Question

The task is to emulate the MIDI player work in js, but just for emulate delays between beats. There is an array with beat starting times in beat clock format, for example [960, 1280, 2200, …], and the formula I’m using for calculate millisecond time for each beat tick:

var beatTickTime = 60000 / (tempo * 96);

The problem is in very slow tick real time generation. Even if I use 1 second delay, it is still very slow. Here is how it was implemented:

var tickTimer = setInterval(function() {
     ...
     tickCount += 1; 
}, beatTickTime); // or just 1 ms

Should I pass some beat ticks doing tickCount += someNumber? Or there is more common way to solve this problem? Also I’m not sure about 96 (PPQ * 4 time) in my formula.

P. S. beat ticks comes from parsed guitar pro file

Answer 1

There’s no guarantee that setInterval() will run as fast as you ask it to. Even if the timeout is set to 1, you can’t count on the function being called 1,000 times each second.

You’ll most likely need to do something like the following:

var startTime = (new Date()).getTime();
setInterval(function() {
  var relTime = (new Date()).getTime() - startTime;
  // relTime is now the number of ms since the script started

  /*
  In here, you'll need to see if relTime is large enough to indicate the next
  beat has been reached. So that means keeping some sort of external marker
  to indicate the most recent beat that has occurred -- when relTime is big
  enough to move that marker to the next beat, also run any code that is
  necessary to handle that beat.
  */
}, 1);

The loop runs “as fast as it can” but still much more slowly than desired. It measures the current time relative to the start of the script, and determines the difference at each iteration. You could divide that time by the tempo of the song, and you’ll have an indicator of where in the song you currently are.