Video Overlay with ATmega8 Microcontroller

The project was designed to create a video overlay board that will allow the display of GPS data and call-sign on an NTSC video signal.

The device locks onto the sync of an external NTSC signal from a video camera where the video line is then pulled high at the correct times to render text on the screen. A 4800 baud NMEA-formatted data stream input is also taken by the device from a GPS receiver to update the GPS fields on screen.

In the design, a clamp and biasing circuit are used by the overlay board to raise the level of the NTSC sync pulses to just above 0V. One of the inputs of the on-board analog comparator of ATmega8 is fed by this signal while the other input of the comparator is connected to a voltage reference established by diode and resistor divider. The positive output will be given by the comparator upon the arrival of each sync pulse which causes an interrupt to fire.

One output line to print white text and another line to dim the background behind the text for ensuring readability are the two parts of output circuit.

Rest of the project