Ink jet printers and methods

A method of operating a printing apparatus for printing a desired pattern on a substrate, comprising: discharging a continuous stream of drops (5) of liquid ink from a nozzle (2; 21) along the axis (3) of nozzle towards the substrate (4); and loading (6) selectively said liquid ink drops with loads Multilevel to deflect selectively different amounts with respect to the nozzle axis to thereby direct some of the liquid ink drops to different locations (9a-9n) on the substrate for printing said desired pattern thereon, while other liquid ink drops which are not printed are intercepted by a gutter (8) before reaching the substrate (4), wherein said method comprises dividing the stream of ink drops discharged from the nozzle (2; 21) into two streams by charging pulses of two charging levels and of appropriate phases; detecting (10, 11; 40, 50) optically the two streams of ink drops for determining velocity errors, and / or setting errors load phase between pulses of respective loading and formation timing physical drop in the stream exiting the nozzle (2; 21); and controlling (25) the charging pulses and / or the formation timing to correct drop such errors.