how to extract frequency associated with fft values in python
I used fft
function in numpy which resulted in a complex array. How to get the exact frequency values?
np.fft.fftfreq
tells you the frequencies associated with the coefficients:
import numpy as np
x = np.array([1,2,1,0,1,2,1,0])
w = np.fft.fft(x)
freqs = np.fft.fftfreq(len(x))
for coef,freq in zip(w,freqs):
if coef:
print('{c:>6} * exp(2 pi i t * {f})'.format(c=coef,f=freq))
# (8+0j) * exp(2 pi i t * 0.0)
# -4j * exp(2 pi i t * 0.25)
# 4j * exp(2 pi i t * -0.25)
The OP asks how to find the frequency in Hertz.
I believe the formula is frequency (Hz) = abs(fft_freq * frame_rate)
.
Here is some code that demonstrates that.
First, we make a wave file at 440 Hz:
import math
import wave
import struct
if __name__ == '__main__':
# http://stackoverflow.com/questions/3637350/how-to-write-stereo-wav-files-in-python
# http://www.sonicspot.com/guide/wavefiles.html
freq = 440.0
data_size = 40000
fname = "test.wav"
frate = 11025.0
amp = 64000.0
nchannels = 1
sampwidth = 2
framerate = int(frate)
nframes = data_size
comptype = "NONE"
compname = "not compressed"
data = [math.sin(2 * math.pi * freq * (x / frate))
for x in range(data_size)]
wav_file = wave.open(fname, 'w')
wav_file.setparams(
(nchannels, sampwidth, framerate, nframes, comptype, compname))
for v in data:
wav_file.writeframes(struct.pack('h', int(v * amp / 2)))
wav_file.close()
This creates the file test.wav
.
Now we read in the data, FFT it, find the coefficient with maximum power,
and find the corresponding fft frequency, and then convert to Hertz:
import wave
import struct
import numpy as np
if __name__ == '__main__':
data_size = 40000
fname = "test.wav"
frate = 11025.0
wav_file = wave.open(fname, 'r')
data = wav_file.readframes(data_size)
wav_file.close()
data = struct.unpack('{n}h'.format(n=data_size), data)
data = np.array(data)
w = np.fft.fft(data)
freqs = np.fft.fftfreq(len(w))
print(freqs.min(), freqs.max())
# (-0.5, 0.499975)
# Find the peak in the coefficients
idx = np.argmax(np.abs(w))
freq = freqs[idx]
freq_in_hertz = abs(freq * frate)
print(freq_in_hertz)
# 439.8975
Frequencies associated with DFT values (in python)
By fft, Fast Fourier Transform, we understand a member of a large family of algorithms that enable the fast computation of the DFT, Discrete Fourier Transform, of an equisampled signal.
A DFT converts a list of N complex numbers to a list of N complex numbers, with the understanding that both lists are periodic with period N.
Here we deal with the numpy
implementation of the fft.
In many cases, you think of
- a signal x defined in the time domain of length N, sampled at a constant interval dt,
- its DFT X (here specifically
X = np.fft.fft(x)
), whose elements are sampled on the frequency axis with a sample rate dw.
Some definition
-
the period (aka duration) of the signal
x
, sampled atdt
withN
samples is isT = dt*N
-
the fundamental frequencies (in Hz and in rad/s) of
X
, your DFT aredf = 1/T dw = 2*pi/T # =df*2*pi
-
the top frequency is the Nyquist frequency
ny = dw*N/2
(and it's not
dw*N
)
The frequencies associated with a particular element in the DFT
The frequencies corresponding to the elements in X = np.fft.fft(x)
for a given index 0<=n<N
can be computed as follows:
def rad_on_s(n, N, dw):
return dw*n if n<N/2 else dw*(n-N)
or in a single sweep
w = np.array([dw*n if n<N/2 else dw*(n-N) for n in range(N)])
if you prefer to consider frequencies in Hz, s/w/f/
f = np.array([df*n if n<N/2 else df*(n-N) for n in range(N)])
Using those frequencies
If you want to modify the original signal x
-> y
applying an operator in the frequency domain in the form of a function of frequency only, the way to go is computing the w
's and
Y = X*f(w)
y = ifft(Y)
Introducing np.fft.fftfreq
Of course numpy
has a convenience function np.fft.fftfreq
that returns dimensionless frequencies rather than dimensional ones but it's as easy as
f = np.fft.fftfreq(N)*N*df
w = np.fft.fftfreq(N)*N*dw
Because df = 1/T
and T = N/sps
(sps
being the number of samples per second) one can also write
f = np.fft.fftfreq(N)*sps