comnum.select_channel#

comnum.select_channel(x, fs_in, offset_hz, fs_out, cutoff_hz, *, n_taps=129)#

One channel of a capture brought to baseband at the rate of its receiver: the mean of the capture removed (the DC offset of the converter), the channel offset_hz above the tuning frequency shifted to 0 Hz, then resampled by the exact ratio fs_out/fs_in (integers in hertz) with a low-pass filter of cut-off cutoff_hz (half amplitude): a sinc under a Dolph–Chebyshev window of 80 dB, about n_taps taps at the input rate (a transition band about 4 fs_in / n_taps wide), without delay.

Parameters:

  • x (array_like) – the complex samples of the capture.

  • fs_in (float) – the sampling rate of the capture (Hz).

  • offset_hz (float) – the centre of the channel minus the tuning frequency (Hz).

  • fs_out (float) – the sampling rate of the output (Hz).

  • cutoff_hz (float) – the cut-off of the filter (Hz), at most fs_out / 2.

  • n_taps (int) – the length of the filter at the input rate.

Returns:

ceil(len(x) fs_out / fs_in) samples (complex128).

Return type:

numpy.ndarray

Raises:

ValueError – for rates that are not whole numbers of hertz, a ratio that reduces to an interpolation by more than 1024, an offset outside the band, a cut-off above fs_out / 2, or no tap.

Examples

>>> import comnum, numpy as np
>>> n = np.arange(15360)
>>> x = np.exp(2j * np.pi * 2e6 * n / 15.36e6)       # a tone 2 MHz up
>>> y = comnum.select_channel(x, 15.36e6, 2e6, 1.92e6, 0.8e6)
>>> len(y), round(float(abs(y[960])), 3)
(1920, 1.0)