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

对用于HDTV带宽压缩的时间轴变换（TAT）系统进行了实验研究。四模TAT模式分配算法正常工作，通过预处理和TAT将HDTV信号带宽成功压缩到8.1MHz。对于色度信号的带宽压缩，将其时间压缩到1/4，然后与亮度信号在一行中交替复用。因此，用于模式分配的色度信号的块大小是亮度信号的块大小的8倍。然而，在重建图像中没有观察到块畸变。在预处理中，以64.8MHz的采样频率对HDTV信号进行采样，然后通过场偏移子采样去除一半像素。剩余的像素通过TAT进一步减少到一半。将这些减少的采样数以16.2MHz的采样频率转换为模拟信号，得到压缩带宽为8.1MHz的模拟信号。场偏移子采样是一种很好的预处理方法，几乎不会引起明显的退化。采用2:1次采样模式（I模式）的TAT与不采用I模式的TAT具有几乎相同的失真功率。差异仅为0.1-0.3dB。但从主观上看，前者的重构图景要比后者略胜一筹。帧间插值模式（F模式）对于整体结构非常精细的静止图像是有效的。因此，具有I和F模式的4模TAT适合HDTV压缩。在这里开发的TAT系统中，模式的分配是为了最小化重构图像中的畸变功率。研究的下一步将是对模式分配的精细控制，使其与人类视觉相匹配。

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.