How to use pwelch for PSD?

I would like to use pwelch to perform PSD on my long term acoustic data. The data is 120 seconds long, sampled at 144kHz.

How do I decide what to set nfft as? Is it correct that I have set it as the length of my sampled data?

path=('C:\Users\datafolder');

d=dir(fullfile(path, '*.wav')); %list all .wav files in path folder

files=length(d); %number of files in folder

for i=1:files    %for each file
    disp(d(i).name);
    filename=fullfile(path, d(i).name); %get full filename
    [xbit, fs]=audioread(filename); %and then sampled data and sampling 
                                    %freq (fs) from audioread function
    cal=-176.2; %for ST5101, adapt this to use switch and have diff cal for 
                %diff ST used, should it be negative or not?
    cal=power(10, cal/20); %calibration correction factor
    calxbit=xbit*cal; %apply calibration
end

%% pwelch
%[Pxx,f]=pwelch(x, window, noverlap, nfft,fs)
%'x' is signal 
%'window' divides the signal into segments.
%Default window length for PamGuide is N=Fs, yielding a frequency
%resolution of 1Hz.
%'noverlap' must be a positive integer less than length of window. If you do
%not specify noverlap the default number of overlapped samples is 50% of
%the window length.
%'nfft' number of discrete Fourier transform points to use in the DFT
%estimate. 
nfft=length(calxbit);


%In our case, x is a vector (for each file). 
[pxx,f]=pwelch(calxbit, segmentLength, 0.5, nfft, fs);
%[] is nfft, how to decide what to put here?

plot(f, 10*log10(pxx));

When I run this code, I get the error:

Warning: Integer operands are required for colon operator when used as index.

> In welch>localComputeSpectra (line 316)

In welch (line 99)

In pwelch (line 162)

...multiple times!

Thanks for your help!

Answer 1

[emehmetcik]

The way you call "pwelch" function is erroneous:

[pxx,f]=pwelch(calxbit, segmentLength, 0.5, nfft, fs);

When "pwelch" is called in this manner it expects the inputs to be as follows;

[pxx,w] = pwelch(x, window, noverlap, nfft, fs)

"noverlap" must be an integer less than nfft.

See the help page for pwelch for details.

Regarding your question about selecting "nfft" value depends on your application/signal. I suggest to study the corresponding chapter in Stoica's spectral estimation book.