The perspectives of wide RF-band digital HDTV emission systems (W-HDTV) via satellite in the 20-GHz range, capable of providing near-studio quality, are examined in the light of the current progress on bit-rate reduction algorithms based on hybrid discrete-cosine transforms (DCT) and of advanced modulation and channel coding techniques.

These W-HDTV systems are evaluated, for different bit-rates (140, 105 and 70 MBit/s), in the context of a possible "common frequency allocation" of the frequency range 21.4-22 GHz; this would allow very efficient use of the spectrum by assigning the overall bandwidth to all service areas. The systems are compared by computer simulations and laboratory evaluations, over a typical satellite channel affected by co-channel and adjacent-channel interference.

An example for a W-HDTV broadcasting satellite service at 22 GHz in climatic zones K and L confirms the suitability of the proposed systems for operation in regions affected by severe propagation conditions.

Following a brief explanation of the principal bit-rate reduction techniques employing discrete cosine transforms (DCT) which are applicable to wideband HDTV, the article describes their implementation in three codecs currently under development in Europe.

The picture quality achieved by these codecs, which reduce the gross bit-rate of a studio-quality HDTV signal by a factor of eight, or more, is typified by that of the EU256 system which was the subject of detailed performance tests early in 1991. These tests confirm the claim that DCT-based codecs are capable of a performance which is virtually transparent to the quality achieved in the studio.

After a presentation of the main principles involved in sub-band coding for HDTV, the authors describe two practical implementations.

The first codec is designed for the digital transmission of HDTV signals in a 140-Mbit/s channel, such as the switched broadband network (VBN) of the Deutsche Bundespost. The pictures have an aspect ratio of 16:9, 1250 lines per frame, 50-Hz field rate and 2:1 interlacing, and after elimination of the blanking intervals a bit-rate reduction factor of 8 has to be achieved.

The second example is being developed for the digital distribution of high-definition progressive (HDP) video signals of 16:9 aspect ratio in a 140-Mbit/s channel such as the future broadband-ISDN conveying network signals in asynchronous transfer mode (ATM). The HDP pictures have 1920x1152x50 active pixels per second (in 50-Hz countries), and a bit-rate reduction factor of 15 is necessary.

One important consideration regarding the suitability of sub-band techniques for HDTV source coding is compatibility between a number of decoding schemes offering a range of picture definitions but all using a common coding strategy.