scratch-audio/src/effects/PitchEffect.js

/**
* A pitch change effect, which changes the playback rate of the sound in order
* to change its pitch: reducing the playback rate lowers the pitch, increasing the rate
* raises the pitch. The duration of the sound is also changed.
*
* Changing the value of the pitch effect by 10 causes a change in pitch by 1 semitone
* (i.e. a musical half-step, such as the difference between C and C#)
* Changing the pitch effect by 120 changes the pitch by one octave (12 semitones)
*
* The value of this effect is not clamped (i.e. it is typically between -120 and 120,
* but can be set much higher or much lower, with weird and fun results).
* We should consider what extreme values to use for clamping it.
*
* Note that this effect functions differently from the other audio effects. It is
* not part of a chain of audio nodes. Instead, it provides a way to set the playback
* on one SoundPlayer or a group of them.
*/
class PitchEffect {
    constructor () {
        this.value = 0; // effect value
        this.ratio = 1; // the playback rate ratio
    }

    /**
    * Set the effect value
    * @param {number} val - the new value to set the effect to
    * @param {object} players - a dictionary of SoundPlayer objects to apply the effect to, indexed by md5
    */
    set (val, players) {
        this.value = val;
        this.ratio = this.getRatio(this.value);
        this.updatePlayers(players);
    }

    /**
    * Change the effect value
    * @param {number} val - the value to change the effect by
    * @param {object} players - a dictionary of SoundPlayer objects indexed by md5
    */
    changeBy (val, players) {
        this.set(this.value + val, players);
    }

    /**
    * Compute the playback ratio for an effect value.
    * The playback ratio is scaled so that a change of 10 in the effect value
    * gives a change of 1 semitone in the ratio.
    * @param {number} val - an effect value
    * @returns {number} a playback ratio
    */
    getRatio (val) {
        return this.intervalToFrequencyRatio(val / 10);
    }

    /**
    * Convert a musical interval to a frequency ratio.
    * With thanks to Tone.js: https://github.com/Tonejs/Tone.js
    * @param {number} interval - a musical interval, in semitones
    * @returns {number} a frequency ratio
    */
    intervalToFrequencyRatio (interval) {
        return Math.pow(2, (interval/12));
    }

    /**
    * Update a sound player's playback rate using the current ratio for the effect
    * @param {object} player - a SoundPlayer object
    */
    updatePlayer (player) {
        player.setPlaybackRate(this.ratio);
    }

    /**
    * Update a sound player's playback rate using the current ratio for the effect
    * @param {object} players - a dictionary of SoundPlayer objects to update, indexed by md5
    */
    updatePlayers (players) {
        if (!players) return;

        for (const md5 in players) {
            if (players.hasOwnProperty(md5)) {
                this.updatePlayer(players[md5]);
            }
        }
    }
}

module.exports = PitchEffect;