Asked by Muhammad Usman Saleem
on 15 Jul 2017

I want to find equation of this signal.

I plot this time series data and filter it with sSgolay filter. Now I want to get equation of this signal. Is this possible in matlab??

I have attached data of this plot with this post

Answer by Star Strider
on 15 Jul 2017

I am not certain what you intend by getting an equation for it.

You can filter out the noise to see the general trend:

fidi = fopen('data.txt','rt');

D = textscan(fidi, '%s%f', 'Delimiter','\t', 'CollectOutput',1);

t = datenum(D{1});

s = D{2};

Ts = mean(diff(t)); % Sampling Interval

Fs = 1/Ts; % Sampling Frequency

Fn = Fs/2; % Nyquist Frequency

L = numel(t); % Signal Length

sm = s-mean(s); % Subtract Mean To Make Amplitudes At Frequencies>0 More Prominent

FTs = fft(sm)/L; % Fourier Transform

Fv = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector

Iv = 1:length(Fv); % Index Vector

Phs = angle(FTs);

figure(1)

plot(Fv, abs(FTs(Iv))*2)

grid

xlabel('Frequency (Days^{-1})')

ylabel('Amplitude')

set(gca, 'XLim',[0 0.05])

Wp = [0.0045]/Fn; % Passband Frequencies (Normalised)

Ws = [0.0055]/Fn; % Stopband Frequencies (Normalised)

Rp = 10; % Passband Ripple (dB)

Rs = 50; % Stopband Ripple (dB)

[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs); % Filter Order

[z,p,k] = cheby2(n,Rs,Ws); % Filter Design

[sosbp,gbp] = zp2sos(z,p,k); % Convert To Second-Order-Section For Stability

figure(2)

freqz(sosbp, 2^16, Fs) % Filter Bode Plot

s_filt = filtfilt(sosbp,gbp, s); % Filter Signal

figure(3)

plot(t, s, '-b')

hold on

plot(t, s_filt, '-r', 'LineWidth',1.5)

hold off

xlabel('Time (Days)')

ylabel('Amplitude')

legend('Original', 'Lowpass Filtered')

Star Strider
on 20 Jul 2017

@Muhammad Usman Saleem —

The point of my comment is that the neural network did not work.

I encourage you either to thoroughly study meteorological models and their complexity, or to abandon this idea and consider a more tractable problem.

Muhammad Usman Saleem
on 24 Jul 2017

Thank you sir so much!

Star Strider
on 24 Jul 2017

As always, my pleasure!

If my Answer helped solve your problem, please Accept it.

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## 4 Comments

## John D'Errico (view profile)

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## Greg Heath (view profile)

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## Muhammad Usman Saleem (view profile)

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## Greg Heath (view profile)

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