波形频率低于音符频率？

Question

I'm trying to extract the frequency of a note from an mp3 file that includes a synthesized sample of an A3 note, which should be 220 Hz.

This is part of the waveform I obtain using librosa :

As you can see, the wave seems to repeat itself every 400 samples. Therefore, by dividing the sampling rate, which is 22050 Hz, by 400 I should get the frequency of the waveform. However, I get 55.125 Hz instead of 220. Am I missing something or making a mistake?

EDIT : Here's the code I'm using

import librosa
from matplotlib import pyplot as plt
import numpy as np
%matplotlib notebook

y, sr = librosa.load("Simple_synth/A3-saw.mp3")

plt.figure(figsize=(18,6))
plt.plot(y[2000:3000])

note_freq = sr/400

Link to the audio file: https://www.filefactory.com/file/7aqmrvq375n9/A3-saw.mp3

Answer 1

For the given audio sample

import librosa
from matplotlib import pyplot as plt
import numpy as np

y, sr = librosa.load("A3-saw.mp3")

it is possible to calculate fourier transform (see how to extract frequency associated with fft values in python )

# calculate fast fourier transform
w = np.fft.fft(y)

# frequencies associated to the fourier transform
freqs = np.fft.fftfreq(len(y))

And then find the highest peak in the fourier transform and its frequency in Hz

idx = np.argmax(np.abs(w))
freq = freqs[idx]
freq_in_hertz = abs(freq * sr)
print(freq_in_hertz)

54.90196078431373

There are also higher harmonics involved in the sample, which can be seen by plotting more peaks

plt.plot(sr*freqs[0:500],abs(w[0:500]))

plt.plot(sr*freqs[0:2000],abs(w[0:2000]))