The invention is particularly applicable to displays for which pixels are controlled analogically. Unit brightness levels called grey levels, corresponding to a received video signal, are achieved by applying corresponding analogue voltage levels onto pixel columns, defined by a grey scale. To display a given grey level, the active element of a pixel is activated for one row period to transfer the corresponding analogue voltage level onto the pixel capacitance. The liquid crystal is then oriented in a direction that depends on the applied analogue value. Input light bias passing through this liquid crystal is then modified and analysed by a polariser.
The display performance depends particularly on the brightness that depends on the pixel illumination time. This illumination time depends on the addressing time necessary to transfer analogue voltage levels onto each pixel in a row of the matrix, and the liquid crystal stabilization time that depends on the previous analogue voltage level and the current analogue voltage level.
These constraints are accentuated in the case of a display addressed in sequential colour mode. Each frame comprises several coloured sub-frames, with one for each primary colour. Thus, there are usually three coloured sub-frames per frame. Thus, all pixels in the matrix have to be addressed at least three times during one frame period, to display the video information corresponding to each primary colour.
In some liquid crystal displays of the sequential colour type, the colour display frequency is increased to solve the well known colour break-up problem particularly due to the liquid crystal stabilisation time. Thus for example, there are two coloured sub-frames per primary colour in each video frame. The upper limit of the display addressing time is fixed by the duration of the video frame. Doubling the video frequency halves the duration of the video frame, which causes problems. In particular, the pixels of the last rows in the panel may not have enough time to reach their new video set value. If the previous addressing phase was for a red sub-frame, then there may be some data corresponding to the previous red sub-frame at the bottom of the panel in the new sub-frame, for example the green sub-frame.
In these different displays, the response time of the liquid crystal is a very strict constraint. This response time depends on the analogue voltage level memorised during the previous sub-frame and the analogue voltage level to be charged during the new sub-frame. In this case, the rows are scanned very quickly. For example, there are 3 milliseconds per coloured sub-frame to scan all rows. The liquid crystal switching time is of the order of 1 millisecond. Thus, in practice the first row addressed in the sub-frame has from 1 to 2 milliseconds to switch from the video level of the previous sub-frame to the video level of the current sub-frame, while the last row has hardly 1 millisecond. Depending on the previous video level and the current level, the changeover times vary and the time available for the pixels in the last rows in the frame may be insufficient to allow these pixels to converge towards their new target level.
Taking the example of a TN (Twisted Nematic) type liquid crystal display, the changeover time for the liquid crystal to change from one grey level to another is much greater than the changeover time to change from the black level to the white level, or from the white level to a grey level. The black level is the liquid crystal mode in which the potential difference between the pixel and the counter electrode is maximum. The white level is the liquid crystal mode in which the potential difference between the pixel and the counter electrode is minimum. Light greys correspond to an applied potential difference similar to the value applied to obtain white, while dark greys correspond to a potential difference similar to that applied to obtain black. The changeover time to change from the white level to a light grey level may be one and a half times longer than the changeover time to change from the black level to this same light grey level. The changeover time from a light grey level to the black level is very short. In one example with TN type liquid crystals, the following changeover times were measured: 0.2 milliseconds to change from white to black; 1 millisecond to change from black to white; 3.25 milliseconds in the worst case measured between two grey levels.
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