It makes no difference whether you use the iPad/iPhone headphone jack or an audio interface that by-passes the iOS DAC.
This happens around every 40 seconds or so - depending on the settings chosen. So it seems as you play you run out of -loaded samples and when it refreshes a crackle sound is heard.
It is the same sound you can hear when you go into options and choose a different sample rate/audio buffer and samples are forced to reload. Periodically there is a crackle that I think is probably the reloading of samples. I was wondering if anybody else who has this piano is having the same problems. This piano has a very nice sound but still has a few glitches that need fixing.
