Interpretation of fft plot

Question

I am trying to understand how to interpret a spectrogram image plot: (1) what does the yellow band and its corresponding color code with a high positive value indicate? There are so many yellow bands in comparison to blue. The legend shows that the yellow bands have higher positive values, so is that higher frequency?

(2) I used the following code to generate the spectrogram. The X axis contains 50 data points because the method splits the signal into segments of length 50. I arbitrary used 10 into the spectrogram function. Is there a specific rule as to how to determine the number of points?

x(1) = rand;
for n = 1:500
    x(n+1) = 4*x(n)*(1-x(n));
end


specto=spectrogram(x,10,0,256); %using 256 fft points with no overlap


logspecto=(20*log10(abs(specto)));

Answer 1

I'm assuming you're using Matlab. The Matlab function and its parameters are:

spectrogram(x,window,noverlap,nfft)

1. The color indicates the power at a given frequency. Typically, this is given in dB. In your plot, the power is concentrated in the low frequencies a the bottom of the plot. The higher frequencies at the top of the plot have less power.

2. The value of 10 you used is the width (number of samples) of your windowing function. It takes a vector of n samples and applies weights to the samples such that the ends of the vector taper to zero. This is done to limit leakage (a loss of frequency resolution/introduction of artificial frequencies) during the DFT process. You choose the width of your window according to the frequency resolution you desire (more samples give a finer frequency resolution).

I hope this helps.