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**Simple Compressor Example

;(set-control-srate 100) ;(set-sound-srate 100) ;(setf xx (pwl 0 1 1 0 1.1 1 1.8 0 2 1 3 0 5)) ;(setf xx (pwl 0 1 1 .2 1.1 1 1.8 .2 2 1 3 0 5)) ;(setf yy (snd-follow xx 0.1 0.25 1.0 30)) ;(setf db-factor (/ 1.0 (log 0.00001))) ; COMPRESS-MAP -- constructs a map for the compress function ; ; The map consists of two parts: a compression part and an expansion part. ; The intended use is to compress everything above compress-threshold by ; compress-ratio, and to downward expand everything below expand-ratio ; by expand-ratio. Thresholds are in dB and ratios are dB-per-dB. ; 0dB corresponds to an amplitude of 1.0 ; If the input goes above 0dB, the output can optionally be limited ; by seting limit-flag to T. This effectively changes the compression ; ratio to infinity at 0dB. If limit-flag is NIL, then the compression-ratio ; continues to apply above 0dB. ; It is assumed that expand-threshold <= compress-threshold <= 0 ; The gain is unity at 0dB so if compression-ratio > 1, then gain ; will be greater than unity below 0dB ;(defun compress-map (compress-ratio compress-threshold expand-ratio ; expand-threshold limit-flag) ; (let () ; ( ;; I'm not sure if the rest of this function was lost due to version ;; problems, or it never existed. Email to rbd@cs.cmu.edu if you would ;; like some help with dynamics compression. ;; ;; Also, I had a really great 2-stage compressor for speech -- it did ;; something like a noise gate with a short time constant, and an automatic ;; gain control with a long time constant. Each one varied the gain by ;; about 12 dB -- any more would cause really ugly noise pumping, but ;; without the combined actions of both, there was not enough control. ;; Again, email me if you are interested. Lately, I've been using ;; more sophisticated multiple band noise reduction in Cool Edit. They ;; obviously put a lot of work into that, and I don't plan to redo the ;; work for Nyquist. -RBD (defun compress (input map rise-time fall-time) ; take the square of the input to get power (let ((in-squared (mult input input))) ; compute the time-average (sort of a low-pass) of the square (setf avg (snd-avg in-squared 1000 500 OP-AVERAGE)) ; use follower to anticipate rise and trail off smoothly (setf env (snd-follow avg 0.001 0.2 1.0 20)) ; take logarithm to get dB instead of linear (setf logenv (snd-log env)) ; tricky part: map converts dB of input to desired gain in dB ; this defines the character of the compressor (setf shaped-env (shape logenv map 1.0)) ; go back to linear (setf gain (snd-exp shaped-env)) ; return the scaled input sound, ; another trick: avg signal will be delayed. Also, snd-follow ; has a delayed response because it's looking ahead in sound ; 20 = the number of samples of lookahead from snd-follow ; 88.2 = 44,100 (sample rate) / 500 (the step-size in avg) ; in other words, 44100/500 is the sample rate of the control ; signal looked at by follow ; "44100" should be replace by the signal's sample rate ; = (snd-srate input) (mult (seq (s-rest (/ 20.0 88.2)) (cue input)) gain)))

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**Simple Compressor Example** /
Speech Compressor Example
Audacity Support Forum /
Audacity and Nyquist /
Nyquist Reference Manual /
Nyquist Examples and Tutorials
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