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High-definition TV may still be far from the norm in many countries, but that isn't stopping engineers at Japan's public broadcaster from pushing ahead with development of Super Hi-Vision, a system they hope will eventually replace HDTV. Last week engineers at NHK's (Nippon Hoso Kyokai) Science and Technical Research Laboratories showed off their most recent work on the technology.

A Super Hi-Vision (SHV) picture is made up of 4,320 horizontal picture lines and 7,680 vertical lines. That's four times the horizontal and vertical resolution of current HDTV or, put another way, a single Super Hi-Vision image is equivalent to 16 tiled HDTV screens.

In the year since it was last demonstrated, the company has developed an image sensor for use in TV cameras that can shoot an entire SHV screen.

Until now the sensors in NHK's prototype SHV camera had half the resolution of an SHV image. Three were used, one each for red, blue and green, and an extra green sensor was added to effectively double the resolution possible and match that of SHV.

Using the new sensor an entire SHV screen can be captured with a single sensor, as NHK demonstrated on Friday. A prototype SHV camera fitted with the sensor was set up about 3 meters away from a scene that included a newspaper. It was possible to read the stories printed on the newspaper on a monitor displaying the image. The same thing would be difficult on today's high-definition systems.

The first sensor is monochrome and NHK says a color image is possible by simply using three in a camera, one for each primary color. Today's TV cameras and some high-end consumer video cameras use three sensors for better image quality.

Shooting an image in Super Hi-Vision is only one of the challenges facing broadcasters before such a system can be introduced.

An uncompressed SHV signal has a bit-rate of 24G bps (bits per second) and that's unmanageable for broadcasting systems. It needs to be compressed. But real-time encoding and decoding of such a high-bandwidth signal is also a challenge.

NHK and Fujitsu Ltd. are working on the problem and have at least solved the real-time part. By linking 16 encoders in parallel an SHV signal can be compressed to around 1/200th of its size using MPEG-4 AVC/H.264 compression. The result is a Super Hi-Vision image of 128M bps, which is still about six times the bandwidth of today's high-definition broadcasting in Japan but within the realms of possibility for future broadcasting systems.

Right now the encoder system stands almost as tall as a person and requires a similarly large unit on the reception end. Engineers will have to shrink this into a handful of chips before its ready for a television set.

NHK isn't saying when Super Hi-Vision services might begin but the broadcaster has a history of being quick off the mark with high-definition.

It began regular broadcasts of HDTV in the mid-nineties using an analog system developed at the same laboratories as Super Hi-Vision. That system, broadcasts of which are scheduled to end this year after the start of digital HDTV in 2000, was first demonstrated to the public in 1969. NHK has achieved a number of firsts in HDTV history, including taking an HDTV camera on the space shuttle in 1998 and live broadcasts from the Antarctic in 2003.