A method for converting between relatively dose field rates of perhaps 60 Hz and 59.94 Hz, monitors the phase difference between input and output field rates. Initially, output fields are created by synchronising input fields. As the measured phase passes a threshold, the method switches to interpolation mode for a short, fixed interval and then returns to synchronization. The method offers much of the sharpness associated with simple synchronization but avoids the visible discontinuities of field drops or repeats.