I knew that a TV cartridge for the gamegear existed. I though, maybe the all the cartridge does is apply the video signal from the tuner to the right contact on the cartridge conector?
I found a file (ggpins.txt) which identified the contact #43 as 'TV cartridge signal'. Unfortunately, when I applied a composite video signal to this contact, it did not work.
Since it was not so simple, I bought a TV cartridge on ebay. After looking inside the cartridge, I think it approximately works like this:
- The cartridge pulls contact #43 to ground, taking control of the LCD.
- The tuner sends the analogic video signal to a custom chip which displays video on the LCD.
Better buy a second hand TV cartridge or forget about using a Gamegear as display since the image on a Gamegear is not very good:
Update (2011-09-08): Here's a page with more information about the TV cartridge: