Circuit Notebook 87 - A Flashing Cursor for ATV

When on the air with ATV it is often useful to point to a piece of equipment, or show a circuit or PC board. This sometimes involves reaching over the camera and pointing with an out-of-focus finger to the item of interest, or when sitting in front of the camera holding up a PC board, trying to point to a component which appears upside down and reversed left to right.

This is where the ATV cursor comes in useful, you point the camera at the scene and use the cursor to highlight the item of interest. The position of the cursor is controlled by a 'joystick' and can be switched on continuously, off, or flashing. See Figs. 1a. & 1b.

The circuit is shown in Fig. 2. It consists of the usual LM 1881 sync separator to provide line and field pulses. Two mono-stable circuits determine the position of the cursor on the screen, in the vertical direction by IC2a, together with the vertical position control of the joystick and in the horizontal direction by IC3a with the horizontal position of the joystick.

The end of the (field) timing period of IC2a triggers IC2b. IC2b generates a pulse of about 1.2ms, equal to about 20 lines. The end of the (line) timing period of IC3a triggers IC3b, but only during the 1.2ms pulse from IC2B, the remainder of the time IC3b is held reset. As a result, IC3b generates a pulse of about 2.5us on every line during the 1.2ms period. This appears as the block of highlight which forms the cursor. The output of IC3b is taken to IC4, pin2. The signal passes through this gate to the second gate which acts as an inverter.

Two (upper) gates of IC4 form a 3Hz square-wave generator and with S1, the 'centre off' toggle switch, the cursor may be switched on continuously, off, or flashing at 3Hz.. With the values shown, the circuit produces an approximately square shaped cursor, but it may be necessary to experiment with component values associated with IC2b and IC3b to produce the size of cursor you prefer.

The video signal passes directly in and out of the cursor unit and the cursor signal is resistively added to the through video by the 680R resistor and diode connected to the output of IC4. The amplitude of the cursor is about 0.2V. An advantage of adding the cursor signal in this way, rather than punching a hole in the picture, is that you can still see the detail of the picture through the cursor.

The joystick I used was a 'skeleton joystick' made by Altai, Cat. No. JS110, having a resistance of 100k ohms in each axis, see Fig. 3. This was purchased at a Rally and may no longer be available. A similar one is in the RS Components catalogue as a 'Mini Stick Controller' 252 series, 100k, No. 456-8235 at £4.50 + VAT, or you could make your own, see Fig. 4.

I made the version shown in Fig. 4. almost 40 years ago for a mock 'Thunderbird' control panel for my (then young) son (I think it controlled the thrusters). It used normal potentiometers and but as you can see their movements are restricted to about one fifth of the normal rotation. For the present application it would be necessary to use 500k ohm linear potentiometers to provide 0 to about 100k ohms variation.

References

National Semiconductor Corp. Linear Databook 3 LM1881

Mullard Data Book High Speed CMOS logic 74HC00, 74HC4538

Figures

Fig.1a. Off screen shots of Cursor

Fig.1b. Off screen shots of Cursor

Fig.2. Circuit Diagram

Fig.3. Altai skeleton joystick

Fig.4. GW3JGA 'Thunderbird' joystick