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

Here is an adapted version of this: http://matplotlib.org/examples/pylab_examples/specgram_demo.html which changes the range of frequencies that are plotted.

#!/usr/bin/env python
#### from the example
#### 
from pylab import *

dt = 0.0005
t = arange(0.0, 20.0, dt)
s1 = sin(2*pi*100*t)
s2 = 2*sin(2*pi*400*t)
mask = where(logical_and(t>10, t<12), 1.0, 0.0)
s2 = s2 * mask
nse = 0.01*randn(len(t))
x = s1 + s2 + nse # the signal

NFFT = 1024       # the length of the windowing segments
Fs = int(1.0/dt)  # the sampling frequency

ax1 = subplot(211)
plot(t, x)
subplot(212, sharex=ax1)
Pxx, freqs, bins, im = specgram(x, NFFT=NFFT, Fs=Fs, noverlap=900,
                                cmap=cm.gist_heat)

#### edited from the example
#### 
# here we get access to the axes
x1,x2,y1,y2 = axis()
# leave x range the same, change y (frequency) range
axis((x1,x2,25,500))

show()