nes-emu/apu/channels/TriangleChannel.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
var _synthesis = require("../synthesis");
var _constants = _interopRequireDefault(require("../../constants"));
var _helpers = require("../../helpers");
function _interopRequireDefault(e) { return e && e.__esModule ? e : { default: e }; }
/**
 * The triangle channel produces a quantized triangle wave. It supports:
 *   - note lengths
 *   - an extra high-resolution ("linear") length counter
 */
class TriangleChannel {
  constructor() {
    _helpers.WithContext.apply(this);
    this.oscillator = new _synthesis.TriangleOscillator();
    this.lengthCounter = new _synthesis.LengthCounter();
    this.linearLengthCounter = new _synthesis.LinearLengthCounter();
    this.outputSample = 0;
  }

  /** When a context is loaded. */
  onLoad(context) {
    this.registers = context.apu.registers.triangle;
  }

  /** Generates a new sample. */
  sample(isNewSample) {
    if (!this.isEnabled) return this.outputSample;
    const timer = _helpers.Byte.to16Bit(this.registers.lclTimerHigh.timerHigh, this.registers.timerLow.value);

    // this channel only outputs a sample if the timer is between [2, 0x7ff]
    if (!(timer >= 2 && timer <= 0x7ff)) return 0;
    this.oscillator.frequency = _constants.default.FREQ_CPU_HZ / (16 * (timer + 1)) / 2;
    // from nesdev: f = fCPU / (16 * (t + 1))
    // (the pitch is one octave below the pulse channels with an equivalent timer value)
    // (i.e. use the formula above but divide the resulting frequency by two).

    const isActive = !this.lengthCounter.didFinish && !this.linearLengthCounter.didFinish;
    if (isActive) this.outputSample = this.oscillator.sample(isNewSample);
    return this.outputSample;
  }

  /** Updates linear length counter. */
  quarterBeat() {
    this.linearLengthCounter.clock(this.isEnabled, this.registers.linearLCL.halt);
  }

  /** Updates length counter. */
  halfBeat() {
    this.lengthCounter.clock(this.isEnabled, this.registers.linearLCL.halt);
  }

  /** Returns a snapshot of the current state. */
  getSaveState() {
    return {
      outputSample: this.outputSample,
      oscillator: this.oscillator.getSaveState(),
      lengthCounter: this.lengthCounter.getSaveState(),
      linearLengthCounter: this.linearLengthCounter.getSaveState()
    };
  }

  /** Restores state from a snapshot. */
  setSaveState(saveState) {
    this.outputSample = saveState.outputSample;
    this.oscillator.setSaveState(saveState.oscillator);
    this.lengthCounter.setSaveState(saveState.lengthCounter);
    this.linearLengthCounter.setSaveState(saveState.linearLengthCounter);
  }

  /** Returns whether the channel is enabled or not. */
  get isEnabled() {
    return this.context.apu.registers.apuControl.enableTriangle;
  }
}
exports.default = TriangleChannel;