A raster-scanned CRT device includes auxilliary vertical deflection means (11) operable during each scan line of the raster to deflect the electron beam (9) selectively between adjacent pairs of uniformly spaced image lines (14) on the screen (2). Control logic (12) operates in response to video information representing an image to be displayed to write image pels (16) on the screen during two successive field scans of the raster (10). The auxilliary deflection means (11) is controlled so that the entire image is formed on the image lines (14), and the field scan in which te pels (16) are allocated for display is selcted on-the-fly for successive portions of the input video in such a way that pel imbalance between the two fields constituting the final image is minimised.