Atari 7800:Video Output Notes
All NTSC Atari 7800 consoles feature RF out as their sole audio/video output, as that was the predominant form of video output at the time of its design (1983/1984). Several video modifications exist for the 7800, ranging from simple composite video mods which tap the video output to the RF modulator, to S-video mods which use the separate chroma and luma signals generated by the MARIA and TIA chips.
All standard PAL 7800 consoles also only have RF out, and can be modified in the same way as their NTSC counterparts.
Presumably to cut costs, Atari opted not to sell a separate SECAM 7800 for France, and instead modified a number of PAL 7800s with RGB output. These French 7800s use a 13-pin DIN connector in place of the RF out (borrowed from the ST computers) and have a separate board in place of the RF modulator. This board is based around a Sony V7021 video decoder, which takes a composite video signal and decodes it into RGB out with sync. Because of this, the French 7800 does not have true RGB output and the video quality is roughly comparable to composite out from one of the more advanced video modifications. Despite this, the French 7800 is still sought after due to its superior video quality over standard PAL consoles and the ease with which it can be integrated into SCART-based setups.