SNES scart cable using 75 ohm coaxial cable

[FoxMontage]

Hello,
I'm new to shmups and I'm looking for some help with a project I'm working on.

I'm making some DIY RGB scart cables for my PAL SNES, and I've just picked up some 75 ohm coaxial cable. My previous version of the DIY cable used discrete 75 ohm resistors to ground on the RGB lines, but I'm wondering, since the cable is 75 ohm impedance, should I leave these resistors out now?

I'm working off this circuit http://members.optusnet.com.au/eviltim/ ... t.htm#snes except I'm using sync from luma (pin 7) instead of sync on composite video (from pin 9).

Any help understanding this would be greatly appreciated.
Thanks
Fox

[borti4938]

The “75ohm” of the cable is the characteristic impedance of the cable. This is not a resistance you can measure to GND here.
An analog video cable should have a characteristic impedance of 75ohm as the standard termination in a TV is 75ohm. This means the signal wave won’t be reflected back to the transmitter (in your case the SNES).

To give you another example a typical data cable (e.g. Ethernet or simple LiYY cables) has a characteristic impedance of 100ohm.

Back to your question; you still need the additional 75ohm to GND in your SCART plug to match the output resistance of a PAL SNES.

[FoxMontage]

Thanks for the reply!

Does that mean that the signal is "matched" for the length of the 75 ohm coax, and that the coax/SCART pins interface is not impedance matched?

I'm just trying to get my head around all this stuff, as I find it all very interesting. Cheers.