Cutting of unused frequencies in specgram matplotlib

Question

I have a signal with sampling rate 16e3, its frequency range is from 125 to 1000 Hz. So if i plot a specgram i get a pretty small colorrange because of all the unused frequencys

Answer 1

specgram() returns (Pxx, freqs, bins, im), where im is AxesImage instance [1]. You could use it to set the limits of your plot:

Pxx, freqs, bins, im = plt.specgram(signal, Fs=fs)
im.set_ylim((125,1000))

[1] http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.specgram