SNES MINI. RGB Amplifier Chip Mod or Standard 3 Wire Mod??

Would you mind pointing out the spot in this video I should be looking for? And do you mean the "Rare" logo, not the "age" logo?:

[video=youtube_share;MC9BQ4J191U]http://youtu.be/MC9BQ4J191U[/video]

I have to do that when switching between stock consoles as well. SMS, Saturn and Jaguar's brightness are all different...at least the ones I own, on my setup.

 

If you have to turn the brightness down you're losing detail. End of story. This probably means your other consoles have issues with brightness levels too and shouldn't be assumed to be normal or good.

I have this issue with one of my PVMs that I strongly suspect is a PVM issue and not a cabling issue as the cables work perfectly on another PVM. There is an immense loss of detail with the brightness being way too high at the "standard" brightness setting.

 

2 seconds in, the bit with the number 4 (sorry, looks like UK age logo and its been some time since I saw it)

The area around the 4 has red bits in it. Using the amp circuit with only the 75ohm resistors, every console ive tried it on - the area is completely white or the red is barely visible, which isnt correct and shows you are losing detail. They have all needed adjusted resistor values to correct it. You can compare it with composite on the same TV to rule out the modification messing things up.

 

I took that video today with my N64 that has the new amp chip it in, no input resistors. I'll make another one later on with the same system, but via composite to compare.

 

Whats in your cable *exactly*?

You arent supposed to use "NTSC" cables - there isnt supposed to be a cap on the output. The fact the consoles are NTSC is irrelevant once you have added your own amp to the mix. The only reason the cables vary is because of the circuit (which you are bypassing) thats already in the console.

However, if the caps do indeed fix the problem - thats good news. But it needs to be made clear you need them in the cable - as thats not what the reference circuit shows, nor any of the guides that use it.

 

I just recorded that video quickly, so I'd know what to look for. I used a custom cable I made with no components at all: It's literally going strait from the amp circuit, into a switch, into a capture card.


When I have extra time (tonight or tomorrow), I'll take a bunch of screenshots for you (unless you prefer videos...seems like a waste though): I'll repeat today's test, do it again with a SNES RGB SCART cable (with the capacitors) and again with both cables on a second N64 unit (with the same custom amp circuit). Then I'll use official Nintendo composite cables (and I think I even have an S-Video cable), just to compare.


I hope none of my posts are sounding snippy, I really do appreciate everyone's help and opinions. This kind of nitpicking is exactly why I started RetroRGB.com: If people like us didn't care about every little detail, no one would have even figured out how to mod N64's for RGB in the first place!

 

Sorry Ape, I didn't notice your post until now.

I realize this is 100% subjective, but all the SMS consoles I've seen have always outputted slightly dimmer then Saturns. Not a lot and not enough that I'd have to turn the brightness down, but I tend to play my games at night in a dark room, so I like to always adjust the brightness to fit. For consoles modded with the amp circuit, if I left the brightness alone, the colors aren't washed out or anything...it's literally just a bit brighter then stock (about the same difference as the Saturn to SMS). Obviously my monitors will be different then someone else's, but I have access (via friends) to 6 RGB monitors, an XRGB Mini and a capture card and this looks fine on all. If I thought there was any chance of someone loosing detail using this circuit, I'd have never listed it for sale on my site.


I think the issue is we're dealing with 20+ year old equipment. If we were talking about brand new devices, I'd agree with you 100%...but even if you buy a brand new XRGB Mini, a brand new RGB cable and a brand new TV, you're still using old consoles and have to compensate for any aging the internal components have done.



I wish you guys lived in the NYC area...I'd organize some kinda of meetup, invite you all down, supply a shitload of beer and mess with this stuff till we were all convinced of the answers!

 

First off, thank you for starting RetroRGB.com - there is a ton of useful information on it!

I have an SNES mini, and I modded it for RGB output. RGB out is going to the xRGB mini through a JP21 cable with 220uF caps on the RGB lines. I bought the cable from retro_console_accessories as suggested.

I followed the procedure on the site using the THS7314 IC. This did not work on my mini. I was getting no color on the image, and the image was not syncing properly. For instance, on Link to the Past, the Nintendo logo would show up, and that was it. I was originally using pin 18 on the S-RGB for composite sync. I moved it over to pin 7, and got some better results. It would now sync ok (the bottom of the screen would not), but there was no color. I also noticed I had no color on composite video, either. I removed the amp, soldered to the RGB output lines on the S-RGB, installed 75 ohm resistors on the lines, still used pin 7 for sync, and haven't had a problem. Composite video returned to normal. It could have been that I fried the chip when soldering, but I was pretty careful.

I noticed that all the photos you have of the mini show the S-RGB A chip, but mine has the plain S-RGB chip. Not sure if the pinout is the same for both (I would think it would be), but when I ohmed out from the holes you chose in the photos, to the RGB inputs on the S-RGB chip, they were not dead shorts, but showed about 500Kohm resistance (could have been more, don't remember off the top of my head). I would think they should be connected, since you are just taking the inputs and bypassing the S-RGB chip, correct?


Also, as far as brightness is concerned, I wouldn't so much put the blame on the consoles and aging internal components, but more on the TV aging. A CRT will lose its brightness gradually over its lifetime. The phosphor wears down, and the R, G, and B guns need to be adjusted for convergence.

 

Thanks!

Since it's now working fine, it sounds like an issue with the installation. Did you tap the RGB lines from the bottom of the board? If so, the wires would be sticking up through the top. If they were touching on the top of the board, it might cause the problem you described.

They should be identical. I've seen more "S-RGB A" than "S-RGB", but I've modded both (using both the amp & non-amp'd method) and both worked fine.


I'm not sure about the resistance. If I have time later, I'll measure mine and check.

 

I didnt use a CRT

 

I was referring to retrorgb's post talking about 20+ yr old equipment. Sorry, should have quoted him.

I think the real test would be to measure the voltages coming in and out of the THS7314 amp when you have a screen of full red, full blue, and full green. Then a completely black screen and a completely white screen (I guess with a white screen, you don't need red, blue and green screens). Doesn't RGB signal into a TV need to be 0V - 0.7V? If the amp isn't putting out in this range, the components on the output can be chosen to get these at the multi-out connector. I guess changing the resistance to 75 or 100 ohm can work, but are we still clipping signal, or reducing it too much that colors are off?

According to the data sheet, the THS7314 provides a 2V/V gain to the input signal, plus a +145mV shift. It says the input voltage limits for the THS7314 with a 5V supply is 2.305V range without clipping the signal. Not sure how much "design" went into the RGB mod, but it might need to be redesigned to make sure the signal is not clipped at all. Looks like an o-scope needs to be on the inputs to the amp, or the outputs from the PPU.

 

Found this:

Amplitude of the SNES Mini RGB

Says amplitude is 0.85Vpp.

 

Now I'm a bit confused, I know the reference circuit off the top of my head and you're right there is no 220uF cap anywhere on the schematic. Does that mean this website is incorrect then: http://members.optusnet.com.au/eviltim/gamescart/gamescart.htm

I've never had issue with my 13" PVM but I finally put my 20" to use and it's clearly way too bright no matter what SCART cable I use, and both have caps inside. I figured it was a calibration issue for the PVM itself but this would suggest that the 13" PVM has issues instead.

 

220uF cap is needed to remove the bias (DC Offset) of the video signal coming out of the amp. 75 ohm resistor in series on the amp output is needed to match the impedance of the TV set (or upscaler), which is 75 ohms (in most common cases). If the cap isn't included, the video signal is offset, causing issues with color and brightness. If the 75 ohm resistor is not included, then there will be major video issues, possibly no video.

The problem is that according to the post I linked above, the output RGB coming from the PPU is 0.85Vpp. Normal RGB signal is 0.7Vpp. Is the RGB coming off the PPU offset also, needing a cap on the input? I don't know. More o-scoping of the RGB off the PPU or going into the S-RGB is needed.

The solution that has been found is increasing the 75 ohm on the output of the amp to 100 ohms. The problem with this is there will be an impedance mismatch. Since the TV or upscaler has a 75 ohm impedance, this will cause an issue with the video, such as ghosting, to show up. Some TVs might handle it better than others. Some TVs might handle the increased RGB voltage better than others too.

The solution would have to be a voltage divider. Calpis describes it best in this post, #25 http://www.assemblergames.com/forum...-Help-Have-sound-but-no-Video-xrgb-mini/page2

Here's a video showing the problem of impedance mismatching - http://video.mit.edu/watch/bell-labs-wave-machine-mismatched-impedance-7049/. Imagine that as a video signal, some of the signal gets bounced back to the amp, instead of going to the TV when it should.


Using a voltage divider on the output, we can make sure the output impedance still "looks" like 75 ohms to the TV set, but we can also lower the voltage of the signal to it's acceptable levels. Which means the RGB video should look the same on any monitor, as long as that monitor is calibrated.

 

You sound confused. Those cables are for the circuit inside the console from the factory. Added in your own amp and bypassing the internal circuit = need cables to match the amp you fitted, not "NTSC cables"

I have a scope, several versions of the snes/n64 and a scope. Just lacking time to get around to make the measurements.

But as you have basically agreed with - the stock circuit is no good in the snes mini as the output is too high/bright.

 

Here's the screen captures. As you can see, each look really similar in brightness:



Here's the full-sized pic: http://retrorgb.com/images/N64-RGBvsComposite_THS7314_Comparison-Large.png

Can anyone post a picture of it acting different? Bad_Ad84, you're the one that seems to have the most trouble with this amp, can you post any pics of it looking wrong? I only have NTSC consoles to test, but they all look similar...meaning they all look fine and aren't the slightest bit over-saturated.

 

Im not the only one having issues. Its documented that the values are incorrect in the link above. As well as a "fix" (using 100ohm resistors on output, which still really isnt the correct way to do it).

Its not a matter of preference, its a matter of the output being wrong.

It would seem your capture card doesnt seem to care about specs much judging from your captures though.

While I do not mean to offend you with this next statement, it probably will....

This is the problem with someone who doesnt actually understand the fundamentals behind what they are explaining. Your site is portrayed as a "all you need to know about RGB", when you dont actually understand the theory and electronics behind it. So you just throw up guides/mods without them actually being correct. Then when someone is telling you its wrong, you just reply with "it looks OK to me".

Starting to realize what Calpis feels like.

 

I'm not offended, I'm just exhausted. Every time I turn around, I have to argue with people on forums about pages on my site. Everything I post is tested by me personally multiple times. Before I listed the amp for sale, I checked with two EE friends, who said it looked fine. What else do you want me to do?

Please don't forget the most important point I'm trying to get across: This circuit won't damage anyone's equipment. Period. Verified by TI. Even Calpis admitted that. We may find ways to improve it, but as is it works fine and won't hurt anything. Don't mistakenly lead people to believe differently.

Is there anything else I can do to help test? Instead, would you just prefer that I stop posting here and leave you to it?

 

Retrorgb,

First off, don't take offense, we are just trying to help. Your site is used by a lot of people installing this mod and others, and we are trying to help get the right information on there.

The RGB mod circuit being used is not designed correctly for the application, and changing values on the fly and adding resistors randomly to adjust brightness is not the proper way to fix it.

It might look OK to you, and it might look OK on whatever setup you have, and OK through your video capture card. Everyone's viewing setup varies. And with the way the circuit is designed now, everyone will have different results. Some setups might be able to tolerate the incorrect RGB output better than others.

If the circuit is designed correctly, then it should work for everyone. Why? Because the RGB signals will be within the proper specifications. No part of the video signal will be clipped, black will be black and white will be white. If people still complain using it, it's because their monitor or setup is not properly calibrated.

I have an o-scope too, I can check some test points, but I don't have a flash cart. Best way would be to run the SNES Test Program and test it with solid colors.



This post by Calpis should be put in stone:

 

The solution would be to add a voltage divider on the inputs. This will be 6 more resistors. Problem is, we don't know yet what values are needed. One post I linked above says 0.85Vpp. It should be 0.7Vpp. This 0.85Vpp needs to be verified for R, G, and B, then we can figure out what resistor values we need to use.

I've hooked it up using 75 ohm resistors and I like the picture. I don't think its too bright, but others do and making a "revision" to the mod by adding resistors should make everyone happy.

Here's a good article I found: Understanding Analog Video Signals