I've seen it with two,
this one by Coidak, and
this one by ADJ. Owing to the Coidak's much inferior low-end curve, it is more pronounced on that
instrument than on the ADJ. If my guesses are correct (and they may all be hogwash, as I am no kind of expert on theatrical lights at all), you wouldn't be likely to see this if you are using a commercial controller. I'm using some open-source software that, from my review of the code, does nothing to synchronize its interpolation loop and its
DMX transmission loop. (Sorry if I seem coy about naming it. It's a pretty good program in many other ways, and I don't want to disparage anyone else's hard work. FWIW, it also shows up in a very simple dimming program I wrote myself, which also does nothing to synchronize those two loops.)
Note that simultaneity of changes on two instruments would not be affected by this phenomenon. Assuming that all interpolated levels are computed for the same moment in time, levels that should match on two different instruments would still be computed to match at that moment. Likewise, both computed levels would be transmitted in the same
DMX frame, with no more
latency between them than would be introduced by the time the
DMX spec imposed between the transmission of those levels to their appropriate channels. To the extent that one
level was transmitted visibly after the moment for which it was computed, so would all other levels in that frame be transmitted after the moments for which they were computed, by the same interval (plus the additional
latency introduced by the
DMX spec, which is not part of this phenomenon). The issue only arises when, due to a lack of synchronization, levels are computed for a given moment in time, are subsequently transmitted at a
later moment in time,
and the interval between computation and transmission is not constant.