Narrow-bandwidth Television Association

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Chapter 6

Adding sync pulses to the video signal

This section contains details for providing a composite video output from the camera. In stand alone there is no requirement for synchronising as both the camera and monitor share the same disc. In order to produce a video signal to drive an external receiving device we must add synchronising pulses to the video output emanating from the camera head. Two further circuits must be added as described below as well as some degree of modification to the disk and motor drive circuitry. An extra circle of 32 holes will need to be drilled around the disc's circumference in order to generate the required synchronising pulses. A phototransistor and LED or a ready made opto fork will require fitting to sense these holes. The club shop may be able to assist you sourcing the disc.

We recommend a more stable speed control than just a potentiometer in series with the motor. Also the disc speed should be adjusted as close as possible to 12,5 rps. This is important for ensuring the external receiving device 'locks' well over an extended period of time. The late Deryck Aldridge, who was renowned for his skill in circuit design, designed both of the circuits shown below.

The first circuit is the video/sync mixer. Here a negative video signal from the amplifier head feeds the first transistor via a gain (contrast) potentiometer. You might need to interchange the two connections of the dome sensor to get the negative video. The LED driver input now can be connected to the output of this new circuit as it inverts the polarity of the video signal again.

The first resistor and capacitor are used as a filter to attenuate high frequencies emanating from the high gain stages of the head pre-amp. This 'noise' from the solar cell shows up as 'grass' on an oscilloscope screen. The trimming resistor of 2k2 adjusts once more the amplification of the first stage. The amplified and now positive video is DC restored by the diode and zener diode circuit and fed to the first mixing transistor. This arrangement produces the black level and all other voltages in the video waveform to be positive to the 5,6 volt zener voltage. On the connection point of the two resistors of 820ohm, video and sync signals are mixed. Then via a diode clipper the mixed signal arrives on an emitter follower circuit coupling the composite video to the output line.

How has it become composite video? The second mixing transistor is used to inject the sync pulses, which are picked up from the extra circle of holes or slots in the disc. A separate circuit (see below) is used to detect, amplify and square them up before arriving at this point. When a pulse arrives the second mixing transistor is driven hard on. The sync pulses are too deep in proportion to the video signal and to maintain the normal 7 to 3 ratio it is necessary to cut off the bottom of the sync pulse. This is the function of the diode and the potentiometer that sets the actual depth of the sync pulses. An oscilloscope would be handy to make this adjustment. Check that the monitor connected to the output gives the same contrast and black level as when it is connected to the line output of the CD player running the club CD's.

The sync pulse detector circuit uses a so called opto-fork or an LED on one side of the disc and a phototransistor on the other side. This circuit will produce a positive going pulse when a hole or slot is detected. Setting the preset potentiometer should be done in the dark to obtain an optimum setting.

Any general-purpose NPN and PNP transistors may be used in this circuit. Both circuits can be constructed on standard strip board. Should the demand be high producing a printed circuit board could be considered.

 

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