Modified BTC algorithm for satellite image coding

BTC (block truncation coding) codes and their variations are lost-block codes often used in narrowband systems with a lower sensitivity for transported picture impairment. Our preferred modification of these codes goes to better compression with a very low picture impairment against original BTC methods. The modification uses the convenience of low-level changes of successive pictures, which are inserted into the "difference byte". In this way the whole level need not be transported. In the case of big differences between levels, this code uses nonlinear quantisation for the level differences and it brings approximate capacity decrease of 30%.