From Trammell Hudson's Projects
NTSC video generation
It is possible to generate 125 ns resolution monochrome NTSC (actually RS-170) using the SPI hardware on the AVR. Two resistors act as a voltage divider to create multiple voltages. The font is a 5x7 fixed width font, stored in program memory and clocked out using SPI.
The output from the AVR can overlayed on video by using a LM-1881 video sync separator chip to sync the start of frame with the camera's video.
Source is available: github/osresearch/avr-video