FREQUENCY-DOMAIN DOWN-CONVERSION OF HDTV USING AN OPTIMAL MOTION COMPENSATION SCHEME

In the MPEG-2 Test Model 5, the down-conversion of an interlaced seque nce is obtained by prefiltering and subsampling each field of the imag e sequence after it has been fully decoded. Although the quality is ve ry good, the cost of such a system is quite high owing to large memory requirements. As a result, low-resolution decoders have been proposed to reduce some of the costs incurred by this scheme. Here, incoming D iscrete Cosine Transform (DCT) blocks are subject to a down-conversion process within the decoding loop; hence, the motion compensation is p erformed using the down-converted images. In past work, it has been pr oven that the optimal filters for performing this motion compensation are intimately related to the method of down-conversion. Therefore, th e choice of down-conversion filter is viewed as the primary variable a ffecting the quality of the down-converted sequence when such an optim al motion compensation scheme is considered. In the conventional metho d of frequency domain down-conversion, the 4 x 4 low-frequency coeffic ients are extracted from each 8 x 8 block. Two problems arise from thi s method: First, the discarding of high frequency data will introduce a disturbing amount of drift; and second, severe blocking artifacts wi ll result in areas of large motion. To remedy the drift problem, a new method of down-conversion which better preserves high-frequency data is presented. This method is referred to as frame-based frequency synt hesis. Then, to overcome the blocking artifacts, the previous method i s extended to a field-based frequency synthesis method. Our simulation results clearly indicate that the amount of drift can be significantl y reduced by retaining high-frequency data and that severe blocking ar tifacts are eliminated by using the field-based method. In addition, t he quality achieved by the proposed field-based frequency synthesis is much closer to the high-quality results produced after full-resolutio n decoding. (C) 1998 John Wiley & Sons, Inc. Int J Imaging Syst Techno l, 9, 274-282, 1998.