SNES brightness and Scart cable signal termination.

[Syntax]

Ive just been looking at the output of the SNES and the cables used for RGB output.

If i do the maths, I end up with 772.3mvpp

From Snes to TV jungle

1.4v source
30R inline
2x 75R Termination in parallel


If you made the termination resistor in the SNES cable a 50R then it brings the video levels down to 694.7mv, bang on where we want it right?

So I guess im asking why are 75R used for termination in the scart cable instead of 50R?

SNES Circuit
http://tinyurl.com/yc8zd3j2

With 50R termination in the scart cable
http://tinyurl.com/ybvb2kwx

[kel]

It's supposed to look like this http://tinyurl.com/y7k43wgp but nintendo used 39ohm resistors instead so it's more like this http://tinyurl.com/y9fyhs7u, 30ohms is definitely not right though.

The 75ohm parallel resistor in the cable is used to impedance match the 37.5ohm/39ohm resistor in the console.

[rama]

Is this a PAL console?

I did some measurements of PAL and NTSC consoles, with their correct cables and fully terminated / TV running and showing the picture.
All older SNES show well below 0.7Vpp on the receiving end.
1Chip models are mixed, but tend to be 0.77Vpp on average.

Generally, older consoles have too weak, and 1Chip models have too strong RGB. This is disregarding the region (as long as the correct cable is used for each).

[Syntax]

It not any console, I was just trying to work out the end value after seeing this on Tims site.

http://members.optusnet.com.au/eviltim/ ... _snesp.png

But i guess 30R was wrong hey? Makes sense.

[Syntax]

The main reason it came up is because im modding a TV thats strictly .5vpp input, so I need to add a series resistor to fix my brightness issue.

But finding a good value to use across the board is not easy.

A Sega Genesis would need a 60R series before my sets termination. http://tinyurl.com/ybgys388

The Snes needs something closer to 39R http://tinyurl.com/ydddgtad

[kel]

I had the 1CHIP SNES in mind when I said it is 39ohms. It looks like Tim's diagram is correct for the 3CHIP SNES, I'm not sure what's going on there then.

[Syntax]

SO for all 3 chip SNES it would make sense to change the RGB output termination resistors to 50R or is it not worth the impedance mismatch?

If not it looks like it would sit closer to .8vpp than .7vpp

So confusing when Ramas real world findings contradict your info regarding 30-39R across different consoles Kel

Really the 1chip with its 39R should sit around 0.68vpp

Whilst the 3 chip with its 30R should sit around 0.77vpp

[kel]

I'm really not sure regarding the 3CHIP SNES circuit. You could give it a try but I couldn't say for sure how it would work out and if the transistor is ok driving that load or not let alone what the diode part is in it all as I was under the impression that the diodes were only used for ESD protection but looking at the schematic it has been connected differently for some reason. Maybe you will get away with the impedance mismatch though, who knows.

[rama]

That diode throws me off as well. It will have a forward voltage drop but it will be frequency dependent.

I guess you don't have a scope available? It'd be the best way to dial in a fitting replacement for the 30 Ohm resistor (3Chip model).
For 1Chips, you should adjust the SNES DAC amplitude by varying the resistor near the recently found C11 capacitor fix.

What kind of video mixer requires 0.5Vpp anyway? Is this surely the maximum?
You could replace the TV end termination resistors for an easy fix. Maybe replace the 3x75Ohm by 3x47Ohm or so.
It will drive the consoles a little harder, but they should be fine.

Edit:
And it reads like maybe you mixed up my given values? 1Chips by default are too hot, 3Chips too low.

[Syntax]

Nah I didnt mix them up, thats the point i was trying to make.
We know its a fact 1 chips are hot and 3 chips are cold but when you look at the output circuits it reads the other way around.


Most of every TV I mod is fine with .7vpp even if the chip states .5vpp. But this 50" Hitachi rear pro was way too bright after I was done and couldnt be controlled via the normal menu.
I ended up tweaking PIP brightness settings in service menu which didnt effect normal tv inputs which is nice. ( I replaced the PIP RGB input with my consoles)

So yeah the Tvs sorted but while I was getting there I opened this old can of Nintendo worms......

[rama]

Okay.
So we kind of know what to do with 1Chips if this should ever be required again.
But 3Chip systems are really a bit neglected, it seems.
Modding material on them is very limited, and people don't often talk about it.

At least good guy borti has us covered! ;p
https://github.com/borti4938/SNES-AddOn ... 3chip-snes