Ciro Santilli
OurBigBook.comOutput:
With our understanding of the discrete Fourier transform we see clearly that:
sin(t)
fft
real 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10
rfft
real 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 0 0 0 0 0 0 0
sin(t) + sin(4t)
fft
real 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 -10 0 0 0 0 0 0 0 0 0 0 0 10 0 0 10
rfft
real 0 0 0 0 0 0 0 0 0 0 0
imag 0 -10 0 0 -10 0 0 0 0 0 0- the signal is being decomposed into sinusoidal components
- because we are doing the Discrete Fourier transform of a real signal, for the
fft, so there is redundancy in the. We also understand thatrfftsimply cuts off and only keeps half of the coefficients
numpy/fft.py
#!/usr/bin/env python
import math
import numpy as np
def printi(X):
# Round to int otherwise there is a lot of small numerical noise.
print('real ' + ' '.join(str(int(np.real(x))) for x in X))
print('imag ' + ' '.join(str(int(np.imag(x))) for x in X))
def analyze(x):
# FFT
X = np.fft.fft(x)
print('fft')
printi(X)
# Real FFT
Xr = np.fft.rfft(x)
print('rfft')
printi(Xr)
N = 20
print('sin(t)')
x = np.array([math.sin(i * 2 * math.pi / N) for i in range(N)])
analyze(x)
print()
print('sin(t) + sin(4t)')
x = np.array([math.sin(i * 2 * math.pi / N) + math.sin(i * 2 * math.pi * 4 / N) for i in range(N)])
analyze(x)
print()