Narrow-bandwidth Television Association

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Chapter 3

Beginners start here

The following three chapters contain the constructional details for two simple projects. The first is a stand-alone monitor which will display a picture from a standard NBTV source, i.e. a club CD, whilst the second project, in Chapter 4, is a combined camera / monitor which will allow you to take and view live standard NBTV signals. Further modifications to the latter will provide an external video output from the unit. Chapter 5 describes a useful lens unit.

Your first NBTV monitor

As with most hobbies the beginner's first question is where do I start? We recommend you build first a disc monitor and use the club CD as a signal source. From then on it's up to you how comfortable you are tackling the project. Remember not to take on too much at a time.

When you have a working system you may wish to move on to a camera or an electronic pattern generator. All the circuits in this book have been selected for their proven track record. Where possible printed circuit boards are available to ease construction. The project may be as 'minimalist' as the one shown here. Peter Smith has constructed a monitor to show how simple it can be.

Disc monitor ... as built by Peter Smith

Finished Peter Smith monitor as shown at the 2001 Convention	Rear view of the monitor. In the centre the cassette recorder motor.

And here you see the typical layout of the different components of a Nipkow disc monitor in top view.



The next chapter in this handbook describes an NBTV camera-monitor. There you will also find suggestions for the mechanical construction. The camera section in the left half of the cabinet could be skipped or reserved for later. You may however wish to spend even more time and produce a more elegant version as shown here.

A replica of a Telehor receiver built by Denis Asseman of Belgium.	A replica of the Baird Televisor built by Peter Smith.

The picture quality of all monitors would be similar. Further enhancements would be to introduce a belt drive motor system to improve the control of the motor for easier locking on to the signal. The original Baird televisors were constructed as a piece of furniture, you may feel you wish to produce some elegant cabinetwork in which to house your monitor.

The most important part of the monitor is the disc; this will determine the picture quality. Pre-cut discs are available from the club; these will give a good picture as they have been produced on a numerically controlled machine to the highest accuracy.

The construction of the monitor is not critical, the cassette DC motor is affixed to the disc in the centre by a push fit plastic coupling available from the club shop, this is then mounted on a frame which allows the disc to rotate freely.

Power supply

Here is a proposal for the power supply. It provides +12 volt and –12 volt and a variable voltage to run the cassette motor on. As pointed out earlier extreme care must be taken when working with mains voltages. If unsure use an 'off the shelf' power supply or seek advice from a more experienced constructor.



Two separate 15-volt windings are fed into diode bridges. The rectified AC is smoothed and passed through 12-volt regulators. The +12 volt supply feeds the LED driver circuitry. The -12 volt can be used for extra circuits.

A motor voltage is generated from the other supply and two potentiometers provide both course and fine speed control. A changeover switch allows the motor direction to be reversed.

The motor speed can be checked with the aid of a stroboscopic disc glued on the centre of the scanning disc. See also Peter Smith' monitor at the beginning of this chapter. For 50 Hz countries the stroboscopic disc should have 8 black and 8 white sectors. When illuminated with TL or neon light the spokes will seem to halt when the disc rotates 12½ revolutions per second. In 60 Hz countries use a 10-spokes stroboscopic disc. Preset the fine speed control in the middle of its range and adjust the coarse control for 12½ rps. Now the speed can be precisely controlled by means of the speed control fine resistor.

A more sophisticated motor drive circuit follows at the end of this chapter.

The LED light source

LED's or Light Emitting Diodes play an important role in the construction of mechanical NBTV monitors. These now replace the now obsolete large neon lamps. Choosing the correct LED's is almost as important as producing a good quality disc. Whilst most LED's on the market will produce a picture of some sort, bright LED's that modulate without a tinge will produce a superior picture. The luminous intensity of an LED is measured in millicandelas (mcd) normally quoted at a typical forward current of 20 mA. Also the opening angle of the beam of light is important. The total amount of emitted light is defined by both. The higher the number of mcd's multiplied by the opening angle of the beam, and the more LED's of course, the brighter the display will be.

The latest Al-In-Ga-P ultra bright LED's are well suited to NBTV operation. They can replicate many small conventional bulbs due to their brightness. These LED's carry warnings as the output light is strong enough to injure eyes when viewed without a diffuser. Although relatively expensive, due to their efficiency a cluster can be made of just six devices.

LED's are current operated devices. Normally they are wired in series with a current limiting resistor. Each LED has a forward voltage drop that depends on its light colour. For red it is about 1½ volt, for yellow 2 volts and for blue and white LED's about 4 volts. Orange high brightness LED's combine a high efficiency with a good visibility and they match more or less the colour of the ancient neon lamps. When placed in series it is important that the driving power supply has a potential high enough to overcome the voltage drop of all the LED's in the chain, else they will not illuminate. The diagram at the left shows a suggested arrangement for yellow or orange ultra-bright LED's, two chains of each three devices. Three yellow or orange LED's in series drop 6 volts, so with a 12 V supply enough voltage remains for operating the driver transistor. These LED's will withstand 50 mA in each chain, so the driver has to provide 100 mA for max brightness.

The latest Ga-N devices give pure white display. For white LED's the voltage drop is higher and the max current per diode is lower, 20 mA. In this case it is better to place the six LED's in parallel, see at the right.

With the absence of a printed circuit board for this part of the project a piece of 40 mm square stripboard will suffice. A spot face cutter is needed to isolate the copper area not required. Holes drilled outside the display area will facilitate mounting.



The LED assembly is mounted behind the disc. A suitably placed diffuser between the LED's and the viewing area close to the spinning disc will allow uniform illumination of the display area. Careful positioning of the LED's and the diffuser is important to ensure the illumination is constant over the display area. Mount a suitable magnifying glass in front of the disc.

A black viewing tunnel prevents surrounding light to reflect on the glass surfaces.

Sync separator and LED driver

A composite video is fed into the video input; this can either from a CD player using one of the clubs CD's or any NBTV source described in this handbook. Please note when using a portable CD player to use the 'line out' signal and not the headphone output as some of the quality is lost in the output audio amplifier. The description of this circuit is as follows. IC1a and the1N4148 diode provide a negative clamp on pin 2 of IC1a. The clamp level is set by the potential divider on pin 3 of IC1a and thus is independent of the characteristic of the 1N4148 diode. The sync slice level can be set by the 220 ohm preset pot to be about 100mV above the bottom of the sync pulse. A clean positive-going sync can be taken from pin 7 of IC1b to the motor drive circuit mentioned later.



Because the sync is clamped, the black level can be offset by the 5K preset so that the picture signal from black to white is used to drive the LED's.

The brilliance control is adjusted to give about one volt peak-to-peak picture signal across the source resistor. This resistor can be changed in value to give the LED current required.

Gamma correction

Although the current/brilliance characteristic of an LED is almost perfectly linear, some correction is provided by two extra source resistors, each in series with a diode. The 1N4001's resistor boosts the current at the top end (0.6 v to 1.0 v) of the signal whilst the Schottky diode's resistor affects the mid-range. You may place fixed resistors with the given value or use trimming resistors and optimise the value yourself. The PCB supports both.

The LED driver circuit will work without these diodes, however they improve the grey scale appearance of the picture. The chapter on gamma correction later in this handbook explains this intentional non-linearity.

Automatic speed control circuit

* values can be changed to suit different motors.

This circuit compares the speed of the disc from a series of 32 holes in the disc with a reference signal from the sync separator and LED driver board. The motor speed is automatically adjusted to match the incoming video. We need to mask off one of the 32 holes on the disc, the hole that passes the opto fork when the spiral of scanning holes jumps to a new frame.

The opto sensor can be made out of an 'opto fork'. Here a combined sensor and transmitter are enclosed in a single module separated by a slot. Sawing the module into two sections allows each of these to be placed either side of the disc. A mounting hole is supplied for ease of mounting. If the sync holes are near the edge of the disc then the fork can be used without sawing it in two.

Motor control circuit PCB ......and...........	.......Copper side of the PCB

Putting the monitor to work

After building up the circuits and interconnections apply 12 Volts DC from the power supply. Apply a suitable NBTV source to the input and the monitor should burst into life. Take particular safety precautions with the spinning disc. Once the video source has been applied the motor should speed up to synchronise with the incoming video signal and some sort of picture should be seen. Optimum picture quality can be achieved by the adjustment of the Gamma potentiometers. Suggested final values are given in the circuit diagram. Attention should be taken with the positioning of the opto sensor and transmitter with relation to the synchronising holes around the disc.

Should the circuit not work, check circuitry and verify with an oscilloscope. Check at the output of IC1b of the LED driver board that there is a 400Hz signal present for a 32-line system (375Hz for a 30-line system). Adjustment of the 220 ohm pot may be necessary. Verify a corresponding signal is being received by the motor drive system from the sync holes in the disc.

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