natshaw wrote:1. How to remove it? If I just use an inline cap, how do I determine which original voltage level will become the output's 0V?
Exactly right, a capacitor in series. The output will be at a nominal 0V - if you think of a signal swinging between 2 to 6 volts, this will become a signal of 2VAC. When it crosses through 4V on the input it will be 0V on the output. There's no set original voltage that will become 0V on the output; 0V will be any point where the signal is at a constant DC voltage at the input.
The issue will be determining the correct value C - too low and you'll attenuate low frequencies, too high and you may get some leakage issues (because that's starting to imply electrolytics!).
I'm more used to dealing with audio than video, so I'd need to do some research on the calculation for the best value of C.
natshaw wrote:2. How to add a - or + DC offset if I need one?
Feed the signal through a cap (to block DC getting back to the source) then a potential divider between rails.
natshaw wrote:- Remove DC bias
- Amplify (or attenuate) signal to 1Vpp
- Add DC bias
There's no point adding DC bias unless you've got some other circuitry after things that needs it. If it's heading out of your circuit then next the next piece of equipment add bias if it needs it.
You certainly need some kind of buffering - this can provide the amplification or attenuation, but also protects the TMS9929 from the outside world. Let a $2 IC become the sacrificial element if anything goes wrong, not whatever's buried in the guts or your Sega.
If you haven't got a split rail (say +/- 12V) then things become more difficult, I see you're reading up on operating op-amps from a single-ended supply, the other option is to use an op-amp already designed for this - e.g. the LM324 but it may not be fast enough for video.
natshaw wrote:I know some TVs will have AC coupling caps on their inputs (mine seems to), but I do want to make this work to the standard, as opposed to just on my TV
Most gear will have coupling caps, it's just safest to assume that they don't. You don't want DC floating around where it isn't needed!