How accurately (in terms of time) does Windows play audio?

Question

Let's say I play a stereo WAV file with 317,520,000 samples, which is theoretically 1 hour long. Assuming no interruptions of the playback, will the file finish playing in exactly one hour, or is there some occasional tiny variation in the playback speed such that it would be slightly more or slightly less (by some number of milliseconds) than one hour?

I am trying to synchronize animation with audio, and I am using a System.Diagnostics.Stopwatch to keep the frames matching the audio. But if the playback speed of WAV audio in Windows can vary slightly over time, then the audio will drift out of sync with the Stopwatch-driven animation.

Which leads to a second question: it appears that a Stopwatch - while highly granular and accurate for short durations - runs slightly fast. On my laptop, a Stopwatch run for exactly 24 hours (as measured by the computer's system time and a real stopwatch) shows an elapsed time of 24 hours plus about 5 seconds (not milliseconds).

Is this a known problem with Stopwatch? (A related question would be "am I crazy?", but you can try it for yourself.) Given its usage as a diagnostics tool, I can see where a discrepancy like this would only show up when measuring long durations, for which most people would use something other than a Stopwatch.

If I'm really lucky, then both Stopwatch and audio playback are driven by the same underlying mechanism, and thus will stay in sync with each other for days on end. Any chance this is true?

Update: I just did the math, and if Stopwatch drifts by 5 seconds over 24 hours, this means it will drift by 10 milliseconds after just 172 seconds. So in 3 minutes the animation will start being perceptably out of sync.

I'm experimenting with periodically (every 10 seconds or so) re-starting the timer from the waveOutWrite callback, but this isn't working because then the whole next set of timer events is offset by whatever the inaccuracy of the callback happened to be. Sucks to be me.

Answer 1

No clock will measure time "exactly", as all physical devices are bound to have some variations and measurement errors. This means that ALL clocks will be slightly too fast or too slow (though the amount of error may differ wildly, depending on the clock).

In your case, the audio output is driven by the clock on the soundcard which drives the DAC. I don't know the .NET platform, but i assume that Stopwatch is some kind of system timer, which means it is driven by a DIFFERENT clock (the one on your motherboard, presumably).

Now in general, two different physical clocks will NEVER run at exactly the same speed - for the reasons outlined above. That is where the discrepancy you got came from. The same thing will happen to your animation - you can absolutely never assume the system clock and the soundcard DAC clock are the same - they will differ!

This means that if you want to keep two streams (video and audio) synchronized, they must be driven by the same clock. As you cannot change the clock that drives your soundcard, it's a good bet to sync everything to the soundcard.