I/p 频率为 8000 Hz 那么为什么以下代码的输出显示峰值为 2000 Hz？

Question

The i/p frequency is 8000 Hz. Why does the o/p of the following code shows, a peak in spectrogram at 2000 Hz? I am new to the spectrogram, can anyone please explain?

from scipy import signal
import matplotlib.pyplot as plt
import numpy as np


time = np.linspace(start = 0, stop = 10, num = 100000) 

t1 = time[0,]
t2 = time[1,]

del_t = t2-t1

fs = 1e4 #np.ceil(1/del_t)

ip_f = 8000

x = np.sin(2*np.pi*ip_f*time) 

# plt.plot(time,x)

op_f, op_t, op_Sxx = signal.spectrogram(x, fs)

plt.pcolormesh(op_t, op_f, op_Sxx, shading='gouraud')
plt.ylabel('Frequency [Hz]')
plt.xlabel('Time [sec]')
plt.show()

Answer 1

If your sample rate is only 10 kHz then 8 kHz is above the Nyquist Frequency (5 kHz). It will alias to 2 kHz, as you have seen.