How to use axes metadata

[ChristianFrick-Kistler]

Hi @danielhrisca
I’m currently trying to use the changes introduced by the scale_axis branch but I am struggling a bit to define the dtype metadata. I’d like to save the data of a spectrogram and attach the FFT frequency information as axis, see following code:

import numpy as np
from asammdf import MDF, Signal

file = "outputs/axis_example.mf4"

fs = 2.5e3           # sampling frequency
n_fft = 1024         # number of FFT bins
n_spectra = 10       # number of FFT frames
dt = 0.1             # time step between FFTs

freqs = np.linspace(0, fs / 2, n_fft, endpoint=False)
times = np.arange(n_spectra) * dt

fft_amplitude = np.zeros((n_spectra, n_fft), dtype=np.float32)
for i, t in enumerate(times):
    peak_freq = 50 + i * 20
    fft_amplitude[i, :] = 20 * np.log10(1e-6 + 1.0 / (1.0 + ((freqs - peak_freq) / 10.0) ** 2))

axes_meta = {"type": "FIXED_AXIS", "size": n_fft, "values": freqs}
fft_dtype = np.dtype([
    ("FFT_AMPLITUDE", f"({n_fft},)float32")
], metadata=axes_meta)
structured_fft = np.zeros(n_spectra, dtype=fft_dtype)
structured_fft["FFT_AMPLITUDE"] = fft_amplitude

fft_signal = Signal(
    samples=structured_fft,
    timestamps=times,
    name="FFT_AMPLITUDE",
    unit="dB",
    comment="Magnitude spectrum per time step (array channel)",
)

mdf = MDF(version="4.10")
mdf.append(fft_signal, acq_name="FFT spectra")
mdf.save(file, overwrite=True)

I would very appreciate if you can have a look at it and let me know what I’m doing wrong resp. point me to an example where I can find more information.

Best regards
Christian