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The simulation included accurate modeling of all the processes described here. The bit stream was preparsed; channel buffer and picture memory sizes, IDCT processing, and IDCT coefficient selection were all in accord with these explanations; upsampling and downsampling were applied to use the original motion vectors.

The resulting HDTV and downconverted images were examined and compared. Although the downconverted images were of discernibly lower quality than the decoded full-HDTV images, observers agreed that the downconversion process met performance expectations of "SDTV quality."

Conclusions

This paper describes an enhanced SDTV receiver that can decode an HDTV bit stream. The enhancements needed to add HD decoding capability to an SD decoder are modest, even by consumer electronics standards. If all receivers included the capabilities described here, an introduction of SDTV would not preclude later introduction of HDTV because fully capable digital receivers would already be in use. The techniques described in this paper also permit design of low-cost, set-top boxes that would permit reception of the new digital signals for display on NTSC sets. The existence of such boxes at low cost is essential to the eventual termination of NTSC service.

References

Lee, D.H., et al. Goldstar, "HDTV Video Decoder Which Can Be Implemented with Low Complexity," Proceedings of the 1994 IEEE International Conference on Consumer Electronics, TUAM 1.3, pp. 6–7.

Ng, S., Thomson Consumer Electronics, "Lower Resolution HDTV Receivers," U.S. Patent 5,262,854, November 16, 1993.

Notes

1. The "predictions" mentioned here are the P-frames within the GOP sequence. The downsampling and preparsing processes alter the image data somewhat, so that small errors may accumulate if unusually long GOP sequences contain many P-frames. B-frames do not cause this kind of small error accumulation, and so good practice would be to increase the proportion of B-frames in long GOP sequences or to use GOPs of modest length. Receiver processing techniques can also reduce any visible effects, although they are probably unnecessary.

2. In this paragraph, the buffering operation and its associated memory are treated as distinct from the picture-storage memory. This distinction is useful for tutorial purposes, even though the two functions may actually share the same physical, 16-Mbit memory module.

3. Note that the macroblock type, coded block pattern, and run-length information must be decoded.

4. Anchor pictures are I- and P-frames in the MPEG GOP sequence. The downsampling that has been applied to them by the decoding techniques described here means that the motion vectors computed by the encoder can no longer be directly applied.

5. The term "downconverter" as used here applies to hardware that reduces the full HDTV image data to form an SDTV-resolution picture. The appropriately enhanced SDTV decoder described here inherently includes such an HDTV "downconverter."



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