ATC VHF radio simulation for Audacity

vhf.ny

;nyquist plug-in
;version 4
;type process
;name "VHF RTF"
;release "0.2.0"
;control text "Process audio to imitate VHF radio reception"
;control text "Filters derived from pierr3/afv-native"
;control preset "Filter preset" choice "Schmid ED137B,Garex 220,Rockwell Collins 2100" 0
;control pregain "Pre-scale" float nil 1.0 0.0 1.0

;; I have not used Nyquist before, so this is pretty awful

(defun peaking (track centre q gain)
	(let*
		(
			(w0    (/ (* 2 pi centre) *sound-srate*))
			(cosw0 (cos w0))
			(sinw0 (sin w0))
			(alpha (/ sinw0 2 q))
			(a     (expt 10 (/ gain 40)))
		)
		(let
			(
				(b0 (+ 1 (* alpha a)))
				(b1 (* -2 cosw0))
				(b2 (- 1 (* alpha a)))
				(a0 (+ 1 (/ alpha a)))
				(a1 (* -2 cosw0))
				(a2 (- 1 (/ alpha a)))
			)
			(biquad-m track b0 b1 b2 a0 a1 a2))))

(defun apply-filters (track filters)
	(if (null filters)
		track
		(apply-filters (eval (car filters)) (cdr filters))))

(apply-filters
	(scale pregain
		(if (arrayp *track*)
			(scale 0.5 (to-mono *track*))
			*track*))
	(nth preset
		(list
			;; Schmid ED137B
			(list
				'(highpass2 track 310 0.25)
				'(peaking track 450 0.75 12.0)
				'(peaking track 1450 1.0 20.0)
				'(peaking track 2000 1.0 20.0)
				'(lowpass2 track 2500 0.25))
			;; Garex 220
			(list
				'(highpass2 track 300 0.25)
				'(eq-highshelf track 400 8)
				'(eq-highshelf track 600 4)
				'(eq-lowshelf track 2000 1)
				'(eq-lowshelf track 2400 3)
				'(eq-lowshelf track 3000 10)
				'(lowpass2 track 3400 0.25))
			;; Rockwell Collins 2100
			(list
				'(biquad-m track -0.01 0.0 0.0 1.0 0.0 0.0)
				'(biquad-m track 0.0  1.0    0.75316 1.0 -1.7153 0.76139)
				'(biquad-m track 1.0 -2.2928 1.0003  1.0 -1.7163 0.76243)
				'(biquad-m track 1.0 -2.0504 1.0505  1.0 -1.7938 0.90968)
				'(biquad-m track 1.0 -1.9519 0.95194 1.0 -1.7941 0.90982)
				'(biquad-m track 1.0 -1.8255 1.0916  1.0 -1.9390 0.94118)
				'(biquad-m track 1.0 -1.6724 0.91618 1.0 -1.9402 0.94263)))))