The HD-MAC coding system achieves bandwidth reduction by a factor of about 4 through a motion adaptive spatio-temporal subsampling using three branches. Thus, the mode decision procedure is introduced in the first stage of encoder, and affects the visual quality highly. In this thesis, considering the coding strategy, an efficient mode desision technique is investigated. It is found that the conventional mode decision method which utilizes the a priori and the a posteriori decision modules provides poor performance in spite of its complex composition. The proposed methods uses only the a posteriori decision module with some preprocessings of compared signals. Experimental result shows that the proposed method provides more exact mode decision than the conventional one, and thereby improves visual quality.
