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JJY.js: Web Audio API based JJY transmitter

JJY.js: JJY time signal emulation/transmission library


  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 and the headphone volume is turned to the maximum.
  3. From the page, run:
var jjy = JJY()

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:

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).

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


A functional demo (with explanations of internal workings) of JJY.js is available at Fukushima web page. Press any key or click anywhere to start the transmission.

Upd. For the BLE-enabled Casio watch synchronization, see my RCVD project.

Upd 2. This project has finally been ported to Python 3.

JJY.js: JJY time signal emulation/transmission library
Copyright (c) @plugnburn ( 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] = (>>9)&1
timeCode[23] = (>>8)&1
timeCode[25] = (>>7)&1
timeCode[26] = (>>6)&1
timeCode[27] = (>>5)&1
timeCode[28] = (>>4)&1
timeCode[30] = (>>3)&1
timeCode[31] = (>>2)&1
timeCode[32] = (>>1)&1
timeCode[33] =
//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( + 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.webkitNow || function(){return (new Date).getTime()}
secondTick = (function(tm){
var px = 0, py = 0, dx = 0;
return function(cb){
px =
dx = (py = - px - tm
px = py
}, tm - dx/2)
// 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]]
if(typeof module !== 'undefined' && module.exports && this.module !== module)
module.exports = JJY()
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plugnburn commented Apr 29, 2024

I came from the fukushima website. On the website it says

feel free to contact the author on GitHub.

I believe it would be more appropriate to replace the link with one to this gist,

Hello, yes, that page is quite old, will be updated soon. Also, a Python port (with pyaudio or something) is being planned to use this algorithm on headless systems.

PS. Honestly I still don't get how the harmonics are maximized.

* The Japanese website linked from says some overdrive distortion is required to make the sine look a little more square, but since we have no instruction to maximize the volume I can't be sure that this sort of distortion happens. It also doesn't explain why square and triangle, which should contain more of that 3rd fundamental, doesn't work.

* I don't quite get why the distorter should default to a value that's a multiple of the sample rate either. A higher sample rate should make the system better at reproducing a sine, which... I think should mean a stronger (lower number) distorter should be used??

I'm not sure either, this came out as a result of experiments with some Casio watches: pure sine and pure square don't work as expected, the best performance is achieved somewhere in between. Feel free to experiment with other waveforms. But yes, it is assumed that the headphone volume is put to the max. I've updated this gist to reflect this requirement.

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