Video boosters VS resistors and capacitors

[samlieu]

Hi,

I'm in the process of part-consolising my CPS2 and am working on a video circuit that will go from JAMMA edge to chassis mount BNC connectors for RGBS.

It's well known that the CPS2 output TTL level RGBS (as does Super Nintendo and Sega Megadrive.) In the console schematics for the consoles we can lower the TTL levels to 75ohm by adding resistors and capacitors.

In Voultars video https://www.youtube.com/watch?v=khl6x0S_bV0&t=386s he explains why we shouldn't add a video buffer THS 7314 as this effectively doubles the voltage.

My question is, if CPS2 is outputting TTL level RGBS, why are modern Superguns (including the Minigun) built with THS7374, instead of adding say, 75ohm resistor and 220uf capacitors?

 

[nem]

(as does Super Nintendo and Sega Megadrive.)

Both of those can output TTL level sync. However, the video signals are still 0.7VPP. Arcade boards output 3-5V.

I won't touch your video buffer question, because I have no clue. @Frank_fjs, @RGB or @invzim can answer that.

 

[samlieu]

Can we reduce the arcade voltage down using resistors instead of through a buffer?

In layman terms, doesn't buffering by default, introduce latency?

[EDIT] I can see the schematics of the minigun does have 75ohm resistor and 330uf capacitor on the RGB out. Can we do away with the THS7374 altogether?

 

[Frank_fjs]

The video is attenuated on input to the THS so it's not simply doubling the video levels. On output it's 75 Ohm terminated and halved.

The reason for the THS is to buffer the signal, place the arcade board under the correct load and impedance match the signal to 75 Ohm on output.

You don't need to use a video buffer but you forfeit the benefits mentioned above.

 

[invzim]

My question is, if CPS2 is outputting TTL level RGBS, why are modern Superguns (including the Minigun) built with THS7374, instead of adding say, 75ohm resistor and 220uf capacitors?

To understand why, you need an intuitive grasp of what impedance is. I'm sure you can find a lot of youtube stuff about it - I think of it as signal strength. The lower the impedance, the the higher the strength. A PSU outputs 5 volts, but with incredibly low impedance - i.e. if you use a 75ohm resistor from the output from the PSU to GND, you will still measure about 5 volts. The higher the impedance the less it takes to mess with it. For Jamma boards, the impdance is not standardized, but is supposed to be 1000 Ohm. Without a video amplifier (buffer), there is no way to get it down to 75 ohms.