It would be cool if the hardware did some Fourier analysis to resample the buffers it gets (so it could make them run for longer if the buffers run dry), but it probably would cause some kind of latency issue. I reckon that just avoiding buffer underruns is less overhead.

Quoting from the Waldorf microwave XT synthesizer FAQ:

The brightness of the click depends on the speed of the level change. The faster the level changes, the brighter is the click. So, the level change speed can be compared with the cutoff of a lowpass filter. There is an easy formula for it:

Let's consider a level change from full to zero (or from zero to full) output from one sample to another on a machine that uses 44.1kHz sample rate. So, we first transfer the sample to milli seconds:

1 sample equals 1/44100 second, which is = 0.02267573696ms.

To calculate the cutoff frequency of the click, just use this formula:

Cutoff (Hz) = 1000 / Level Change Time (ms)

which in the example results in:

44100Hz = 1000 / 0.02267573696ms

Whoops? This the sampling frequency and, err, very bright.

http://faq.waldorfian.info/faq-browse.php?product=xt#116

With zero crossing detection, it would need to know the buffer is running out before the next zero crossing occurs. So if the shutdown signal is given within the last millisecond before the zero crossing, it would ignore all data that comes after the zero, and output zero for the remaining partial wave. On starting up again, it would wait until the signal is at zero before beginning to output. At least that's how the common audio codecs I've seen that implement that feature work. The most common application is volume control, where you don't want a sudden change in the amplitude of the wave to result in a glitch, so you adjust amplitude at zero crossings.