hello
you may play with the colormap and also with the displayed dB range to focus on your signal tones and not the background
here I prefer the jet colormap with a dB range limited to 30 dB (from the max amplitude to 30 dB lower then everything below that lower limit is displayed with dark blue color)
my code uses my own spectrogram function but you can achieve the same result with the regular spectrogram function
folder = pwd;
filename = '27o2.csv';
data = readmatrix(fullfile(folder, filename));
t = data(3:end, 1);
x = data(3:end, 2);
fs = 1/mean(diff(t));
nfft = 400;
overlap = 0.75;
dB_range = 30; % display this dB range on y axis
[S,F,T] = myspecgram(x, fs, nfft, overlap); % overlap is 75% of nfft here
sg_dB = 20*log10(abs(S)); % expressed now in dB
% saturation of the dB range : the lowest displayed level is spectrogram_dB_scale dB below the max level
min_disp_dB = round(max(max(sg_dB))) - dB_range;
% plot
figure(1);
imagesc(T,F,sg_dB)
caxis([min_disp_dB min_disp_dB+dB_range]);
hcb = colorbar('vert');
set(get(hcb,'Title'),'String','dB')
set(gca,'YDir','Normal')
xlabel('Time (secs)')
ylabel('Freq (Hz)')
title('Specgram')
colormap('jet');
% colormap('hot');
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [fft_specgram,freq_vector,time] = myspecgram(signal, Fs, nfft, Overlap)
% FFT peak spectrogram of signal (example sinus amplitude 1 = 0 dB after fft).
% signal - input signal,
% Fs - Sampling frequency (Hz).
% nfft - FFT window size
% Overlap - buffer overlap % (between 0 and 0.95)
signal = signal(:);
samples = length(signal);
% fill signal with zeros if its length is lower than nfft
if samples<nfft
s_tmp = zeros(nfft,1);
s_tmp((1:samples),:) = signal;
signal = s_tmp;
samples = nfft;
end
% window : hanning
window = hanning(nfft);
window = window(:);
% compute fft with overlap
offset = fix((1-Overlap)*nfft);
spectnum = 1+ fix((samples-nfft)/offset); % Number of windows
% % for info is equivalent to :
% noverlap = Overlap*nfft;
% spectnum = fix((samples-noverlap)/(nfft-noverlap)); % Number of windows
% main loop
fft_specgram = [];
for ci=1:spectnum
start = (ci-1)*offset;
sw = signal((1+start):(start+nfft)).*window;
fft_specgram = [fft_specgram abs(fft(sw))*4/nfft]; % X=fft(x.*hanning(N))*4/N; % hanning only
end
% one sidded fft spectrum % Select first half
if rem(nfft,2) % nfft odd
select = (1:(nfft+1)/2)';
else
select = (1:nfft/2+1)';
end
fft_specgram = fft_specgram(select,:);
freq_vector = (select - 1)*Fs/nfft;
% time vector
% time stamps are defined in the middle of the buffer
time = ((0:spectnum-1)*offset + round(nfft/2))/Fs;
end
