Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p><code>np.fft.fftfreq</code> tells you the frequencies associated with the coefficients:</p> <pre><code>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:&gt;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) </code></pre> <hr> <p>The OP asks how to find the frequency in Hertz. I believe the formula is <code>frequency (Hz) = abs(fft_freq * frame_rate)</code>.</p> <p>Here is some code that demonstrates that.</p> <p>First, we make a wave file at 440 Hz:</p> <pre><code>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() </code></pre> <p>This creates the file <code>test.wav</code>. 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:</p> <pre><code>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 </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POhow to extract frequency associated with fft values in python
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USunutbu
    1. USunutbu
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POhow to extract frequency associated with fft values in python
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. CO@unutbu: thanks a lot. Let me try it:)
      singulars
      1. PO
      1. USria
    2. CO@~unutbu:But can I get the frequency values in Hertz?I want to make wav files.
      singulars
      1. PO
      1. USria
    3. CO@ria: `freq` as defined by `np.fft.fft` is unitless, and always in the interval [-1/2, 1/2]. I believe to convert to Hertz, you multiply by the frame rate and take the absolute value.
      singulars
      1. PO
      1. USunutbu
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload