我有一个采样率为16e3的信号，其频率范围为125到1000 Hz。 因此，如果我绘制一个规格，我会得到一个非常小的颜色范围，因为所有未使用的频率。
香港专业教育学院试图通过设置斧头限制来修复它，但这不起作用。
有没有办法切断未使用的频率或用NaN替换它们？
将数据重新采样到2e3将不起作用，因为仍有一些未使用的频率低于125 Hz。
感谢您的帮助。
答案 0 :(得分：16)
specgram（）正在为您完成所有工作。如果你在specgram函数中查看axes.py，你可以看到它是如何工作的。原始功能在我的计算机上Python27\Lib\site-packages\matplotlib\axes.py。
<snip>
Pxx, freqs, bins = mlab.specgram(x, NFFT, Fs, detrend,
window, noverlap, pad_to, sides, scale_by_freq)
Z = 10. * np.log10(Pxx)
Z = np.flipud(Z)
if xextent is None: xextent = 0, np.amax(bins)
xmin, xmax = xextent
freqs += Fc
extent = xmin, xmax, freqs[0], freqs[-1]
im = self.imshow(Z, cmap, extent=extent, **kwargs)
self.axis('auto')
return Pxx, freqs, bins, im
您可能需要对此进行建模并剪切Pxx数据以满足您的需要。
Pxx, freqs, bins = mlab.specgram(x, NFFT, Fs, detrend,
window, noverlap, pad_to, sides, scale_by_freq)
# ****************
# create a new limited Pxx and freqs
#
# ****************
Z = 10. * np.log10(Pxx)
Z = np.flipud(Z)
Pxx是一个二维数组，形状为（len（freqs），len（bins）
>>> Pxx.shape
(129, 311)
>>> freqs.shape
(129,)
>>> bins.shape
(311,)
>>>
这将限制Pxx和频率
Pxx = Pxx[(freqs >= 125) & (freqs <= 1000)]
freqs = freqs[(freqs >= 125) & (freqs <= 1000)]
这是一个完整的解决方案 - my_specgram（） - 与图库中的specgram_demo一起使用。
from pylab import *
from matplotlib import *
# 100, 400 and 200 Hz sine 'wave'
dt = 0.0005
t = arange(0.0, 20.0, dt)
s1 = sin(2*pi*100*t)
s2 = 2*sin(2*pi*400*t)
s3 = 2*sin(2*pi*200*t)
# create a transient "chirp"
mask = where(logical_and(t>10, t<12), 1.0, 0.0)
s2 = s2 * mask
# add some noise into the mix
nse = 0.01*randn(len(t))
x = s1 + s2 + +s3 + nse # the signal
NFFT = 1024 # the length of the windowing segments
Fs = int(1.0/dt) # the sampling frequency
# modified specgram()
def my_specgram(x, NFFT=256, Fs=2, Fc=0, detrend=mlab.detrend_none,
window=mlab.window_hanning, noverlap=128,
cmap=None, xextent=None, pad_to=None, sides='default',
scale_by_freq=None, minfreq = None, maxfreq = None, **kwargs):
"""
call signature::
specgram(x, NFFT=256, Fs=2, Fc=0, detrend=mlab.detrend_none,
window=mlab.window_hanning, noverlap=128,
cmap=None, xextent=None, pad_to=None, sides='default',
scale_by_freq=None, minfreq = None, maxfreq = None, **kwargs)
Compute a spectrogram of data in *x*. Data are split into
*NFFT* length segments and the PSD of each section is
computed. The windowing function *window* is applied to each
segment, and the amount of overlap of each segment is
specified with *noverlap*.
%(PSD)s
*Fc*: integer
The center frequency of *x* (defaults to 0), which offsets
the y extents of the plot to reflect the frequency range used
when a signal is acquired and then filtered and downsampled to
baseband.
*cmap*:
A :class:`matplotlib.cm.Colormap` instance; if *None* use
default determined by rc
*xextent*:
The image extent along the x-axis. xextent = (xmin,xmax)
The default is (0,max(bins)), where bins is the return
value from :func:`mlab.specgram`
*minfreq, maxfreq*
Limits y-axis. Both required
*kwargs*:
Additional kwargs are passed on to imshow which makes the
specgram image
Return value is (*Pxx*, *freqs*, *bins*, *im*):
- *bins* are the time points the spectrogram is calculated over
- *freqs* is an array of frequencies
- *Pxx* is a len(times) x len(freqs) array of power
- *im* is a :class:`matplotlib.image.AxesImage` instance
Note: If *x* is real (i.e. non-complex), only the positive
spectrum is shown. If *x* is complex, both positive and
negative parts of the spectrum are shown. This can be
overridden using the *sides* keyword argument.
**Example:**
.. plot:: mpl_examples/pylab_examples/specgram_demo.py
"""
#####################################
# modified axes.specgram() to limit
# the frequencies plotted
#####################################
# this will fail if there isn't a current axis in the global scope
ax = gca()
Pxx, freqs, bins = mlab.specgram(x, NFFT, Fs, detrend,
window, noverlap, pad_to, sides, scale_by_freq)
# modified here
#####################################
if minfreq is not None and maxfreq is not None:
Pxx = Pxx[(freqs >= minfreq) & (freqs <= maxfreq)]
freqs = freqs[(freqs >= minfreq) & (freqs <= maxfreq)]
#####################################
Z = 10. * np.log10(Pxx)
Z = np.flipud(Z)
if xextent is None: xextent = 0, np.amax(bins)
xmin, xmax = xextent
freqs += Fc
extent = xmin, xmax, freqs[0], freqs[-1]
im = ax.imshow(Z, cmap, extent=extent, **kwargs)
ax.axis('auto')
return Pxx, freqs, bins, im
# plot
ax1 = subplot(211)
plot(t, x)
subplot(212, sharex=ax1)
# the minfreq and maxfreq args will limit the frequencies
Pxx, freqs, bins, im = my_specgram(x, NFFT=NFFT, Fs=Fs, noverlap=900,
cmap=cm.Accent, minfreq = 180, maxfreq = 220)
show()
close()
答案 1 :(得分：2)
现在，与提出问题相比，您可以更轻松地执行此操作：您可以使用str1 <- "C2_Sperd20A_XXX_20170301_20170331"
将matplotlib.pyplot.axis和ymin设置为所需的频率。这很容易;这是我程序中的一个片段：
ymax答案 2 :(得分：1)
specgram（）返回（Pxx，freqs，bins，im），其中im是AxesImage 实例[1]。您可以使用它来设置绘图的限制：
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
答案 3 :(得分：0)
以下是此版本的改编版本：http://matplotlib.org/examples/pylab_examples/specgram_demo.html 它改变了绘制的频率范围。
#!/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()