prepare_analog_filter#

gwpy.signal.filter_design.prepare_analog_filter( filt: FilterCompatible, *, unit: str | UnitBase = 'Hz', normalize_gain: bool = False, ) → tuple[Literal['ba', 'zpk', 'sos'], BAType | ZpkType | SosType][source]#

Prepare an analog filter by parsing and converting units.

This handles:

Parsing filter specification
Converting Hz → rad/s for ZPK filters (if unit=’Hz’)
Applying gain normalization (if requested)

Does NOT:

Convert to digital
Extract gain for SOS
Apply prewarping

Parameters:

filtfilter specification

Filter as (b, a), (z, p, k), sos array, or lti object.

unitstr, optional

For analogue ZPK filters, the units in which the zeros and poles are specified. Either 'Hz' or 'rad/s' (default).

normalize_gainbool, optional

Whether to normalize the gain when converting from Hz to rad/s.

False (default): Multiply zeros/poles by -2π but leave gain unchanged. This matches the LIGO GDS ‘f’ plane convention (plane='f' in s2z()).
True: Normalize gain to preserve frequency response magnitude. Gain is scaled by \(|∏p_i/∏z_i| · (2π)^{(n_p - n_z)}\). Use this when your filter was designed with the transfer function \(H(f) = k·∏(f-z_i)/∏(f-p_i)\) in Hz. This matches the LIGO GDS ‘n’ plane convention (plane='n' in s2z()).

Ignored when unit='rad/s'.

Returns:

formstr: Filter form: 'ba', 'zpk', or 'sos'.
filttuple or numpy.ndarray: Filter coefficients in the identified form.