editing signal in frequency domain and converting back to time domain tia sal22

[Rick T]

editing signal in frequency domain and converting back to time domain tia sal22

Greetings All

I would like to be able to edit different aspects of a signal

(frequency, magnitude) in the frequency domain is this possible?

I do know about fft and ifft  which work great but I'm a little

confused as how to edit the data in the frequency domain and properly 

get it back out in the time domain.  I can get it out using ifft but when

I try and edit the signal in the frequency domain and plot it in the time domain the values 

are very different.

Example: I create an array with the variables frequency, magnitude.  

created from the frequency domain I then want to change the magnitude of one of the frequencies

and then convert the signal back to the time domain.

I was reading up on it and it had mentioned using phase some how 

phase = unwrap(angle(ya));  the problem is that the phase array

is twice as long as the array for the frequency and magnitude.

code below:

%signal from frequency domain to time domain

clear all,clf

%addpath("/home/transform/"); %add path to location of functions

%create time domain

Fs = 1000;                    % Sampling frequency

t=linspace(0,1,1000);

%1a create signal

ya = .5*sin(2*pi*10*t) + 1*sin(2*pi*50*t); 

ya_fft = fft(ya);

%Original data in frequency domain

[xfreqa,yampa]=rtplotfft(ya,Fs);

yampa1=(yampa(:,1)/max(abs(yampa(:,1)))*1); %keep at 1, amplitude levels adjustied

phase = unwrap(angle(ya_fft));

%Create array with Freq,magnitude

freq_mag_phase=[xfreqa yampa]; %can't add phase because length is to large

%Edit data in frequency domain

freq_mag_phase(52,2)=[.1] ; %changes amplitude to .1

[xfreqb,yampb]=rtplotfft(ya,Fs); %not sure what to replace ya with?

yampb1=(yampb(:,1)/max(abs(yampb(:,1)))*1); %keep at 1, amplitude levels adjustied

%3a frequency back to time domain

ya_ifft=real(ifft(ya_fft));

%1b time domain plot

subplot(2,2,1),plot(t,ya)

title('1) Orginal Signal ')

ylabel('amplitude')

xlabel('time domain')

%2b frequency domain plot.

subplot(2,2,2),plot(xfreqa,yampa) 

title('2) Orginal signal in Frequency domain')

xlabel('Frequency (Hz)')

ylabel('amplitude')

%3b rebuilt time domain from frequency (ifft)

subplot(2,2,3),plot(t,ya_ifft)

title('3) rebuild time domain using ifft but it hasnt been edited')

xlabel('time domain ')

ylabel('amplitude')

%4b rebuilt of signal in frequency (ifft) 

subplot(2,2,4),plot(xfreqb,yampb)

title('4) rebuild in frequency domain not edited')

xlabel('Frequency (Hz)')

ylabel('amplitude')

fprintf('done');

I've also included the function rtplotfft that is used 

to create the frequency and magnitude plot:

function [x,freq]=rtplotfft(vp_sig_orig,Fs)

vp_sig_orig=vp_sig_orig';

vp_sig_len=length(vp_sig_orig); %get sample rate from vp fs_rate needs to be an even number?

% Use next highest power of 2 greater than or equal to length(x) to calculate FFT.

nfft= 2^(nextpow2(length(vp_sig_orig))); 

% Take fft, padding with zeros so that length(fftx) is equal to nfft 

fftx = fft(vp_sig_orig,nfft); 

% Calculate the number of unique points

NumUniquePts = ceil((nfft+1)/2); 

% FFT is symmetric, throw away second half 

fftx = fftx(1:NumUniquePts); 

% Take the magnitude of fft of x and scale the fft so that it is not a function of the length of x

mx = abs(fftx)/length(vp_sig_orig); 

% Take the square of the magnitude of fft of x. 

%mx = mx.^2; rem'd out to get amplitude to work

% Since we dropped half the FFT, we multiply mx by 2 to keep the same energy.

% The DC component and Nyquist component, if it exists, are unique and should not be multiplied by 2.

if rem(nfft, 2) % odd nfft excludes Nyquist point

   mx(2:end) = mx(2:end)*2;

else

   mx(2:end -1) = mx(2:end -1)*2;

end

freq=mx;

% This is an evenly spaced frequency vector with NumUniquePts points. 

freq_vect = (0:NumUniquePts-1)*vp_sig_len/nfft; 

x=freq_vect';

tia sal22

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