MATLAB Answers

plot fails with -Inf

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Jonathan
Jonathan on 23 Mar 2020
Commented: Jonathan on 23 Mar 2020
I need to plot a fft of an audio file. It appears that if any of the fft bins are zero, the conversion to dB produces an -Inf. Then the plot function does draw the figure but does not plot any data.
Is there an efficent way to trap the 0 value bins in the conversion to dB? For my work -Inf PSD would be equiv to zero.
The test audio file was a code generated audio file, 100Hz Sine for 10 seconds. Since only one fft bin will have a value, all the other bins are zero.
Thanks.
%Code Fragment:
[yS, Fs] = audioread(PathFN,'double');
L = length(yS);
Y = fft(yS,L);
% Version One
Px = Y.*conj(Y); % PSD Power of each freq components
Px = Px(1:L/2);
Px = 10*log10(Px); % Convert to dB
fVals = Fs * (0:(L/2)-1)/L;
plot(fVals, Px); % Plot

  3 Comments

Peng Li
Peng Li on 23 Mar 2020
log(0) = -Inf so I don't personally understand why you think -Inf PSD should be zero.
If you want to show the log(0) part in figure, you can add a small offset to Px such as Px = 10*log10(Px + eps).
Jonathan
Jonathan on 23 Mar 2020
Hi, thank you for the response. The problem is that the plot function seems to failing because some of the y data is -inf
Peng Li
Peng Li on 23 Mar 2020
did you try my solution? If you add a small number like eps to Px, there won't be any -inf.

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Answers (1)

Stijn Haenen
Stijn Haenen on 23 Mar 2020
you can use Px(isinf(abs(Px)))=0; to set all -Inf values to 0

  3 Comments

Jonathan
Jonathan on 23 Mar 2020
Hi, thank you for your response. Your suggestion does catch the -inf values. However, the plot still does not work. If I take out Px = 10*log10(Px); then it will plot. Any ideas? Thanks.
Stijn Haenen
Stijn Haenen on 23 Mar 2020
did you place it after the convertion to dB?
like this:
Px = Y.*conj(Y); % PSD Power of each freq components
Px = Px(1:L/2);
Px = 10*log10(Px); % Convert to dB
Px(isinf(abs(Px)))=0;
fVals = Fs * (0:(L/2)-1)/L;
plot(fVals, Px); % Plot
Does your Px also contains non real values?
Jonathan
Jonathan on 23 Mar 2020
Yes, it was placed there and no just real values. aside from a typo, it does plot now. The single positive is at 100Hz as expected. However, it plots also at every 40Hz a spectra that is at/about -250db for the full range of Fs/2 except where the -Inf were. I didn't think it possible to have a negative PSD. Round off/precision errors from the program that generated the audio file? Thanks.

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