Hello,
The "spectrogram" function in MATLAB returns the Short-Time Fourier Transform(STFT) of the input signal. It is used to analyze how the frequency content of a signal changes over time. It gives the variation of signal in the frequency domain over time. In this case, since the signal is a constant frequency signal the spectrogram is a straight line with the maximum value at 100 Hz. Which is the frequency of the signal "x". The "colorbar" varies up to 110 because the maximum value of "S_magnitude" is around 110. As shown in the code below the maximum value of "S_magnitude" for different time values is 114.97 and from the "F" vector we can see that it occurs at around 100Hz frequency.
clear all; clc;
% Sample signal
fs = 1000; % Sampling frequency (Hz)
t = 0:1/fs:2; % Time vector
x = 2*cos(2*pi*100*t); % Example signal
% Compute the spectrogram
window = hann(256); % Window function
noverlap = 128; % Number of overlapping samples
nfft = 256; % Number of FFT points
[S, F, T] = spectrogram(x, window, noverlap, nfft, fs);
S_magnitude = abs(S); % Magnitude of the spectrogram
% Plot the spectrogram
figure;
imagesc(T, F, S_magnitude);
axis xy;
xlabel('Time (s)');
ylabel('Frequency (Hz)');
title('Spectrogram (Linear Scale)');
colorbar;
[val,idx]=max(S_magnitude)
F(idx(1))
