Video display system.

摘要

In order to compensate for image distortion introduced into a digitally controlled raster-scan CRT by the finite video amplifier rise and fall times, the digital video drive waveform is subjected to selective pulse stretching to extend the duration of each pel which is immediately preceded by a pel of lower light intensity value (brightness). This is achieved by means of a comparator (14) which examines each current pel PEL(N) in the video data stream in relation to its immediately preceding neighbour PEL(N-1) in order to detect pel boundary transitions therebetween representing an increase in brightness from the preceding pel to the current pel. Re-timing means (17, 19, 21, 22) is operably under control of the comparator output to cause advancement in time by a predetermined ammount (dt) of any such pel boundary transition relative to remeining pel boundary transitions.

In practive, two copies of the basic or nominal pel clock are produced, phase shifted from each other by a small predetermined amount corresponding to the required time advancement (dt) of detected transitions. The pel clock copies are themselves phase shifted with respect to the nominal pel clock so that the clocking transitions of the two clock copies occur about midway through the nominal pel clock cycle. A multiplexor 21 forming part of the re-timing means (Continuation next page)pel clock copies whenever the comparator indicates that the current pel is brighter than its immediately preceding neighbour but selects a pel clocking transition from the later of the two copies whenever the comparator indicates that the current pel is not brighter than its immediately preceding neighbour. Each clocking transition of the resulting re-timed pel clock (and not a clocking transition of the nominal pel clock) is used to set a latch with the brightness value of the current pel. The changing output level from the latch oc- curing each re-timed pel clock cycle (shifted half a pel cycle with respect to the nominal block) is the required pulse stretched video drive waveform to be supplied to the video amplifier. An extension for multi-level intensity system is also described.