Different spectrogram between MATLAB and Python

Question

I have a program in MATLAB which I want to port to Python. The problem is that in it I use the built-in spectrogram function and, although the matplotlib specgram function seems identical, I'm getting different results when I run both.

These is the code I've been running.

MATLAB:

data = 1:999; %Dummy data. Just for testing.

Fs = 8000; % All the songs we'll be working on will be sampled at an 8KHz rate

tWindow = 64e-3; % The window must be long enough to get 64ms of the signal
NWindow = Fs*tWindow; % Number of elements the window must have
window = hamming(NWindow); % Window used in the spectrogram

NFFT = 512;
NOverlap = NWindow/2; % We want a 50% overlap

[S, F, T] = spectrogram(data, window, NOverlap, NFFT, Fs);

Python:

import numpy as np
from matplotlib import mlab

data = range(1,1000) #Dummy data. Just for testing

Fs = 8000
tWindow = 64e-3
NWindow = Fs*tWindow
window = np.hamming(NWindow)

NFFT = 512
NOverlap = NWindow/2

[s, f, t] = mlab.specgram(data, NFFT = NFFT, Fs = Fs, window = window, noverlap = NOverlap)

And this is the result I get in both executions:

http://i.imgur.com/QSPvYsC.png

(The F and T variables are exactly the same in both programs)

It's obvious that they're different; in fact, the Python execution even doesn't return complex numbers. What could be the problem? Is there any way to fix it or I should use another spectrogram function?

Thank you so much in advance for your help.

Answer 1

In matplotlib, specgram by default returns the power spectral density (mode='PSD'). In MATLAB, spectrogram by default returns the short-time fourier transform, unless nargout==4, in which case it also computes the PSD. To get the matplotlib behaviour to match the MATLAB behaviour, set mode='complex'