PhaseShiftAllpass.ts

import { Gain } from "../../core/context/Gain";
import { connectSeries, ToneAudioNode, ToneAudioNodeOptions } from "../../core/context/ToneAudioNode";

/**
 * PhaseShiftAllpass is an very efficient implementation of a Hilbert Transform
 * using two Allpass filter banks whose outputs have a phase difference of 90°.
 * Here the `offset90` phase is offset by +90° in relation to `output`.
 * Coefficients and structure was developed by Olli Niemitalo.
 * For more details see: http://yehar.com/blog/?p=368
 * @category Component
 */
export class PhaseShiftAllpass extends ToneAudioNode<ToneAudioNodeOptions> {

	readonly name: string = "PhaseShiftAllpass";

	readonly input = new Gain({ context: this.context });

	/**
	 * The Allpass filter in the first bank
	 */
	private _bank0: IIRFilterNode[];

	/**
	 * The Allpass filter in the seconds bank
	 */
	private _bank1: IIRFilterNode[];

	/**
	 * A IIR filter implementing a delay by one sample used by the first bank
	 */
	private _oneSampleDelay: IIRFilterNode;

	/**
	 * The phase shifted output
	 */
	readonly output = new Gain({ context: this.context });

	/**
	 * The PhaseShifted allpass output
	 */
	readonly offset90 = new Gain({ context: this.context });

	constructor(options?: Partial<ToneAudioNodeOptions>) {

		super(options);

		const allpassBank1Values = [0.6923878, 0.9360654322959, 0.9882295226860, 0.9987488452737];
		const allpassBank2Values = [0.4021921162426, 0.8561710882420, 0.9722909545651, 0.9952884791278];

		this._bank0 = this._createAllPassFilterBank(allpassBank1Values);
		this._bank1 = this._createAllPassFilterBank(allpassBank2Values);
		this._oneSampleDelay = this.context.createIIRFilter([0.0, 1.0], [1.0, 0.0]);

		// connect Allpass filter banks
		connectSeries(this.input, ...this._bank0, this._oneSampleDelay, this.output);
		connectSeries(this.input, ...this._bank1, this.offset90);
	}

	/**
	 * Create all of the IIR filters from an array of values using the coefficient calculation.
	 */
	private _createAllPassFilterBank(bankValues: number[]): IIRFilterNode[] {
		const nodes: IIRFilterNode[] = bankValues.map(value => {
			const coefficients = [[value * value, 0, -1], [1, 0, -(value * value)]];
			return this.context.createIIRFilter(coefficients[0], coefficients[1]);
		});

		return nodes;
	}

	dispose(): this {
		super.dispose();
		this.input.dispose();
		this.output.dispose();
		this.offset90.dispose();
		this._bank0.forEach(f => f.disconnect());
		this._bank1.forEach(f => f.disconnect());
		this._oneSampleDelay.disconnect();
		return this;
	}
}