Natural Motion — Television images

Television pictures are composed of dots or 'pixels' (picture elements) which are too small to be seen individually by the human eye, at a normal viewing distance.

The pixels are not transmitted simultaneously, but in succession. This occurs via what are called 'scanning lines'. The number of lines is chosen so that the human eye cannot perceive them individually. The European PAL (Phase Alternating Line) transmission system utilizes 625 lines. With a picture width-to-height ratio of 4:3, there are approximately 800 pixels per line.

At least 25 pictures or ‘stills’ per second are necessary in order to reproduce natural movement realistically. This frequency is much too low for very bright pictures. In order to reduce large-area flicker, the still is shown not just once, but twice. The electron beam which makes the fluorescent layer of the picture tube glow, firstly scans 312.5 (‘odd’) lines. It then shoots back to the top of the picture before scanning 312.5 (‘even’) lines, which are alternately sandwiched or interlaced between the lines already scanned. The two matching half-pictures or ‘fields’ complement each other to complete the picture.

Large-area flicker is thus reduced by interlacing even and odd lines. Movements appear more natural with this method. It is almost as if 50 instead of 25 stills per second are being shown. The scanning frequency of 'normal' television is therefore 50 hertz, conforming with the 50 fields per second.

With black and white television, only one signal is involved, namely the luminance signal. With color television, there are also color 'hue' and color 'saturation' signals to contend with. Combining this varied information into one signal at the transmitter, and unraveling it at the television receiver, is a complicated process for which different countries have different systems.

America uses the NTSC (National Television System Committee) system, named after the body which laid down the standard. NTSC is based on 525 lines and a frequency of 60 Hz. The picture is rather crude and is subject to color variations, which earned the system the nickname 'Never The Same Color'. Televisions that work to the NTSC standard always have a color adjustment facility.

PAL, which is the system used widely in Europe, is not or is hardly ever affected by color variations. Color discrepancies, which can occur during signal transmission, are electronically corrected. An even more advanced version of PAL, is PAL-plus. This gives sharper pictures with fewer color discrepancies. PAL-plus also enables television pictures to be received in 16:9 wide-screen format.

A standard television shows 50 fields per second. This still makes pictures flicker, especially when you look at a large screen out of your eye corner, or view it under artificial lighting. A much improved picture is achieved with the 100 Hz technique, which was developed in the 80's by Philips Research. The picture on a 100 Hz set is free of large-area flicker due to a doubling of the picture frequency, using digital techniques.

Philips Research developed a special memory chip which reads transmitted information alternately 'in' and 'out'. The signal from the first one-fiftieth of a second (the even lines) is stored in the memory and appears on the screen twice in succession during a period of one-hundredth of a second. The same occurs with the odd lines from the next one-fiftieth of a second. With 100 Hz TV, not 50 but 100 fields per second appear on the screen. The picture is therefore much more acceptable to the eye.

The picture memory and digital techniques also make it possible to 'freeze' images, to enlarge them, or to show them as a 'picture-in-picture' (PIP). This makes it possible to view other programs simultaneously. Pictures can also be viewed from external sources, such as an optical-disc player, satellite tuner or CCTV (closed-circuit TV) camera.

At the end of the 80's, 100 Hz TV was the state-of-the-art. But technical experts at Philips Research were not satisfied. The 100 Hz TV still had the line flicker which affected the 50 Hz TV. This happens when a light horizontal line constantly jumps between the odd and even lines. This disturbance occurs once every twenty-fifth of a second and therefore has a frequency of 25 Hz. Particularly horizontal lines from the patterns on sports-fields or the textile patterns in television presenters' suits, are potential sources of irritation for the viewer.

Thanks to a new invention from Philips at the beginning of the 90's, line flicker is also a thing of the past. Using digital techniques, it is now possible to switch from even to odd lines. Instead of the usual doubling of stills, as represented by AABB (two even lines followed by two odd lines), an ABAB sequence can be applied, which solves the line flicker problem. Unfortunately, this gives rise to the problem of judder in moving images. For this reason, a technique called 'Digital Scan' was developed, in which the information from the previous period is compared with information from the next. In the event of a static image, the ABAB sequence is applied and the flicker stops. In the event of a moving image, the classical AABB sequence is applied. A fast processor selects the right mode within one-billionth of a second. The only thing that a viewer realizes, is that the picture is ideal.

An ideal picture, however, is not obtained when viewing motion pictures created for the cinema, on TV. They are subject to judder, owing to the fact that each still is transmitted twice. Instead of a picture sequence A1A1B1B1, A2A2B2B2, etc., the situation calls for a sequence A1A1*B1B1*, where by A1* is a picture between A1 and B1, and B1* is a picture between B1 and A2. If the new stills were to be inserted precisely as mentioned, you would obtain motion which looks completely natural. Natural motion is now a fact. Thanks to the development of a motion estimator and a special chip, Philips succeeded last year in producing a system for 'Natural Motion'. The best imaginable picture is obtained when 100 Hz TVs are fitted with this motion estimator. They are completely flicker-free and judder-free when it comes to moving images. Pictures from these TVs are always razor sharp.