Pellet ELM pacemaking is one of the recently proposed methods to mitigate the ELM caused heat load on plasma facing components of an ITER class tokamak. The increase of the ELM frequency by frequent injection of shallow penetrating cryogenic pellets seems to be a working technique which was demonstrated on ASDEX Upgrade and JET. To optimize this tool the understanding of the underlying physical processes of the pellet ELM triggering is inevitable. The basic question to be answered is the nature and the location of the seed perturbation- introduced by the ablating pellet - which is indispensable for triggering a type-I ELM. To answer this question the interaction of cryogenic pellets with hot plasma has to be understand in details. To study this interaction (pellet caused perturbations, ELM triggering and pellet cloud dynamics) fast visible imaging by using different fast framing cameras (CCD and CMOS) was recently developed and installed both at ASDEX Upgrade and JET. The pelletplasma interaction was investigated on both machines in different scenarios to exploit the fast camera diagnostics, results are presented in the following.