editing signal in frequency domain and converting back to time domain the code is matlab / open source octave code
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.
%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';