A display system for converting N bit signals each representing 2 gray scale levels, to M bit signals representing 2 gray levels (where N is an integer larger than or equal to 2 and M is an integer satisfying N>M>/=1) comprises a register (1) for separating each of the N bit signals into higher order M bits and N-M bits. 2 tables (5,6,7,8) each store a distinctive set of PxQ modification values satisfying PxQ>/=2 . A control device (12) selects one of the tables as a function of the N-M bits. A comparator (31) detects a difference between a first set of modification values and a second set of modification values of the selected table. An exchanger (32) exchanges the first set of modification values and the second set of modification values to generate a modified table from the selected table. An adder (9) adds the M bits of one N bit signal and each of the modification values of the selected table to generate a first set of PxQ M bit signals, and adds the M bits of next N bit signal and each of the modification values of the modified table to generate a second set of PxQ M bit signals. The first and second sets of M bit signals are then supplied to a display device having 2 gray levels. This permits the addition of pel lines to the picture displayed on the LCD device to bring the aspect ratio of the LCD device into line with that of a CRT device without introducing inconsistent gray levels into the picture.