how to acces the points on the fft plot.

i have the following code.i want the access of value of z1 at fs1=1000.

[z, f] = wavread('z.wav');

>> n=length(z)

n =

      137728

>> fs1=(0:n-1)*(f/n);

>> z1=abs(fft(z));

plot(fs1,z1);

i have the values of z1 and fs1 separately.but i dont know how to find the value of z1 at specific fs1.plss help me with this.

would be really thankful.

Answer 1

[Wayne King]

Hi, If the signal is real-valued, it is better to not use the whole result of fft() since you are looking at the absolute value.

The spacing of the DFT bins is Fs/length(input) as you note in your code above.

t = 0:0.001:1-0.001;
Fs = 1000;
x = cos(2*pi*100*t)+randn(size(t));
xdft = fft(x);
xmag = abs(xdft(1:length(xdft)/2+1));
freq = 0:Fs/length(x):Fs/2;

Now, how do we find the value of xmag that corresponds to 100 Hz?

The spacing between the DFT bins is Fs/length(x), so

   df = Fs/length(x);
   freqbin = round(100/df)+1;
   xmag(freqbin)

Of course if you want the actual Fourier coefficient at that frequency

    xdft(freqbin)

[777]

cool!! thanks a lot