Electronic – Clamper for easy DC-restore in video-composite

The fundamental question here is how fast you need to update the sign's image.

If you don't need to update it more often than once every few seconds, DVI is WAAAAAY overkill. (And placing something that shows video next to a highway seems like a very bad idea)

You're probably best off using a frame-buffer in your hardware, and using something like a FT232 or FT245 to dump images to it. The FT2** IC Gives you ~8 MBps of interface bandwith over USB, and a dead-easy software driver you can talk to.

Alternatively, depending on your sign's resolution, you could use a simple serial interface.

I would assume you would be sending raw bitmaps to the sign (it makes the software end easier). Then you just cache them in the hardware, and then serve them up to whatever interface the sign implements from your hardware.

You could use a depletion-mode FET like CPC5602 or DN3135. But the obvious way to connect it would require the use of a negative gate voltage to open the circuit. The negative voltage doesn't have to supply much current however, so you can probably arrange a simple charge pump with a square wave and a couple diodes and caps. When the square wave stops, the transistor will turn on.