我在 SciPy 中使用过滤器并注意到一些我不太熟悉的东西：

我正在使用代码：

import numpy as np
from scipy.signal import butter, lfilter, freqz
import matplotlib.pyplot as plt


def butter_lowpass(cutoff, fs, order=5):
    nyq = 0.5 * fs
    normal_cutoff = cutoff / nyq
    b, a = butter(order, normal_cutoff, btype='low', analog=False)
    return b, a

def butter_lowpass_filter(data, cutoff, fs, order=5):
    b, a = butter_lowpass(cutoff, fs, order=order)
    y = lfilter(b, a, data)
    return y


# Filter requirements.
order = 6
fs = 30.0       # sample rate, Hz
cutoff = 3.667  # desired cutoff frequency of the filter, Hz

# Get the filter coefficients so we can check its frequency response.
b, a = butter_lowpass(cutoff, fs, order)

# Plot the frequency response.
w, h = freqz(b, a, worN=8000)

取自：https ://stackoverflow.com/a/25192640/4959635

并对给定的 freqz 结果进行一些绘图：

plt.plot(w,h)
plt.show()

plt.plot(w,abs(h))
plt.show()

那么为什么我必须采取abs来获得“我想要的情节”呢？第一个具有负振幅的是什么？