---
name: "rtf_create_fir_lowpass"
section: "3"
platform: "linux"
package: "librtfilter-dev"
version: "1.1-4+b3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/rtf_create_fir_lowpass"
---

# rtf\_create\_fir\_lowpass(3)

Creates FIR lowpass, highpass and bandpass windowed sinc filters

## NAME

rtf\_create\_fir\_lowpass, rtf\_create\_fir\_highpass, rtf\_create\_fir\_bandpass - Creates FIR lowpass, highpass and bandpass windowed sinc filters

## SYNOPSIS

**#include \<rtf\_common.h\>**

**hfilter rtf\_create\_fir\_lowpass(unsigned int** *nch***, int** *proctype***,**\
                                **double** *fc***, unsigned int** *hlen***,**\
                                **KernelWindow** *wndtype***);**\
 **hfilter rtf\_create\_fir\_highpass(unsigned int** *nch***, int** *proctype***,**\
                                **double** *fc***, unsigned int** *hlen***,**\
                                **KernelWindow** *wndtype***);**\
 **hfilter rtf\_create\_fir\_bandpass(unsigned int** *nchann***, int** *proctype***,**\
                                 **double** *fc\_low***, double** *fc\_high***,**\
                                 **unsigned int** *hlen***,**\
                                 **KernelWindow** *wndtype***);**\

## DESCRIPTION

**rtf\_create\_fir\_lowpass**() creates a lowpass windowed sinc filter processing *nchann* channels of data type specified by *proctype* with *fc* as normalized cutoff frequency and *hlen* as the half length of the window.

**rtf\_create\_fir\_highpass**() is the same as **rtf\_create\_fir\_lowpass**() but creates a highpass filter.

**rtf\_create\_fir\_bandpass**() is the same but creates a bandpass filter with *fc\_low* and *fc\_high* as respectively the lowpass and highpass normalized cutoff frequencies.

A normalized frequency is the ratio between the absolute frequency and the sampling frequency (i.e. a value of 1.0 refers to the sampling frequency).

The parameter *proctype* has the same meaning as in [**rtf\_create\_filter**(3)](/rtf_create_filter/3.md). Refers to its manpage for more details.

The parameter *wndtype* specifies the type of window applied to the impulse response and can take the following values: **BLACKMAN\_WINDOW**, **HAMMING\_WINDOW** or **RECT\_WINDOW** (see a book on signal processing for details about their meaning).

## RETURN VALUE

Returns the handle to the created filter in case of success, NULL otherwise.

## SEE ALSO

[**rtf\_create\_filter**(3)](/rtf_create_filter/3.md), [**rtf\_destroy\_filter**(3)](/rtf_destroy_filter/3.md)