首先給一個matlab自帶的example
Fs = 1000; % Sampling frequency T = 1/Fs; % Sample time L = 1000; % Length of signal t = (0:L-1)*T; % Time vector % Sum of a 50 Hz sinusoid and a 120 Hz sinusoid x = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t); y = x + 2*randn(size(t)); % Sinusoids plus noise plot(Fs*t(1:50),y(1:50)) title('Signal Corrupted with Zero-Mean Random Noise') xlabel('time (milliseconds)')
NFFT = 2^nextpow2(L); % Next power of 2 from length of y Y = fft(y,NFFT)/L; f = Fs/2*linspace(0,1,NFFT/2+1); % Plot single-sided amplitude spectrum. plot(f,2*abs(Y(1:NFFT/2+1))) title('Single-Sided Amplitude Spectrum of y(t)') xlabel('Frequency (Hz)') ylabel('|Y(f)|')針對以上程序,你會發現在用fft求信號的幅度譜的時候要乘以2,除以N
乘以2是因爲我們研究的實信號的頻譜是偶函數,但是負頻率是沒有意義的,乘以2是爲了把雙邊譜轉換爲單邊頻譜。
因爲IDFT中含有1/N這一項,X(K)/N代表信號x分解後各頻點的係數。
還有,當補零後,fft後的值不是除以N,而是除以L,這又怎麼解釋呢?