BUTTER

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Apply a butterworth filter to an input signal. It is designed to have a frequency response that is as flat as possible in the pass band. Inputs ------ default : OrderedPair The data to apply the butter filter to. Params: filter_order : int The intensity of the filter. critical_frequency : int The frequency where the filter takes effect. For lowpass and highpass, it takes a scalar. For bandpass and bandstop, it takes an array with the length of 2. btype : select The type of the filter. sample_rate : int The sample rate of the input signal. Returns: out : OrderedPair x: time domain y: filtered signal

Python Code

from scipy import signal
from flojoy import flojoy, OrderedPair
from typing import Literal


@flojoy
def BUTTER(
    default: OrderedPair,
    filter_order: int = 1,
    critical_frequency: int = 1,
    btype: Literal["lowpass", "highpass", "bandpass", "bandstop"] = "lowpass",
    sample_rate: int = 10,
) -> OrderedPair:
    """Apply a butterworth filter to an input signal.

    It is designed to have a frequency response that is as flat as possible in the pass band.

    Inputs
    ------
    default : OrderedPair
        The data to apply the butter filter to.

    Parameters
    ----------
    filter_order : int
        The intensity of the filter.
    critical_frequency : int
        The frequency where the filter takes effect. For lowpass and highpass, it takes a scalar.
        For bandpass and bandstop, it takes an array with the length of 2.
    btype : select
        The type of the filter.
    sample_rate : int
        The sample rate of the input signal.

    Returns
    -------
    OrderedPair
        x: time domain
        y: filtered signal
    """

    sig = default.y
    order: int = filter_order
    wn: int = critical_frequency  # hz
    btype: str = btype
    fs: int = sample_rate  # hz

    sos = signal.butter(N=order, Wn=wn, btype=btype, fs=fs, output="sos")
    #    sos = signal.butter(10, 15, "hp", fs=1000, output="sos")
    filtered = signal.sosfilt(sos, sig)

    return OrderedPair(x=default.x, y=filtered)

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Example App

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In this example, BASIC_OSCILLATOR node generates a signal that’s 10 seconds long with a sample rate of 50. The generated signal is a sawtooth with a starting frequency of 1.

It is then sent to a BUTTER node that filters out frequencies above 5 with a filter_order of 4. Since we want the lower frequencies to pass through, the lowpass option is selected for the btype option.