How to calculate SNR using Matlab?

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Nur Fauzira Saidin
Nur Fauzira Saidin 2015-11-14
评论： PRANAV BIBRA 2022-1-3
Hi. I want to calculate SNR of the noisy signal and the filtered signal. I used this following code but I am not sure whether the result is correct or not. Can somebody help me?
%generate the noisy signal which will be filtered
x= cos(2*pi*12*[0:0.001:1.23]);
x(end) = [];
[b,a] = butter(2,[0.6 0.7],'bandpass');
filtered_noise = filter(b,a,randn(1, length(x)*2));
y = (x + 0.5*filtered_noise(500:500+length(x)-1))/length(x)*2;
%Use matlabs built-in buttord function to get the optimum order to meet a specification
[N,Wn] = buttord(0.1, 0.5, 5, 40)
%use the N and Wn values obtained above to design the filter in the usual way
[b,a] = butter(N, Wn, 'low');
%filter the signal and plot the ouput of the filter
Y_filtered = filter(b,a,y);
%calculation of snr before filtering
snr_before = mean(x.^2) / mean(filtered_noise.^2);
snr_before_db = 10 * log10(snr_before) % in dB
%calculation of snr after filtering
residual_noise = x - Y_filtered;
snr_after = mean(x.^2) / mean(residual_noise.^2);
snr_after_db = 10 * log10(snr_after)
Result that I got is:
snr_before_db =
6.8725
snr_after_db =
0.0132

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