RGB resistors for a supergun setup?

[Drewman21]

I'm putting together what I hope will be a Supergun/RGB home-system box for my TV.  After looking at all the diagrams that i think i need I've  wondered if it is necessary for a resistor to be used with the RGB output of most home systems.  I have here from posts and guides that most of the time ( or if not all) that a 75ohm resistor should be used with a RGB monitors that people are using.  Well I have been looking at and bought a Jrok encoder card for my supergun and I am looking to put on 1k potentiometers to control the RGB input into the encoder.  Would this be the same type of thing?

I'm intrested in trying to get my home systems to a rgb setup and if the 75ohm resistor will get it closer where i need to be for stable use with this card then great that I won't have to tune the potentiometers in to much.  

Any ideas on this?

Drewman21

[Guest]

The 75ohm resistor is to set the output impedance (for impedance matching) it's quite necessary and it has nothing to do with the RGB brightness.

[Drewman21]

Ok, that makes sense then.  So if i do that I should in theory then be able to use this encoder card with most if not all home systems instead of a arcade board then?  Any one have any luck or any other insights with this type of setup?
The Jrok.

Drewman21

[viletim!]

Drewman21,
You shouldn't have any problems connecting your PCBs directly to the Jrok encoder because it seems that's exactly what it's designed for -- the video input on this card is not the same as you'll find on a standard tv/monitor. There are some trimpots already on the PCB for adjusting the video input level. I suppose if you were going to connecting lots of different boards to the thing, and you want  to be able to adjust the contast easily then you could remove the trimpots and replace them with normal pots....


Guest,
That's not right at all. The output impedance of an arcade board is undefined and usualy lies somewhere between a few hundred ohms to a few kiloohms (arcade monitors have a highish imput imp. too ~5k typ.) . When connecting an arcade board to a standard 75 ohm rgb input you can forget about impedance matching, just concentrate on getting the video level (contrast) right. A resistor in series with the video will make a divider with the 75 ohm resistor  inside the TV/Monitor. In the case of a 75ohm you'll get 'input'/2. The problem here is  that 'input' is totaly unpredictable because of the undefined output impedance (and to some extent amplitude) of the arcade board's video.

My point is that the resistor has everything to do with video level (this what you meant by brightness?).The best value of resistance to put in series with the video is something between 0 and 400 ohms. It will change depending on the game board but I've got no idea by how much.