JJY.js: Web Audio API based JJY transmitter

README.md

JJY.js: JJY time signal emulation/transmission library

Usage

1. Make sure that the watch/clock is configured to receive JJY 40 KHz signal (for most Casio Waveceptor/G-Shock watches, the easiest way is to enter the engineer menu by pressing Mode+Light+Receive and select J 40 reception mode, for all other watches you need to set the home city to Tokyo)
2. Make sure your device clock is in sync before running the emulator.
3. From the page, run:

var jjy = JJY()
jjy.run()

The library automatically transcodes your local time into Japanese timezone regardless which timezone you're in.

To transmit a single timecode (Date object must be constructed in UTC) steadily: jjy.run(DateObject)

To change the waveform distortion parameter (for debug/wave experiment purposes), pass a numeric float value above 2 to JJY constructor (default is equal to 1/3 of current AudioContext sampling rate).

4. Start the reception and place the watch/clock close to device speaker or headphones.

Demo

A functional demo (with explanations of internal workings) of JJY.js is available at Fukushima web page.

jjy.js

/*
JJY.js: JJY time signal emulation/transmission library

Copyright (c) @plugnburn (831337.xyz) 2017
@license ISC
*/

function JJY(distorter) { //JJY constructor
  distorter = parseFloat(distorter)

  //create pre-filled constant array for efficient copying
  var codeProto = '200000000200000000020000000002000000000200000000020000000002'.split('').map(Number)

  function toBCD(val) {
    return val%10 + (((val/10)%10)<<4) + ((val/100)<<8)
  }

  function calcParity(val){
    var i = 0;
    while(val) {
      i ^= val & 1
      val >>>= 1
    }
    return i
  }

  function jjyTimecode(timeObj) { //generate a JJY timecode from any given Date object
    var ts = {
      min: toBCD(timeObj.getUTCMinutes()),
      hour: toBCD(timeObj.getUTCHours()),
      day: toBCD(Math.floor((timeObj - new Date(timeObj.getUTCFullYear(),0,0,0,0))/864e5)),
      year: toBCD(timeObj.getUTCFullYear() % 100),
      weekDay: timeObj.getUTCDay()
    }, timeCode = codeProto.slice();

    //populate minute
    timeCode[1] = (ts.min>>6)&1
    timeCode[2] = (ts.min>>5)&1
    timeCode[3] = (ts.min>>4)&1
    timeCode[5] = (ts.min>>3)&1
    timeCode[6] = (ts.min>>2)&1
    timeCode[7] = (ts.min>>1)&1
    timeCode[8] = ts.min&1
    //populate hour 
    timeCode[12] = (ts.hour>>5)&1
    timeCode[13] = (ts.hour>>4)&1
    timeCode[15] = (ts.hour>>3)&1
    timeCode[16] = (ts.hour>>2)&1
    timeCode[17] = (ts.hour>>1)&1
    timeCode[18] = ts.hour&1
    //populate day number
    timeCode[22] = (ts.day>>9)&1
    timeCode[23] = (ts.day>>8)&1
    timeCode[25] = (ts.day>>7)&1
    timeCode[26] = (ts.day>>6)&1
    timeCode[27] = (ts.day>>5)&1
    timeCode[28] = (ts.day>>4)&1
    timeCode[30] = (ts.day>>3)&1
    timeCode[31] = (ts.day>>2)&1
    timeCode[32] = (ts.day>>1)&1
    timeCode[33] = ts.day&1
    //populate parity bits
    timeCode[36] = calcParity(ts.hour)
    timeCode[37] = calcParity(ts.min)
    //populate year
    timeCode[41] = (ts.year>>7)&1
    timeCode[42] = (ts.year>>6)&1
    timeCode[43] = (ts.year>>5)&1
    timeCode[44] = (ts.year>>4)&1
    timeCode[45] = (ts.year>>3)&1
    timeCode[46] = (ts.year>>2)&1
    timeCode[47] = (ts.year>>1)&1
    timeCode[48] = ts.year&1
    //populate day of the week
    timeCode[50] = ts.weekDay>>2
    timeCode[51] = (ts.weekDay>>1)&1
    timeCode[52] = ts.weekDay&1
    return timeCode  
  }

  function getJJYTimeCode(timeObj) {
    var timeRep = timeObj || new Date(Date.now() + 324e5);
    return {
      bitCode: jjyTimecode(timeRep),
      cs: timeRep.getUTCSeconds()
    }
  }
  
  return {
    getTimeCode: getJJYTimeCode, //exposed for getting timecode information in non-browser environments
    run: function(tObj) { //browser-only function, requires Web Audio API support 
      var ctx = new (window.AudioContext || window.webkitAudioContext)(),
          sr = ctx.sampleRate,
          opFreq = 40000/3,
          rp = opFreq/sr, bufSet = [], pwm = [.8, .5, .2];
      if(isNaN(distorter) || distorter < 2)
        distorter = sr/3

      ctx.createBuffer = ctx.createBuffer || ctx.webkitCreateBuffer;
      ctx.createBufferSource = ctx.createBufferSource || ctx.webkitCreateBufferSource;
      performance.now = performance.now || performance.webkitNow || function(){return (new Date).getTime()}

      secondTick = (function(tm){
        var px = 0, py = 0, dx = 0;
        return function(cb){
          px = performance.now()
          setTimeout(function(){
            dx = (py = performance.now()) - px - tm
            px = py
            cb()
          }, tm - dx/2)
        }
      })(1000)

      // pre-populate the buffers
      for(var i=0;i<3;i++) {
        var sLen = sr*pwm[i];
        bufSet[i] = ctx.createBuffer(1, sLen, sr);
        var cData = bufSet[i].getChannelData(0);
        for(var k=0;k<sLen;k++)
          cData[k] = Math.floor(distorter*Math.sin(2 * Math.PI * k * rp))/distorter
      }
      // play back the timecode
      var renderMinute = function() {
        var sigInfo = getJJYTimeCode(tObj), currentIndex = sigInfo.cs,
            renderSecond = function() {
              secondTick(currentIndex<59 ? renderSecond : renderMinute)
              var bs = ctx.createBufferSource()
              bs.buffer = bufSet[sigInfo.bitCode[currentIndex]]
              bs.connect(ctx.destination)
              bs.start()
              currentIndex++
            }
        renderSecond()
      }
      renderMinute()
    }
  }
}

if(typeof module !== 'undefined' && module.exports && this.module !== module)
  module.exports = JJY()