HDTV Poses a High-Tech Challenge
A television picture might look like a single, continuous image, but that’s only because the human eye is too slow to see what it really is: a series of rapidly flashing lines of light.
A high-definition television picture, which has a crispness and clarity approaching that of big-screen cinema, is simply a denser set of more rapidly flashing lines of light.
Today’s television format, known as NTSC, provides a picture made up of 525 horizontal lines. Most HDTV formats would double the number of lines, increase the frequency with which the picture is updated and change the ratio of width to height to give TV a wide-screen look.
What this means in practice is finding a way to accommodate far more pictorial and sound information into each piece of the televi sion system. At the source, cameras and videotape recorders must register more information. Then the transmission system--be it cable, broadcast or satellite--must send more information. And the television set itself must be able to process and display more information.
Unfortunately, the best solution for one component of the system is not necessarily the best solution for others, so trade-offs are necessary. The optimal means of displaying a picture, for example, is a technique called progressive scan, in which all lines on the screen are flashed simultaneously every one-sixtieth of a second.
But progressive scan is very hard to record, and video cameras that use progressive scan are less sensitive and require more light than those using the traditional technique known as interlace scan, in which the picture is flashed in two separate fields, each of which contains every other screen line.
By the same token, the Federal Communications Commission has decreed that any HDTV broadcast system must use radio frequencies that are already assigned for television, and must not disrupt existing broadcasts. These constraints mean the HDTV broadcast formally will have limitations that would not exist on a system designed for roomier cable or satellite channels.
These difficulties explain in part why there has been so much argument and confusion over HDTV standards. And to make some sense out of the debate, it’s useful to think of it in terms of two separate standards, one for production of programs and one for the transmission of those programs.
Today, most prime-time television programs and virtually all films are produced with 35-millimeter film, which is itself a high-definition medium and can easily be converted to high-definition video. In addition, Sony has for several years been selling HDTV video equipment that uses the 1,125-line interlace format that was developed as the standard for HDTV in Japan.
Although early proponents of HDTV had promoted it as a production medium that could eventually replace film, most in Hollywood believe that the picture quality still doesn’t have the subtlety and feel of regular film.
The Society of Motion Picture and Television Engineers has endorsed the Sony system as a standard for HDTV production, but many have opposed adopting it until the broadcast standard is established. Sony images created on its production system will be easily converted to any format eventually adopted for broadcast. But others say this is simply untrue, and that the production and broadcast formats should be designed to work together.
The transmission segment--getting the image into the home--is perhaps the most difficult part of HDTV. In Japan and Europe, HDTV is sent over satellite links directly to home receiving dishes. But in the United States, television broadcasters and federal regulators have been eager to have an HDTV system that uses traditional over-the-air broadcasting channels, and that is much more difficult.
Some companies, including General Instrument Corp., Zenith Electronics and American Telephone & Telegraph, believe the answer to HDTV broadcasting lies in digital transmission. This involves converting the HDTV image to the digital language of computers--a series of ones and zeros--and then compressing it through a complex process of mathematical sampling that is carried out by a cluster of computer chips.
The television signal itself thus sends not all the information contained in the image, but rather a series of numerical codes that describe the image. A decoder in the television set then reads the signals and reproduces a full image for display on the screen.
Displaying the HDTV picture in the home poses certain problems. Initially, HDTV sets--like existing TV sets--will use cathode ray tubes, which create the lines of light by firing beams of electrons onto the back of the glass screen.
But HDTV pictures have to be large to be appreciated, and large CRTs are extremely heavy and bulky. And projection TV systems, similar to those available today, do not have the contrast needed for optimal HDTV pictures.
Thus, many believe, HDTV will not really take off until flat screens like those now used on portable computers can be made large enough for big-screen television. And most believe that is at least a decade away.