An image decoder comprises first clock regeneration means (20) for deriving a first clock signal (21) from a time stamp signal (19) extracted from a digital transmission stream (5); decoder means (8) for decoding a digital video stream (7) to provide a first digital video signal (9) at a first frame frequency, the first digital video signal being synchronized with the first clock signal (21); graphic generator means (10); superposition means (12) for superposing the first digital video signal (9) and a graphic signal (11) on each other to provide a first synthesized digital video signal (15) synchronized with the first clock signal (21); and D/A converter means (16) for converting the first synthesized digital video signal (15) to an analog signal (17), wherein there are no missing frames or distortion occurring in the displayed image even though the digital video and graphic signals are superposed.