www.pudn.com > voiceprocessingtoolbox.rar > fft3.m

% Example 3: Add zero padding 
 
N = 64; 
T = 1; 
f = 0.25 + 0.5/N;   % Move frequency off-center by half a bin 
n = 0:N-1; 
 
% Example 3: Add zero padding 
zpf = 8;                   % zero-padding factor 
x = [cos(2*pi*n*f*T),zeros(1,(zpf-1)*N)];  % zero-padded FFT input data 
X = fft(x);                 % Interpolated spectrum 
 
subplot(3,1,1); 
ni = [0:.1:N-1];      % Interpolated time axis 
plot(ni, cos(2*pi*ni*f*T)); 
line(0:zpf*N-1, x, 'color', 'b', 'marker', 'o', 'linestyle', 'none'); 
title('Zero-Padded Sampled Sinusoid'); 
xlabel('Time (samples)'); ylabel('Amplitude'); 
axis tight; 
 
% Plot spectral magnitude 
magX = abs(X); 
nfft = zpf*N; 
fni = [0:1.0/nfft:1-1.0/nfft];   % Normalized frequency axis 
subplot(3,1,2); 
plot(fni,magX,'-'); grid; % With interpolation, we can use solid lines '-' 
title('Interpolated Spectral Magnitude');  
xlabel('Normalized Frequency (cycles per sample))');  
ylabel('Magnitude (Linear)'); 
text(-.11,40,'b)'); 
 
% Same thing on a dB scale 
spec = 20*log10(magX);  % Spectral magnitude in dB 
spec = max(spec,-60*ones(1,length(spec))); % clip to -60 dB 
subplot(3,1,3); 
plot(fni, spec); 
% title('Interpolated Spectral Magnitude (dB)');  
xlabel('Normalized Frequency (cycles per sample))');  
ylabel('Magnitude (dB)');