Locate and position the current Audacity source code, and clear a variety of old junk out of the way into junk-branches

nyquist/follow.lsp

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+;(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)))
+