Tone.js/Tone/instrument/FMSynth.ts

import { Positive } from "../core/type/Units.js";
import { optionsFromArguments } from "../core/util/Defaults.js";
import { RecursivePartial } from "../core/util/Interface.js";
import { Multiply } from "../signal/Multiply.js";
import { ModulationSynth, ModulationSynthOptions } from "./ModulationSynth.js";

export interface FMSynthOptions extends ModulationSynthOptions {
	modulationIndex: Positive;
}

/**
 * FMSynth is composed of two Tone.Synths where one Tone.Synth modulates
 * the frequency of a second Tone.Synth. A lot of spectral content
 * can be explored using the modulationIndex parameter. Read more about
 * frequency modulation synthesis on Sound On Sound: [Part 1](https://web.archive.org/web/20160403123704/http://www.soundonsound.com/sos/apr00/articles/synthsecrets.htm), [Part 2](https://web.archive.org/web/20160403115835/http://www.soundonsound.com/sos/may00/articles/synth.htm).
 *
 * @example
 * const fmSynth = new Tone.FMSynth().toDestination();
 * fmSynth.triggerAttackRelease("C5", "4n");
 *
 * @category Instrument
 */

export class FMSynth extends ModulationSynth<FMSynthOptions> {
	readonly name: string = "FMSynth";

	/**
	 * The modulation index which essentially the depth or amount of the modulation. It is the
	 * ratio of the frequency of the modulating signal (mf) to the amplitude of the
	 * modulating signal (ma) -- as in ma/mf.
	 */
	readonly modulationIndex: Multiply;

	constructor(options?: RecursivePartial<FMSynthOptions>);
	constructor() {
		const options = optionsFromArguments(FMSynth.getDefaults(), arguments);
		super(options);

		this.modulationIndex = new Multiply({
			context: this.context,
			value: options.modulationIndex,
		});

		// control the two voices frequency
		this.frequency.connect(this._carrier.frequency);
		this.frequency.chain(this.harmonicity, this._modulator.frequency);
		this.frequency.chain(this.modulationIndex, this._modulationNode);
		this.detune.fan(this._carrier.detune, this._modulator.detune);
		this._modulator.connect(this._modulationNode.gain);
		this._modulationNode.connect(this._carrier.frequency);
		this._carrier.connect(this.output);
	}

	static getDefaults(): FMSynthOptions {
		return Object.assign(ModulationSynth.getDefaults(), {
			modulationIndex: 10,
		});
	}

	dispose(): this {
		super.dispose();
		this.modulationIndex.dispose();
		return this;
	}
}