Narrowband Television

Narrowband television is a new venture for me, this page comprises some of the things that inspired me to have a go, plus a bit about my first experiments!

Narrowband television is a means of transmitting television using bandwidths of less than 20kHz. John Logie Baird’s original television system was based on the scanning disk of Paul Nipkow. Baird achieved a number of firsts in the field of television, in addition to the first practical demonstration of television itself, he made the first transatlantic transmission of television, the first colour television transmission and the first television recordings. Unfortunately he had serious competition, and it was electronic television developed by EMI and others that was to become triumphant.

The Nipkow disk consists of a disk of opaque material, with a spiral of holes near its edge. If the disk is spun, then any image viewed through it will be split up into a series of curved lines. This is the basis of the original Baird television system. The camera consisted of a Nipkow disk, a lens and a photocell. The receiver was very similar except that the photocell was replaced by a special neon lamp.

Junkbox NBTV monitor

This is the first monitor I have built, my main idea was not to buy anything, just to use what I had in my relatively small junkbox. It is very basic, with no synchronisation.

The first thing I needed was a Nipkow disk If you click on the image and save it, you can print it out to whatever size you need, it is in .gif format. I printed mine on 140gsm card, then pierced the holes with a fine sewing needle. Actually the print wasn’t quite as dark as I wanted, so I went over it with a black permanent marker pen.

Next I needed a small electric motor and something to mount the disk on. In my my collection of bits I found an electric motor that had come from a child’s toy. It ran happily between 1 and 3 volts. (In the UK, Maplin do a similar one cat. no.YG13P for £2.79). Mine had a small pulley on it which was ideal for mounting the disk.

Now I needed to control the speed of the motor, I could have used a simple variable resistor, but I didn’t have a suitable one to hand. Instead I made a simple circuit, using a darlington pair,You could use a ready made darlington pair, but I had these transistors to hand. There’s nothing very critical about them, except that the output device needs to handle up to about an amp with a couple of watts or so dissipation. You may need to alter the values of the resistors and pots to suit your motor, all three in series should total about 1k0 to 10k. I have since added a 0.47uFd capacitor across the motor terminals, it seems to improve the image by smoothing the speed of the motor. (You can download a larger version of the circuit by ‘right clicking’ and then ‘save as’.) I have just added this alternative speed controller using a power mosfet

Next I needed two LEDs I simply selected the brightest orange ones I had, which were rated at 4200mcd, they required 2V drive at about 20mA. No problem there, the NBTVA standard is for 1V video, so a single stage would do, I chose a BC108, again because its what I had, it doesn’t really matter, any small signal audio npn transistor should be fine. A judicious application of ohms law, and we get this circuit. A couple of versions of simple drivers I have seen on the NET use a lower value of capacitor from the pot to the base of the transistor, the 10uFd gave a better greyscale, 100nF simply gave the LED on or off (anything from 1 - 10uFd would do. One of the problems I had was diffusing the light from the LEDs, the solution was a thin piece (2mm) of plastic foam. Its worth experimenting with this, as with the rest of the project

The almost finished project is shown on the left. To get an impression of the size, the disk is the same size as a CD, and the image is about 1cm high! Still it works ok with testcard images played on my PC CD-rom drive and with the NBTVA triple waveform generator. The only real problems are that the motor is not synchronised, so that you need to keep adjusting the motor speed control to keep the image stable, and the accuracy of the holes in the disk. Both of these are problems that were encountered by John Logie baird, so I don’t feel at all disappointed.

The completed version is on the right, you can just see the viewing window on the right of the case. The case is made from black card from the local art shop, it was the simplest way of cutting stray light. The lens was from an old magnifier, and is about 2.5cm in diameter with a fairly short focal length. The image is viewable in normal light

This is a very brief clip of a checkerboard testcard viewed on this monitor. It actually looks a bit better in real life, but I was using a webcam, focused on the disk. The monitor lens was removed for this. The testcard was drawn in MSpaint, saved as a .gif, then converted to 32-line video using Gary Millard’s Video2NBTV. The resulting .wav file was burnt to a CD. This was played through a PC with a Soundblaster 128 giving the video out, which was then fed to the monitor.. The webcam video was saved as a wmv file, and then converted to Flash using Internet Video Converter. A whole lot of processing! It’s surprising that there’s anything left of the video content.

A few more projects are under way, I have just completed a waveform generator, and am currently working on a monitor using the 12” Nipkow disk which can be bought by members of the NBTVA. The other projects underway are a mechanical camera and a circuit to enable images to be viewed on an oscilloscope.

If you want to find out more about narrow bandwidth television then visit the NBTVA website.