如何获得信号的（x，y）坐标，其最大振幅的10％？

Question

How do I extract the (x, y) coordinates of a sinewave, when sinewave is 10% of itss maximum amplitude, as seen in the figure (red dots)? My 'x-values' is the time and the index number of the array.

I have tried something like this, it is not working properly:

sinewave_max = sinewave[0:argmax(sinewave)]
for i,val in enumerate(sinewave_max):
                if i == int(0.1*(len(sinewave_max))):
                    y = sinewave_max[i]
                    x = index(y)  (#Pseudo-Code line)             

Answer 1

Here is one way to do it. The idea is to have a dense mesh of x-points and then define a small tolerance value. Then look for the values in the y-array which are close to 0.1 times the maximum height (=1) within this tolerance

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 10, 1000)
y = np.sin(x)

plt.plot(x, y)
plt.axhline(0, color='k')
tol = 1e-2
ind = np.argwhere(abs(y-0.1*max(y))<=tol)

plt.scatter(x[ind], y[ind], c='r', s=100, zorder=3)
plt.xlabel('Time')
plt.ylabel('Amplitude = sin(time)')
plt.title('Sine wave')
plt.grid()
plt.show()

Answer 2

Since you tagged pandas, you can do so with pandas' cumsum :

x = np.linspace(0, 10, 1000)
y = np.sin(x)
thresh = max(y) * 0.10

s = pd.Series(y>thresh)

# idx contains the jump from y<=thresh to y>thresh
# except possibly the first position
idx = s.index[s.ne(s.shift())]

# check the first position
if y[0] < thresh: idx = idx[1:]

# plot:
plt.figure(figsize=(10,6))
plt.plot(x,y)
plt.scatter(x[idx],y[idx], c='r', s=100)
plt.grid(True)

Plot:

Note : if as you said, the x series is y 's time index, then the code above needs to change to:

s = pd.Series(y>thresh)

# idx contains the jump from y<=thresh to y>thresh
# except possibly the first position
idx = s.index[s.ne(s.shift())]

# check the first position
if y.iloc < thresh: idx = idx[1:]

plt.figure(figsize=(10,6))
plt.plot(x,y)

# we only need to plot y[idx] against idx now
plt.scatter(idx,y[idx], c='r', s=100)
plt.grid(True)

Which gives: