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I have fabricated a circuit card with an ATMEGA328 micro-controller and an LM1881 Sync Separator with a hardware design similar to a combination of an Arduino Uno and Video Experimenter. This design has worked great (thanks to Nootropic for an excellent design) for a variety of video sources. Recently I purchased a low cost camera sensor ($15), and I experienced an identical problem to that described in this post. The video overlay was barely discernible when overlaid on either a bright image or dark image. Looking at the video output from the camera sensor on the scope, I observed a DC bias. The vertical/horizontal sync pulses were at +.25 volts and bright images were running between +1.2 and +1.5 volts DC. These video amplitudes swamped out the overlay pulses that have an amplitude of +.8 volts DC. The solution is to reduce the value of the series 330 ohm resistor connected to pin 13 of the ATMEGA328 (Pin D7 of the Arduino Uno) through a diode. For a supply voltage of +5 volts, a series resistance of 150 ohms will increase the amplitude of the overlay pulses to approximately 1.5 volts (based on the monitor having an input resistance of 75 ohms and a diode in series with the resistor). This value of resistance results in the processor sourcing 20 mA which is well within its rating of 40 mA max.
I purchased a Video Experimenter CCA to overlay simple alphanumerics on a camera output. The software for my project is similar to the overlay demo project software. The alphanumeric display worked great, but I had a problem with a line of extraneous stuff on the bottom of the display frame. After hours of trying to debug the software, I found the root cause to be the Arduino Uno serial interface that I was using as a debug tool. Apparently the serial interface was interfering with the LM1881 chip. After I commented out the serial interface code, the garbage on the display disappeared.
