Narrow-bandwidth Television Association

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Chapter 2

NBTV EQUIPMENT

by Chris Lewis

Examples of common NBTV equipment are shown in figures 1 to 4.   This is not an exhaustive list of all methods used but highlights most utilised systems.

Nipkow Disc

Figure 1 shows the most common system in use, and indeed the one used by the BBC experimenters of the 1920's and 30's period and Baird in his commercial televisors. The Nipkow disc named after the German designer Paul Nipkow comprises of a flat circular disc with equi-spaced holes drilled to form a spiral. When rotated, a modulated light source behind the disc would shine through the scanning holes to display a picture when viewed from the disc front. Should the modulated light source be exchanged for a photo sensor device then the system could be used as a camera. One variation of this system was to fit lenses in the scanning holes. Separate holes around the circumference of the disc facilitate synchronising the disc to the incoming video signal. One limitation of pictures developed or received on a Nipkow disc system was the keystone distortion caused by the curved image that was scanned in an arc.This was not a problem if a similar scanning disc was being used at the transmitting end, but the later BBC 30-line transmissions used a mirror drum scanner scanner which generated a rectangular picture. Most viewers using a Nipkow disc for receiving the early broadcasts just accepted this.

Aperture Drum

Figure 2 illustrates the aperture drum mainly used by experimenters, which is a variation of the Nipkow disc. Here a strip of material is folded around a circular disc circumference. Scanning holes are drilled in a diagonal line across the surface of the strip. As with the Nipkow disc this device can be used as a camera or a monitor.

Mirror Drum

Fig 3 shows the mirror drum, which could be used as a camera or flying spot scanner. A number of accurately located angled mirrors are affixed to the circumference of a drum. Each mirror is inclined at an ever-increasing angle to the previous, which allowed scanning to be performed. A modulated beam of light shone onto the rotating mirror surface is reflected onto a viewing surface. Favoured by the BBC in the 1930's the subject was illuminated by a beam of light reflected off these mirrors, a photosensitive device would then detect the light which had been reflected off the subject. Creative makeup was used on the artists to obtain decent pictures. The flying spot scanner technique is still used in broadcasting today to convert film to video for transmission. This is often referred to as telecine.

Mirror Screw

Figure 4 shows the mirror screw arrangement, a series of metal strips, each polished along one of its long edges, mounted in a spiral staircase arrangement. Normally used as a receiving device it could produce a fairly large display. A modulated light source is projected on to the mirror screw. When the rotating shaft is synchronised to the incoming video source the reflection off the mirrors when viewed from the front of the mirror will produce a picture. We do not recommend beginners taking on this project as the positioning of the mirrors and the quality of the polishing is critical to get good results.

Modern Components

Much of the technology today would not have been available to the original pioneers. High-powered LED's replace neons; Sensitive semiconductors replace the original photo-multipliers. Experiments with CRT's, LCD's and even photosensitive welding goggles have been investigated for use with NBTV!

Many people visiting various historic sites around cannot have missed the beauty of a laser light show or have viewed one in a night club. Commercial laser's originally prohibitively expensive are now available to the experimenter as shown at a recent convention by Colin Hopper. Although we strongly recommend that they are used by persons competent in their use for safety reasons, these can be pressed into operation for NBTV use. By varying the intensity and deflection of the laser a picture can be displayed on a wall.

Original 'tube' based television, although the 'final nail in the coffin' for mechanical television, can now be used for NBTV operation. Turning the CRT on its side to correct the scanning direction and by adjusting the line rate to accept the 30 or 32 line signals. Likewise a standard vidicon camera can be converted for NBTV application.

Personal Computer

A relatively new contender is the Personal Computer (PC). Several early attempts were made using the old Spectrum and BBC computers. With the advent of more powerful PCs many successful programs have been written to display, store and transmit NBTV pictures. With windows based basic packages NBTV can be decoded using the latest PCs. Unfortunately processor speed is offset by the constraints of overheads of windows based programs. It's rather ironic that as with the WW2 Colossus computer the latest PCs could not compete with the original equipment. Producing pictures using a PC requires a fairly up to date machine. Using programs from Graham Lewis and Con Wassilieff using Windows Basic packages it is possible to display live pictures on the screen.

Various attempts have been made using the PCs internal sound card to decode the audio NBTV signal. In order to offload some of the decoding tasks of stripping the sync pulses, A to D conversion and signal buffering Graham Lewis has designed an interface card with a few IC's on to plug into the parallel printer port of a PC. It is however important to be able to replicate results on different computers.

 

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