Abstract: In a digital video effect system, only one one-frame RAM (30) produces a mixed and phase-locked signal (14) for n individually synchronized input television signals (11, 12) when read by horizontal and vertical address signals (48, 49) derived from a reference timing signal (15) in synchronism with horizontal and vertical synchronizing pulses included therein. Digital video signals (39') and vertical address signals (29') derived from n or (n-1) input television signal or signals are written in n or (n-1) four-line RAM (80) by horizontal address signals (28') derived also therefrom and read by horizontal address signals (48 or 28) derived either from the timing signal or one of the n input television signals. Thus horizontally phase-locked digital video signals (39, 83) are written in the one-frame RAM by the last-mentioned horizontal address signals and likewise horizontally phase-locked vertical address signals (29, 84).