Modified Discrete Cosine Transform for Addressing Liquid Crystal Displays

An addressing technique that is based on modified discrete cosine transform (M-DCT) is proposed to drive RMS responding matrix displays. Time taken to switch from one arbitrary gray shade to another is almost independent of initial and final gray shades. 1. Objective and Background Discrete Cosine Transform (DCT) is capable of achieving a high energy compaction ratio and it is extensively used in image compression. We have explored the possibility of using DCT to drive RMS responding matrix displays. There are several addressing techniques for displaying gray shades in passive matrix liquid crystal displays (LCDs). Amplitude modulation [1], pulse height modulation [2], successive approximation techniques [3,4] and wavelets based techniques [5-7] are some of the techniques that can be used to display gray shades. Grayscale to grayscale response times i.e. the time taken to switch from one arbitrary gray shade to another depends on the initial and final gray shades in these techniques. It is preferable to have uniform switching times between gray scales to preserve color purity of images and avoid motion artifacts. Almost uniform response times are achieved when pixels are switched from one arbitrary gray shade to another if the display is scanned with DCT based addressing technique that is proposed in this paper. 2. Discrete Cosine Transform Discrete cosine transform is a subset of discrete Fourier transform. A function is expressed as a sum of cosine functions of several frequencies in discrete cosine transform (DCT) as compared to expressing it with sine and cosine functions in discrete Fourier transform (DFT). A DCT of a signal ) (n s is obtained as follows: