array. The corresponding imaginary parts are, well, zero! Just create an array floats with each value set to zeros!
how would I get the complex data points from the sound file? 
4. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 4:43 AM (in response to 843853)Thanks for the help sabre. I think i'm almost there.
Ive been working at it and now I have the sound file stored in a byte array with endianness accounted for. How would I convert this data into a float array? I imagine its more difficult that a generic byte array to float array conversion? 
5. Re: audio files real and imaginary data for FFT
843853 May 2, 2010 5:20 AM (in response to 801494)ultiron wrote:
Assuming each value is two bytes (that is the only reason the 'endianness' matters) then first convert each pair of bytes to a short using something like
Thanks for the help sabre. I think i'm almost there.
Ive been working at it and now I have the sound file stored in a byte array with endianness accounted for. How would I convert this data into a float array? I imagine its more difficult that a generic byte array to float array conversion?
and then to a floatshort valueAsShort = (short)( (s[i] << 8)  (s[i+1] & 0xff));
Of course I have assumed bigendian signed but only you know what your actual format is.float valueAsFloat = (float)valueAsShort;
You do know that with that FFT implementation you can only process 2^N points!
You do know how to interpret the resultant complex data of the FFT don't you? 
6. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 5:32 AM (in response to 843853)cheers for the help on the floats. I'm working on that now.
Im totally unsure on how to interpret the complex data. I was going to work it out when I got that actual FFT itself working.
If you've got any pointers on how to interpret the data and what exactly the data will represent then they would be greatly appreciated. I need all the help I can get. 
7. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 5:35 AM (in response to 801494)and I did know it can only process 2^n points. I read about zero padding and was planning to use that if the data wasnt exactly 2^n points.
How would I find out how many points the sound file is?? 
8. Re: audio files real and imaginary data for FFT
843853 May 2, 2010 5:40 AM (in response to 801494)ultiron wrote:
Since you don't know how to interpret the complex FFT result, one of the difficulties you will find is knowing whether or not your conversion from bytes to float is correct. The FFT does not care if you put rubbish into it; it will still transform it.
cheers for the help on the floats. I'm working on that now.
Im totally unsure on how to interpret the complex data. I was going to work it out when I got that actual FFT itself working.
>If you've got any pointers on how to interpret the data and what exactly the data will represent then they would be greatly appreciated. I need all the help I can get.
Forums are not a good place to learn the basics of signal processing. You need to get a book and/or tutorial and spend a lot of time reading. I assume that this is a school/college/university project so you should consult your tutor. 
9. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 5:45 AM (in response to 843853)right. Cheers for the help! 
10. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 7:44 AM (in response to 801494)Hi, Sabre, was just looking for your opinion on the folowing code for converting a byte array to a float array?
public float [] bytetofloat(byte [] convert){ float [] data = new float [convert.length/2]; for(int i = 0;i<convert.length;i+=2){ for(int j = 0; j <data.length;j++){ short valueAsShort = (short)( (convert[i] << 8)  (convert[i+1] & 0xff)); float valueAsFloat = (float)valueAsShort; valueAsFloat = data[j]; } } return data; }

11. Re: audio files real and imaginary data for FFT
843853 May 2, 2010 1:05 PM (in response to 801494)ultiron wrote:
Err .. isn't that back to front?
Hi, Sabre, was just looking for your opinion on the folowing code for converting a byte array to a float array?
valueAsFloat = data[j];

12. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 1:16 PM (in response to 843853)yes your absolutely right. The nested loop was wrong as well.
which produced float results ranging frompublic static void bytetofloat(byte [] convert){ float [] data = new float [convert.length/2]; for(int i = 0;i<convert.length;i+=2){ short valueAsShort = (short)( (convert[i] << 8)  (convert[i+1] & 0xff)); float valueAsFloat = (float)valueAsShort; System.out.println(valueAsFloat); } }
1370 to a bout 5000.
If i was to get there values into an array would this be OK to put into the FFT? 
13. Re: audio files real and imaginary data for FFT
843853 May 2, 2010 3:53 PM (in response to 801494)ultiron wrote:
Of course! What would be the point of creating the arrays if you couldn't?
yes your absolutely right. The nested loop was wrong as well.
which produced float results ranging frompublic static void bytetofloat(byte [] convert){ float [] data = new float [convert.length/2]; for(int i = 0;i<convert.length;i+=2){ short valueAsShort = (short)( (convert[i] << 8)  (convert[i+1] & 0xff)); float valueAsFloat = (float)valueAsShort; System.out.println(valueAsFloat); } }
1370 to a bout 5000.
If i was to get there values into an array would this be OK to put into the FFT? 
14. Re: audio files real and imaginary data for FFT
801494 May 2, 2010 4:44 PM (in response to 843853)cheers for this  you've helped me an awful lot!
one more thing  am i right in thinking the float values represent amplitudes of the sound?
if not, what do they represent?