/usr/share/audacity/plug-ins/crossfadetracks.ny is in audacity-data 2.2.1-1.
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 4
;type process
;name "Crossfade Tracks..."
;manpage "Crossfade_Tracks"
;debugbutton disabled
;action "Crossfading..."
;preview selection
;author "Steve Daulton"
;copyright "Released under terms of the GNU General Public License version 2"
;; crossfadetracks.ny by Steve Daulton Nov 2014 / Sep 2015
;; Released under terms of the GNU General Public License version 2:
;; http://www.gnu.org/licenses/old-licenses/gpl-2.0.html .
;control type "Fade type" choice "Constant Gain,Constant Power 1,Constant Power 2,Custom Curve" 0
;control curve "Custom curve" real "" 0 0 1
;control direction "Fade direction" choice "Automatic,Alternating Out / In,Alternating In / Out" 0
(defun crossfade (type dir curve)
(setf fade-out
(case dir
(0 (equal (guessdirection) 'OUT)) ; auto
(1 (oddp (get '*track* 'index))) ; fade out odd
(T (evenp (get '*track* 'index))))) ; fade out even
(mult *track*
(cond
(fade-out
(case type
(0 (pwlv 1 1 0))
(1 (osc (hz-to-step (/ (get-duration 4))) 1 *sine-table* 90))
(2 (s-sqrt (pwlv 1 1 0)))
(T (custom curve 0))))
(T ; else fade in.
; Control envelope sample rate must match sound so that lengths
; match exactly, otherwise we get a click at the end of the fade.
(setf *control-srate* *sound-srate*)
(case type
(0 (pwlv 0 1 1))
(1 (osc (hz-to-step (/ (get-duration 4))) 1))
(2 (s-sqrt (pwlv 0 1 1)))
(T (custom curve 1)))))))
(defun custom (curve inout)
;; 'epsilon' defines the curvature of a logarithmc curve.
;; To avoid log 0 or /0 it must be > 0 and < 1.
(let* ((curve (+ 0.99 (* -0.98 (min 1 (max 0 curve)))))
; magic number 2.7 gives approx 'constant power' curve at 50% setting.
(epsilon (power curve 2.7)))
(if (= inout 0)
(setf logcurve (pwev epsilon 1 1))
(setf logcurve (pwev 1 1 epsilon)))
; Scale and invert curve for 0 to unity gain.
(sum 1
(mult (/ -1 (- 1 epsilon))
(diff logcurve epsilon)))))
(defun guessdirection ()
"If the selection is closer to the start of the
audio clip, fade in, otherwise fade out."
(let* ((start (get '*selection* 'start))
(end (get '*selection* 'end))
(clips (get '*track* 'clips))
(in-dist end)
(out-dist end))
(if (arrayp clips)
(setf clips (append (aref clips 0)(aref clips 1))))
(dotimes (i (length clips))
(setf in-dist (min in-dist (abs (- start (first (nth i clips))))))
(setf out-dist (min out-dist (abs (- end (second (nth i clips)))))))
(if (< in-dist out-dist) 'in 'out)))
(if (< (length (get '*selection* 'tracks)) 2)
"Error.\nSelect 2 (or more) tracks to crossfade."
(crossfade type direction curve))
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