/usr/share/audacity/plug-ins/lowpass.ny is in audacity-data 2.0.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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;version 3
;type process
;categories "http://lv2plug.in/ns/lv2core#LowpassPlugin"
;name "Low Pass Filter..."
;action "Performing Low Pass Filter..."
;info "by Dominic Mazzoni, modified by David R. Sky\nReleased under terms of the GNU General Public License Version 2\nAttenuates high frequencies above your specified cutoff frequency.\nHigher rolloff values give a sharper attenuation of frequencies above\nthe cutoff frequency. If using a rolloff of 12 dB, a [q] value greater than\ndefault 0.7 increases resonance ['ringing'] of the cutoff frequency and\ncould result in clipping.""
;control rolloff-choice " Rolloff [dB per octave]" choice " 6 dB,12 dB,24 dB,36 dB,48 dB" 0
;control q " Filter quality [q] for 12 dB rolloff" real "" 0.7071 .1 20
;control f " Cutoff frequency [Hz]" real "" 1000 1 20000
; note that all Nyquist lowpass functions
; [lp, lowpass2, lowpass4, lowpass6, lowpass8]
; are defined below with -s suffix.
; This enables lowpass functions other than lp
; to deal with stereo selection,
; and dummy q arg for all but lowpass2
; 6dB/octave
(defun lp-s (s f q) ; dummy q arg
(lp s f))
; 12dB/octave
(defun lowpass2-s (s f q)
(if (arrayp s)
(vector (lowpass2 (aref s 0) f q)
(lowpass2 (aref s 1) f q))
(lowpass2 s f q)))
; 24dB/octave
(defun lowpass4-s (s f q) ; dummy q arg
(if (arrayp s)
(vector (lowpass4 (aref s 0) f)
(lowpass4 (aref s 1) f))
(lowpass4 s f)))
; 36dB/octave
(defun lowpass6-s (s f q) ; dummy q arg
(if (arrayp s)
(vector (lowpass6 (aref s 0) f)
(lowpass6 (aref s 1) f))
(lowpass6 s f)))
; 48dB/octave
(defun lowpass8-s (s f q) ; dummy q arg
(if (arrayp s)
(vector (lowpass8 (aref s 0) f)
(lowpass8 (aref s 1) f))
(lowpass8 s f)))
; check function: returns 1 on error
(defun check (arg min max)
(if (and (>= arg min) (<= arg max))
0 1))
; initialize blank error-msg
(setf error-msg "")
; check for erroneous q value
(setf error-msg (if
(and (= rolloff-choice 1)
(= (check q 0.1 20) 1))
(strcat error-msg (format nil
"q value ~a lies outside valid range 0.1 to 20
for your chosen rolloff of 12 dB per octave.~%
" q))
error-msg))
;; check for erroneous frequency cutoff value
(cond ((< f 1)
(setf error-msg
(strcat error-msg (format nil
"Cutoff frequency is set at ~a Hz~%but must be at least 1 Hz." f))))
((> f (truncate (/ *sound-srate* 2.0)))
(setf error-msg
(strcat error-msg (format nil
"Cutoff frequency is set at ~a Hz but must not~%~
be greater than ~a Hz (half of the track sample rate)."
f (truncate (/ *sound-srate* 2.0)))))))
(cond
((> (length error-msg) 0)
(setf error-msg (strcat (format nil
"Error.~%You have entered at least one invalid value:~%
") error-msg))
(format nil "~a" error-msg))
;
(t ; perform lowpass effect
(funcall (nth rolloff-choice '(lp-s lowpass2-s lowpass4-s lowpass6-s lowpass8-s))
s f q)))
; from previous commit
; arch-tag: c2d96e46-b4e2-47c0-9a19-761011418e02
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