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