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Wrong Axis Title and Ticks #26

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opened 2024-12-08 17:38:27 +00:00 by nuluh · 1 comment
nuluh commented 2024-12-08 17:38:27 +00:00 (Migrated from github.com)
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Maximum Frequency Value After Performing The STFT Appears as 0.5 Instead of 512 Hz

Some STFT implementations (like those in libraries such as SciPy) normalize the frequency axis to a range of [0,0.5], which represents the fraction of the Nyquist frequency relative to the sampling rate.
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Solution

  • Extract Frequencies and Convert: The frequencies array returned by scipy.signal.stft represents actual frequencies in Hertz if you specify the sampling frequency (fs) in the stft call. Ensure fs is provided correctly.
## Maximum Frequency Value After Performing The STFT Appears as 0.5 Instead of 512 Hz Some STFT implementations (like those in libraries such as SciPy) normalize the frequency axis to a range of $[0,0.5]$, which represents the fraction of the Nyquist frequency relative to the sampling rate. ![image](https://github.com/user-attachments/assets/0cd8122e-3835-494e-ac33-57fdd816a39a) ## Solution - **Extract Frequencies and Convert**: The frequencies array returned by scipy.signal.stft represents actual frequencies in Hertz if you specify the sampling frequency (`fs`) in the `stft` call. Ensure `fs` is provided correctly.
nuluh commented 2025-03-05 11:52:50 +00:00 (Migrated from github.com)
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The short explanation is that SciPy’s STFT defaults to a normalized frequency range of $[0, 0.5]$ (cycles per sample) if you don’t provide a sampling rate (fs). This is why you see a maximum frequency of 0.5 instead of 512 Hz.

To fix this, pass your actual sampling frequency (e.g., fs=1024) when calling stft. That way, SciPy will correctly scale the frequency axis up to the Nyquist limit (fs/2 = 512 Hz in your case).

from scipy.signal import stft

# Example fix:
f, t, Zxx = stft(x, fs=1024)  # Now your frequency range will go up to 512 Hz

In summary:

  • No fs provided → Frequencies shown in cycles per sample (max = 0.5).
  • fs=1024 provided → Frequencies shown in Hz (max = 512 Hz).

That’s it! Specifying fs in the STFT call ensures your frequency axis matches real-world units, and you’ll see the maximum frequency you expect.

The short explanation is that **SciPy’s STFT defaults to a normalized frequency range of $[0, 0.5]$** (cycles per sample) if you don’t provide a sampling rate (`fs`). This is why you see a maximum frequency of `0.5` instead of `512 Hz`. To fix this, **pass your actual sampling frequency** (e.g., `fs=1024`) when calling `stft`. That way, SciPy will correctly scale the frequency axis up to the Nyquist limit (`fs/2 = 512 Hz` in your case). ```python from scipy.signal import stft # Example fix: f, t, Zxx = stft(x, fs=1024) # Now your frequency range will go up to 512 Hz ``` **In summary**: - **No `fs` provided** → Frequencies shown in cycles per sample (`max = 0.5`). - **`fs=1024` provided** → Frequencies shown in Hz (`max = 512 Hz`). That’s it! Specifying `fs` in the STFT call ensures your frequency axis matches real-world units, and you’ll see the maximum frequency you expect.
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Reference: nuluh/thesis#26
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