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Feb 09, 2013, 01:36 AM
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Video overlay test

After all that speculation & rework, the video overlay was implemented. The only way to get black pixels was shorting the composite signal directly to a GPIO. The input protection diode didn't raise the hsync pulses as much as feared. Getting the whitest pixels required shorting directly to a PNP transistor fed by 3.3V. Both the HSYNC & VSYNC output of the LM1881 were required to get the best sync.

Using the PIC's RC oscillator wasn't good enough to show any text, but good enough for showing a code. The PIC at 32Mhz was fast enough to draw at the limit of the video resolution. Ordinary telemetry could have been sent this way years ago, when the 1st autopilot was being developed with nothing but a 72Mhz radio & the $80 for a set of XBees was unbelievable.

To minimize size, the PNP transistor was eliminated & the overlay was done by toggling a single pin, shorted to the composite signal. The whites weren't whitest, but good enough. The PIC probably can't drive enough current to damage the DSLR.

The circuit added some noise to the video. While you could probably get the sync pulses with discrete parts, the LM1881 is the most compact way.
Last edited by Jack Crossfire; Feb 09, 2013 at 07:30 AM.
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