Manually plot spectrogram: "Surf" the output of the spectrogram function

Question

Joschua Kraus 2018-5-9

2
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/399933-manually-plot-spectrogram-surf-the-output-of-the-spectrogram-function

编辑： Francisco Flores 2022-6-29

Hello, I try to understand the workings of the spectrogram function by reproducing the same plot that the spectrogram function gives by using the output parameters of the spectrogram function. Also I try to understand the difference between Power Spectral Density and Power Spectrum, which are two optional return values of the spectrogram function. Below you can see my code and a picture that shows the output.

% Try to understand what the spectrogram is doing and how to manually plot
% the spectrogram and also what PSD vs power spectrum is 
clc; clear; close all;
win_size = 0.01;
fft_overlap = 0.5;
[signal, Fs] = audioread('../RASTA/speech_5.wav');
signal = signal(:,1); %use only the left channel 
hop_size = Fs*win_size;
nfft = hop_size/fft_overlap;
noverlap = nfft-hop_size;
w = sqrt(hann(nfft)); %use some window 
%Normal Spectrogram plot
subplot(4,1,1);
spectrogram(signal, w ,noverlap, nfft, Fs, 'yaxis' );
colormap jet;
title('Default spectrogram plot');
%Try to plot the spectrogram from the output 
[s, f, t] = spectrogram(signal, w ,noverlap, nfft, Fs);
subplot(4,1,2);
surf(t, f, 20*log10(abs(s)), 'EdgeColor', 'none');
axis xy; 
axis tight; 
colormap(jet); view(0,90);
xlabel('Time (secs)');
colorbar;
ylabel('Frequency(HZ)');
title('surf(t, f, 20*log10(abs(s)), ___)');
%why is the spectrum "cut" , there are no zero values displayed at the
%beginning and at the end. Also it seems different from the normal
%spectrogram plot (more red and fewer blue) 
%Try to utilize the ps output with 'psd' argument
[s, f, t, psd] = spectrogram(signal, w ,noverlap, nfft, Fs, 'psd');
subplot(4,1,3);
surf(t, f, psd, 'EdgeColor', 'none');
axis xy; 
axis tight; 
colormap(jet); view(0,90);
xlabel('Time');
colorbar;
ylabel('Frequency(HZ)');
title('surf(t, f, psd, ___)');
%Its only blue (meaning very small values) but since its the psd why should
%I need to do 10*log10(psd) on it? 
%Try to utilize the ps output with 'power' argument
[s, f, t, power] = spectrogram(signal, w ,noverlap, nfft, Fs, 'power');
subplot(4,1,4);
surf(t, f, power, 'EdgeColor', 'none');
axis xy; 
axis tight; 
colormap(jet); view(0,90);
xlabel('Time');
colorbar;
ylabel('Frequency(HZ)');
title('surf(t, f, power, ___)');
%Whats the difference between the variables power and psd? Spectrum seems
%to be identical..

Please help me to educate myself on the workings of this function and on spectrograms, PSD and Power Spectrum in general. How do I have to fix my code to produce identical plots?