Hann Window Spectrum Analysis Results

Finally, the Matlab for computing the DFT of the Hann-windowed complex sinusoid and plotting the results is listed below. To help see the full spectrum, we also compute a heavily interpolated spectrum (via zero padding as before) which we'll draw using solid lines.

% Compute the spectrum and its alternative forms:
Xw = fft(xw);              % FFT of windowed data
fn = [0:1.0/N:1-1.0/N];    % Normalized frequency axis
spec = 20*log10(abs(Xw));  % Spectral magnitude in dB
% Since the nulls can go to minus infinity, clip at -100 dB:
spec = max(spec,-100*ones(1,length(spec)));
phs = angle(Xw);           % Spectral phase in radians
phsu = unwrap(phs);        % Unwrapped spectral phase

% Compute heavily interpolated versions for comparison:
Nzp = 16;                   % Zero-padding factor
Nfft = N*Nzp;               % Increased FFT size
xwi = [xw,zeros(1,Nfft-N)]; % New zero-padded FFT buffer
Xwi = fft(xwi);             % Compute interpolated spectrum
fni = [0:1.0/Nfft:1.0-1.0/Nfft]; % Normalized freq axis
speci = 20*log10(abs(Xwi)); % Interpolated spec mag (dB)
speci = max(speci,-100*ones(1,length(speci))); % clip
phsi = angle(Xwi);          % Interpolated phase
phsiu = unwrap(phsi);       % Unwrapped interpolated phase

figure(1);
subplot(2,1,1);

plot(fn,abs(Xw),'*k'); hold on; 
plot(fni,abs(Xwi),'-k'); hold off;
title('Spectral Magnitude'); 
xlabel('Normalized Frequency (cycles per sample))'); 
ylabel('Amplitude (linear)');

subplot(2,1,2);

% Same thing on a dB scale
plot(fn,spec,'*k'); hold on; plot(fni,speci,'-k'); hold off;
title('Spectral Magnitude (dB)'); 
xlabel('Normalized Frequency (cycles per sample))'); 
ylabel('Magnitude (dB)');

cmd = ['print -deps ', 'specmag.eps']; disp(cmd); eval(cmd);
disp 'pausing for RETURN (check the plot). . .'; pause

figure(1);
subplot(2,1,1);
plot(fn,phs,'*k'); hold on; plot(fni,phsi,'-k'); hold off;
title('Spectral Phase'); 
xlabel('Normalized Frequency (cycles per sample))'); 
ylabel('Phase (rad)'); grid;
subplot(2,1,2);
plot(fn,phsu,'*k'); hold on; plot(fni,phsiu,'-k'); hold off;
title('Unwrapped Spectral Phase'); 
xlabel('Normalized Frequency (cycles per sample))'); 
ylabel('Phase (rad)'); grid;
cmd = ['print -deps ', 'specphs.eps']; disp(cmd); eval(cmd);

Figure 8.8 shows the spectral magnitude and Fig.8.9 the spectral phase.

Figure 8.8: Spectral magnitude on linear (top) and dB (bottom) scales.

There are no negative-frequency components in Fig.8.8 because we are analyzing a complex sinusoid $x=[1,j,-1,-j,1,j,\ldots\,]$ , which has frequency $f_s/4$ only, with no component at $-f_s/4$ .

Notice how difficult it would be to correctly interpret the shape of the ``sidelobes'' without zero padding. The asterisks correspond to a zero-padding factor of 2, already twice as much as needed to preserve all spectral information faithfully, but not enough to clearly outline the sidelobes in a spectral magnitude plot.