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diff --git a/demos/pitch_change.htm b/demos/pitch_change.htm new file mode 100644 index 0000000..43852e7 --- /dev/null +++ b/demos/pitch_change.htm @@ -0,0 +1,83 @@ +<html> + +<head> +<title>Pitch Change by Resampling Tutorial</title> +</head> + +<body> + +<h1>Pitch Change by Resampling Tutorial</h1> + +<p><b>Roger Dannenberg</b></p> +<p> +This tutorial shows how to change the pitch of a sound using resampling. +In this example, a source sound is resampled to effectively play the sound +faster and/or slower. This changes the perceived pitch. It also makes the +sound play faster and/or slower, so the original duration is not preserved. +<p> +To control the playback speed, a control function is used. The function +specifies the pitch shift in steps, so if the function is zero, there is +no change. If the signal is 1, the signal is shifted up one half-step. +If the signal is -2, the signal is shifted down a whole-step, etc. +<p> +The control signal can of course change over time. The control signal +time corresponds to time in the resulting sound. Thus, if the control +signal jumps from 0 to 5 at time 3, the resulting sound will jump up +a musical fourth (5 half-steps) at time 3. This may or may not be time +3 in the source sound. + +<p> +The code implements VARIABLE-RESAMPLE which takes two parameters: +<ul> +<li>steps -- the pitch control function +<li>snd -- the source sound to be resampled +</ul> +The function works as follows: +First, steps is converted to ratio, which is the speed change expressed +as a ratio. E.g. when steps is 12 (an octave), ratio will be 2 (a 2:1 ratio) +Second, ratio is integrated to get map. The map is a function from real +time to the score. Note that the slope of the map at any time determines +the amount of speedup. +Finally, SND-COMPOSE is used to map the source sound according to the +tempo map. Because SND-COMPOSE is defined to output the sample rate +of the map, we coerce the sample rate of map to *sound-srate*. +<p> +This function could be improved by adding code to handle stereo inputs. +Also, this function should really be implemented using snd-resamplev, +which uses a high-quality resampling algorithm. Unfortunately, there +a bug in snd-resamplev, so until the bug is fixed, snd-compose actually +sounds better. +<p> +The resulting sound ends whenever either steps or snd come to an end. +To control duration, you may want to make sure that the source sound +is much longer than the control function. That way, even if you speed +it up, it will still last long enough to exhaust the control function, +and you know by design how long that is. You can also apply an +envelope to the result to trim it to a known length. +<p> +So, here finally is the implementation: +<pre> +(defun variable-resample (steps snd) + (let ((p1 (/ (log 2.0) 12)) ; p1 helps convert steps to a ratio + ratio map) + (setf ratio (s-exp (mult steps p1))) ; pitch ratio + (setf map (integrate ratio)) ; map from real-time to sound time + (snd-compose snd (force-srate *sound-srate* map)))) +</pre> +Here is an example: the PWL control function is initially zero (no +transposition), but starting at time 1.0, it rises to 2.0 in 0.1s +where it remains until time 3.0. The function ends at time 3.0. +The sound to be modified in this case is a simple sinusoid with a +duration of 8. In this case, the PWL function finishes at 3.0s, well +before the OSC sound comes to an end. +<pre> +(play + (variable-resample (pwl 1.0 0.0 1.1 2.0 3.0 2.0 3.0) (osc c4 8.0))) +</pre> +Note that you can replace (osc c4 8.0) with any sound, including one +read from a sound file using S-READ. + +</body> +</html> + + |