HackRF Shell: Fun with filters

I had added bindings earlier for pulling data from the HackRF, and then more recently a binding to liquid-dsp frequency demodulation. But in between the two, filtering will be needed. So, I added a binding for the firdespm_lowpass function from liquid-dsp. This function gives you the coefficients for a Finite Impulse Response (FIR) filter, using the Parks-McClellan algorithm.

I wanted to see something interesting from that before going to bed, so I threw together a scheme procedure to generate and then print out the coefficients:

;; -*- geiser-scheme-implementation: guile -*-

;; Copyright 2020 Christopher Howard

;; This program is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <https://www.gnu.org/licenses/>.

(define-module (sdr filters)
  #:export (print-firdespm-lowpass-coefficients))

(use-modules (rnrs bytevectors)
             (sdr liquid-dsp))

(define (print-firdespm-lowpass-coefficients filter-len
                                             cutoff-freq
                                             stop-band-atten
                                             fract-samp-offset)
  (let ((output-bv (make-bytevector (* 4 filter-len))))
    (firdespm-lowpass filter-len
                      cutoff-freq
                      stop-band-atten
                      fract-samp-offset
                      output-bv)
    (let lp ((coeff 0))
      (when (< coeff filter-len)
        (display
         (number->string
          (bytevector-ieee-single-ref output-bv (* 4 coeff) (native-endianness))))
        (display "\n")
        (lp (1+ coeff))))))

Then I dumped some coefficients to a file:

(with-output-to-file "out.csv"
  (lambda () (print-firdespm-lowpass-coefficients 201 0.2 60.0 0)))

Plotting out the coefficients, I got this visual:

I don’t really know enough yet to be sure if those are sane results or not, but it looks pretty. I was hoping to pick up a DSP book from the library sometime this week and dive more into the details of FIR filters.

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