An Experimental Study on the TAT System for the HDTV Bandwidth Compression

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The TAT (Time-Axis Transform) system for the HDTV bandwidth compression was examined experimentally. Mode assignment algorithm of 4-modes TAT functioned correctly and the bandwidth of the HDTV signal was successfully compressed to 8.1MHz using a pre-processing and TAT.Concerning to the bandwidth compression of chrominance signals, they are time-compressed to 1/4 and then alternately multiplexed in one line with a luminance signal. Thus, the block size of the chrominance signals for the mode assignment is 8 times that of a luminance signal. However, no block distortion is observed in the reconstructed picture.In the pre-processing, the HDTV signal is sampled at a sampling frequency of 64.8MHz and then one half of the pixels are deleted by field-offset subsampling. The remaining pixels are further reduced to one half by TAT. By converting these reduced number of samples to analogue signal at a sampling frequency of 16.2MHz, an analogue signal with a compressed bandwidth of 8.1MHz is obtained. The field-offset subsampling is a good pre-processing and hardly causes visible degradation.TAT with 2:1 subsampling mode (I mode) has nearly the same distortion power as TAT without I mode. The difference is only 0.1-0.3dB. However, the reconstructed picture of the former is subjectively a little better than that of the latter. Interframe interpolation mode (F mode) is effective for still pictures with very fine structures as a whole. Therefore, the 4-modes TAT with I and F modes is suitable for the HDTV compression.In the TAT system developed here, the modes are assigned to minimize the distortion power in the reconstructed picture. The next step of the study will be a fine control of the mode assignment so that it is matched to human vision.