Metronome

Why a Browser Metronome Is Harder Than It Looks

A metronome has one job, and a web page is unusually badly equipped to do it. The obvious implementation — ask the browser to run a function every so many milliseconds, and play a click each time it fires — produces something that sounds approximately right for about thirty seconds and then falls apart. It is worth understanding why, because the failure is not a bug anyone introduced. It is the ordinary behaviour of a browser meeting a task that needs millisecond accuracy.

Two failures, both invisible at first

The first is drift. Browser timers are best-effort: they fire when the main thread is free, which might be a few milliseconds late, or considerably later if something else is happening on the page. If each click is scheduled relative to the previous one, that lateness is added permanently and the metronome falls further behind real time with every beat. Ten thousand beats into a session it can be seconds adrift — but three beats in it looks perfect, which is exactly why the flaw survives casual testing. The fix is to compute every click's time from a single fixed starting point, so that late arrivals correct themselves instead of compounding.

The second is background throttling. Browsers deliberately slow timers in tabs you are not looking at, to about one callback per second, because most pages doing work in the background are doing nothing useful. For a metronome this is fatal in the most annoying possible way: the click is fine while you watch it and stutters the moment you switch to the tab with your sheet music in it — which is what you opened it to do.

What makes this solvable is that browsers expose a second, far better clock. The Web Audio system runs on its own thread with sample-level accuracy, and it will accept an instruction to play a sound at a precise future moment. So the timer's job changes completely: it stops being the thing that plays the click, and becomes the thing that tops up a queue of clicks already scheduled on the audio clock. Its own jitter no longer matters, because nothing it does is heard. Handle background throttling by queueing further ahead when the page is hidden — several seconds instead of a fraction of one — and a browser that only wakes you once a second still finds plenty already scheduled.

What a metronome measures, and what it does not

The device is younger than most of the music played to it. Winkel built the mechanism and Maelzel patented it in 1815, well after Bach and Mozart, and the Italian tempo words that appear on scores are centuries older still. Those words were never precise: allegro means cheerful, not 132 beats per minute, and the same marking covers a range because it describes a character. Beethoven, who adopted metronome marks early, left figures that many performers still consider unplayably fast — a two-hundred-year argument about what a number can and cannot capture about music.

A limitation worth stating, especially in this region

This tool assumes a Western metrical accent: bars of equal beats with the emphasis on the first. That assumption does not travel everywhere, and it travels particularly badly across Southeast Asia. Javanese and Balinese gamelan, and the related Malay traditions, are organised around colotomic cycles marked by gongs, where the structurally important moment is the arrival at the end of the cycle rather than a stress at its beginning — the opposite emphasis from the one every dot on this page is drawing. A metronome accenting beat one is not a neutral tool for that repertoire; it is a Western one.

Where it does fit the regional context squarely is graded examination practice. The ABRSM and Trinity syllabuses are widely taken across Singapore and Malaysia, and their set pieces carry metronome marks that candidates are expected to work towards rather than hit on the first attempt. The standard method — set the mark, subtract twenty, and creep upwards as the passage becomes clean — is exactly what tap tempo and a fine-grained slider are for.