Video probs w/ SNES RGB (Vertical Bands, etc)

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FBX
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by FBX »

borti4938 wrote:
I did this approximately a year ago and it hasn't changed anything in my system. I removed R59 instead of lifting pin 3, which is much easier. (R59 is 4.7k resistor and close to Q1, Q2 and Q3 under PPU2).
My experience was the same as ramas these days: it was better but still there. So I put the SFC back into its place inside the cabinet.

After reading this, I give it another shot. This time I lifted pin 3 but it's still the same...
Has Voultar documented it somewhere? Have I missed something out? :?
You probably have something else going on then. On mine, lifting the pin was night-and-day difference (as you can see in the pics), though the overall quality of the APU RGB picture is still not good enough for my tastes.

And in regards to Voultar, he mentioned it near the end of the recent Retro Round Table live stream. About an hour and 50 minutes or so in.
Last edited by FBX on Sat Oct 28, 2017 11:49 am, edited 1 time in total.
borti4938

Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by borti4938 »

Well, thank you for the information.

I'm a bit suspicious why the results of 'lifting' pin 3 at -RGB-01 and -APU-01 boards differ that much! At the moment I don't have any clue what could be the reason.
Maybe Voultar? If you read that: do you have an idea what it could further be?
(just for completeness: full recap, new 7805, different PSUs and also a RGB bypass - nothing changes anything in my case)

Of course and nevertheless: great that this method works at least on some -APU-01 boards and great that Voultar put it into public! :D

My SFC is back in its cabinet - when I find the time I will take it out, again...
(So maybe never; I haven't found the time in the past...)
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by rama »

Lifting a pin vs removing the closest component:

What we have here are several Mhz fast buffers or inverters or opamps, that want to push a signal.
At those speeds and with analog power and signals nearby, it matters how they're 'terminated'.

If the pin is just lifted, the termination is a very short piece of metal with no extra components, a simple antenna.

If it's a removed components, it's that same piece of metal but additionally the pad with solder joint, a bit of PCB trace, probably a via, etc.
It's a more complex antenna.

It would be great to have a termination solution that disables the signal without much fuss.
I know we can't just ground it. The power consumption would go way up and it'd create more issues than it solves.
Maybe pull high through a resistor? Pull low through a resistor? Maybe use an SMD coil?
We don't know how those signals are generated internally, but I bet there's ways to reverse it!
paulb_nl wrote: I also removed R54 under PPU2 which is the 5MHz pixel clock but that did nothing.
Ah yea, this is the other clock output pin. You say it does nothing but there is a visible effect if you use a different scaler.
I can never get any 3 Chip SNES to be completely jailbar free without lifting that 5MOUT pin as well.
It's faintly visible on the scope as well.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by paulb_nl »

rama wrote: Ah yea, this is the other clock output pin. You say it does nothing but there is a visible effect if you use a different scaler.
I can never get any 3 Chip SNES to be completely jailbar free without lifting that 5MOUT pin as well.
It's faintly visible on the scope as well.
Do you get vertical bars without lifting that 5MOUT pin? On the screenshot posted by FBX there are also no bars visible anymore. I do see the noise I showed on my video at the top right of his screenshot too. Do you also get that noise with your console? I think its interference from a data line.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Voultar »

Image

I'll have some data, soon.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Voultar »

So GigaBoots' SNES-Mini arrived over the weekend. I've spent a couple of hours with it, and I believe I have a pretty good handle on what's happening. On the select few systems.

We really don't know (unless someone has a service manual or a die shot) of how the DAC is constructed. But by exclusionary evidence, I've removed the 1CHIP completely and did a variety of tests.

Any op-amp based DAC is limited by the saturation level of its transistors. You can get close to 0V (around 50mV maybe) but you can't go to zero.

The 1CHIP DAC can drive to zero. The only thing that can drive to zero is a current steering/stacking DAC.

In a current stacking DAC, you have a compensation node that will minimize voltage variation. This compensation node can be in-accessible as it's often internalized. Based on my assessment of the 1CHIP ASIC, this compensation node isn't internalized on the silicon, but is externally managed. - This is excellent news.

Pin 154 of the ASIC is the "compensation pin", it's supplied with a (C11) 47nF couple cap that goes into +5V.

Now keep in mind, C11 is a 47nF that's 20%. It could be 37.6nF up to 56nF. Depending on the age/performance of this capacitor, it can be inadequate to slow the response down. If the response rate is too fast you're going to generate too much current. This feedback node is going to realize it's generating too much current, too late. This is when you get the "wobblez" at the beginning of the stage until the feedback network finally lines it out. Much like Gigaboots' captures, you can clearly see this occurring.

Okay, so how do we fix this? We can slightly increase the capacitance C11 so that the voltage variation is minimized.

To properly address this, I recommend replacing C11 with a 470-560nF X5R/X7R capacitor. Keep the value under 1uF and above 300nF. Why keep it below 1uF? Because we don't want to affect the slew-rate so much that we degrade our bandwidth. Based on my analysis from today, C11 is the same between 1CHIP variants and the SNES Mini. So this applies to all 1CHIP revisions, (I think).

Image


Gigaboots 1CHIP Mini with stock C11:

Image

Notice the behavior. The capacitor doesn't have the gas to stabilize the voltage variation. The feedback network fights it for a few cycles until it eventually becomes stable, but it's too damned late, and your video already looks like shit, mid screen. Remember the spacing between those bars in Gigaboots' capture of MM:X? Look at the time-base of this distortion loop! I'VE GOT YOU, YOU DIRTY BASTARD!

Gigaboots 1CHIP Mini with C11 replaced with 470nF:

Image

The capacitor is appropriately sized so that it can properly compensate the DAC for consistent stability.

I have replicated this issue across 4 different 1CHIP/SNES Mini systems now. Including Gigaboots' system which was in pretty bad shape, as far as Mega Man X is concerned. No more "bars" and no more "ghosting" across the screen.

I've only looked at this for a couple of hours, but I feel confident in saying:

1) No, this phenomenon isn't a problem with the 1CHIP ASIC itself, or its design. It's simply a matter of an aged component that may have lost performance or was slightly undersized by Nintendo. Cheap bastards..

2) Do I recommend lifting the DAC +Supply (pin 155) and limiting current which also in turn acts as a pseudo filter? I can understand Rama/Borti's theory behind this. But in my opinion, doing such isn't ideal and doesn't address the actual problem. Just replace C11 with a better sized capacitor as noted for the current steering DAC that the 1CHIP employs. We've deduced the "DAC design" here, so from this point it's fairly easy to see what's happening and more importantly, why. Less swing, less ring.

3) Is this a wide-spread issue? Not in my experience. I've sampled several 1CHIP systems when designing my own little doodads for the thing. I know RetroRGB has done extensive testing as well. It's certainly out there and systems can be affected, but I don't believe it's impacting more systems than it isn't.

4)Are bypass RGB boards responsible for this? No. Hasn't anything to do with this problem. This is a voltage stability issue that's the result of the compensating node not being able to properly stabilize the voltage variance. The feedback circuit doesn't work fast enough, so we get these shitty bars/ghosting that appear up to the mid-line of the screen.

It's a really simple thing to fix. It ain't no thang!
Last edited by Voultar on Sun Oct 29, 2017 8:22 am, edited 1 time in total.
borti4938

Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by borti4938 »

@Voultar: FANTASTIC!!! ABSOLUTELY FANTASTIC!!!
(I‘m speechless)
rama wrote: If the pin is just lifted, the termination is a very short piece of metal with no extra components, a simple antenna.

If it's a removed components, it's that same piece of metal but additionally the pad with solder joint, a bit of PCB trace, probably a via, etc.
It's a more complex antenna.
Are you kidding?
Have you ever calculated how long your antenna has to be to transmit the subcarrier?
Or if assuming a perfect square wave to transmit at least 0.001% of its energy?
(Assume you have to use at least a lambda/2 or lambda/4 antenna)
It’s ore capacitive coupling we are talking about here, imo.
Last edited by borti4938 on Sun Oct 29, 2017 9:11 am, edited 3 times in total.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by GigaBoots »

Excellent work Voultar. Thanks again for looking into this.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by FBX »

Voultar wrote:
To properly address this, I recommend replacing C11 with a 470-560nF X5R/X7R capacitor. Keep the value under 1uF and above 300nF.
What should we do for tolerance? The original cap was 20%, but what should we want for the new cap? Is it better to get as low a tolerance as possible like say 1%? Or not worry about it as long as the percentage swing doesn't fall below 470nF?
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Voultar »

FBX wrote:
Voultar wrote:
To properly address this, I recommend replacing C11 with a 470-560nF X5R/X7R capacitor. Keep the value under 1uF and above 300nF.
What should we do for tolerance? The original cap was 20%, but what should we want for the new cap? Is it better to get as low a tolerance as possible like say 1%? Or not worry about it as long as the percentage swing doesn't fall below 470nF?
Meh, I would keep it @ 10%.

You want to keep it as small as possible. I would say anything in the 3xxnF range would be fine, too.

You want to keep the slew nice.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by FBX »

How about this one at 5%?

https://www.digikey.com/product-detail/ ... ND/7536373

Edit: There's also a 470nF version too.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by FBX »

Crap. I hope I didn't shut the thread down asking that question. :?
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by rama »

Wow, this is an excellent analysis!
I'll have to try this on my boards :)
Do you think the PPU2 in older models uses a similar configuration?
It'd be a bomb shell if one replaced capacitor on those would somehow fix the blur they have.
borti4938 wrote: Are you kidding?
Have you ever calculated how long your antenna has to be to transmit the subcarrier?
Or if assuming a perfect square wave to transmit at least 0.001% of its energy?
(Assume you have to use at least a lambda/2 or lambda/4 antenna)
It’s ore capacitive coupling we are talking about here, imo.
Sorry if I don't get the terms right.
As with the DAC current limiting, I'm only good at closing in on root causes, never actually naming them.
In this case, I measure a difference between a lifted pin and a removed component.
All that's different is a few short connections but it does have an effect the image.
That's why I'm asking if anyone knows a good termination strategy for unwanted clock output pins.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by AndehX »

rama wrote: Do you think the PPU2 in older models uses a similar configuration?
It'd be a bomb shell if one replaced capacitor on those would somehow fix the blur they have.
That a very interesting question, and maybe this issue might spur some investigation into this.


Or I could be completely wrong xD
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by rama »

Console #1: SNSP 1Chip-01 with noticeable ghosting
I took out C11 and measured it at 50nF. I replaced it with 100nF, then 200nF, then 470nF, all while looking at the scope reading for a 100% white picture.
Replacing this cap works! The effect starts at 200nF and at 470nF, the wobble is sufficiently reduced.
So yeah, this is great. No more lifting of pins :)

I forgot to take measurements with the old C11 but with 470nF, the RGB voltages into the S-RGB were all below 0.7Vpp.
My hope is that replacing C11 also helps with the overdriven whites. I'll check for that on the next console.

Edit:
Just for characterization, this console has the red channel too high. Red is about 0.7Vpp vs 0.67Vpp for the other 2 channels.
Pulldown resistors are the stock 160Ohm.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by FBX »

rama wrote:
I forgot to take measurements with the old C11 but with 470nF, the RGB voltages into the S-RGB were all below 0.7Vpp.
My hope is that replacing C11 also helps with the overdriven whites. I'll check for that on the next console.
I'm curious if this c11 replacement will negate the need for the 750 Ohm resistors I have soldered to R6, R7, and R8?
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by rama »

Console #2: SNSP 1Chip-02 with noticeable ghosting
I couldn't get to C11 because of an earlier mod board in the way.
Lifted pin 154 and connected it via 1uF to Vcc.
This also works and the console doesn't show ghosting anymore.
Pulldown resistors on this one are adjustable (it's my experiments board ;p).
All 3 colors are 0.7Vpp into the S-RGB at 153Ohm.

Note on the value of my replacement C11:
From Voultar's explanation, I gather that C11 must not be too large, otherwise the rise time on pixel transitions would suffer.
When scoping a checkerboard pattern, this new solution is better than the old DAC current limiting method. This is true even with 1uF for C11.
Please correct me if I got this wrong though!
FBX wrote: I'm curious if this c11 replacement will negate the need for the 750 Ohm resistors I have soldered to R6, R7, and R8?
I don't know yet. Voultar says the S-RGB has a gain > 2, so the resistors could still be necessary.
I measure 0.7Vpp on my upscaler input (so after the cable and all the termination) but I don't trust the GBS8200 analog input design. Not at all.
Colors are full range and not clipped though. It looks great :)
Last edited by rama on Sun Oct 29, 2017 5:06 pm, edited 1 time in total.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Harrumph »

This is awesome, terrific investigation Voultar, and thanks rama for confirming the finding on PAL consoles!
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by FBX »

rama wrote: Voultar says the S-RGB has a gain > 2, so the resistors could still be necessary.
Copy that. I ordered a batch of 10 470nF X7R caps at 5% tolerance. Now although my 1CHIP-03 doesn't have any visible ghosting that I can see, I'll give the replacement cap a try in it anyway.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by AndehX »

So what is the exact recommended replacement capacitor for C11? I'll probably go ahead and do this mod to my 1CHIP 01 just for peace of mind.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Voultar »

AndehX wrote:So what is the exact recommended replacement capacitor for C11? I'll probably go ahead and do this mod to my 1CHIP 01 just for peace of mind.
The Mini uses 0603 and the 1CHIP uses 0805.

At this point, I recommend 3xxnF to 470nF.

Do NOT remove the attenuation, if you're using my design.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by ahaddow »

Hey everyone. Glad that some progress has been made with this problem!

I have an issue as well I wanted to report as well, hoping maybe someone has some experience with this.

My SFC exhibits some scrolling faint horizontal lines on my Trinitron (Japanese model, 480p) when plugged in over both S-Video and RGB.
I have all 1st party cables, including the power brick. I am pretty sure it is a 1Chip based on the serial number, but will try to confirm this later.
I have tried it on my PVM over S-video and did not see the lines, but it's possible they were not as apparent with the scanlines.

Is something not playing nice with the line doubling on the 480p set?

It is extremely hard to photograph this but I drew up a quick example in paint. Basically looks like this with the bars scrolling upwards, or at least the illusion of that happening. It's strongly apparent on solid colours, and when playing at night when the screen goes black, like before starting a level, it looks pretty much like the image below.
https://imgur.com/Sxu3URX
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by AndehX »

Voultar wrote:
AndehX wrote:So what is the exact recommended replacement capacitor for C11? I'll probably go ahead and do this mod to my 1CHIP 01 just for peace of mind.
The Mini uses 0603 and the 1CHIP uses 0805.

At this point, I recommend 3xxnF to 470nF.

Do NOT remove the attenuation, if you're using my design.
Duly noted, Lord Voultar

I have a 1CHIP-01 with no RGB bypass mods. In this case, its just a straight swap for C11 yes? 470nf 0805?
borti4938

Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by borti4938 »

Thanks to the great investigation by Voultar, I have removed my pre-resistor on pin 155 today. :)
Thank you once again!!!

I was suspicious what R3 is for. This resistor is connected to pin 153 and goes to ground. It's a 1.6k resistor.
Actually it is Rset and controls the output current of the DACs. The lower the R-value, the higher the output current (and thus the Vpp) and vice versa.

I calculated, that a 1.7k resistor should be used with a 6dB amp (THS7374 e.g.). But I only had a 1.78k at home.

So, I replaced R3 by a 1.78k resistor and now I measure Vpp = 680mV at the output without any additional load resistors; as expected with 1.78k.
With the stock S-RGB one has to use a slightly higher value; I calculated approximately 1.87k.
As said, with the THS7374, this should be 1.7kOhm.
(both not tested yet due to the lack of appropriate resistors)

Overall to sum up:
by replacing C11 and R3 appropriate sized components every 1Chip-01/02/03 get a very nice picture without any additional components or modifications.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by rama »

It gets better every day, awesome find as well! :D
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by AndehX »

borti4938 wrote:Thanks to the great investigation by Voultar, I have removed my pre-resistor on pin 155 today. :)
Thank you once again!!!

I was suspicious what R3 is for. This resistor is connected to pin 153 and goes to ground. It's a 1.6k resistor.
Actually it is Rset and controls the output current of the DACs. The lower the R-value, the higher the output current (and thus the Vpp) and vice versa.

I calculated, that a 1.7k resistor should be used with a 6dB amp (THS7374 e.g.). But I only had a 1.78k at home.

So, I replaced R3 by a 1.78k resistor and now I measure Vpp = 680mV at the output without any additional load resistors; as expected with 1.78k.
With the stock S-RGB one has to use a slightly higher value; I calculated approximately 1.87k.
As said, with the THS7374, this should be 1.7kOhm.
(both not tested yet due to the lack of appropriate resistors)

Overall to sum up:
by replacing C11 and R3 appropriate sized components every 1Chip-01/02/03 get a very nice picture without any additional components or modifications.
So you recommend also replacing R3 with a 1.87k Ohm resistor if you're not using any RGB mods?
borti4938

Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by borti4938 »

I‘ve ordered a bunch of resistors. So please wait a week or two for measurements.

But I general; if you are replacing C11 anyway, you can replace R3, too. In such a way everything looks like a stock console, which is pretty neat.

However, I have to say that I‘m not a DAC expert. I also don‘t exactly know how Rset works. My calculation is based on similarities and formulars of other known video DAC-ICs. But my first few measurements (reducing R3 a bit, increasing R3 a bit) confirm my simple calculus.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Voultar »

borti4938 wrote:I‘ve ordered a bunch of resistors. So please wait a week or two for measurements.

But I general; if you are replacing C11 anyway, you can replace R3, too. In such a way everything looks like a stock console, which is pretty neat.

However, I have to say that I‘m not a DAC expert. I also don‘t exactly know how Rset works. My calculation is based on similarities and formulars of other known video DAC-ICs. But my first few measurements (reducing R3 a bit, increasing R3 a bit) confirm my simple calculus.
Ste of HD-Rectalvision sent this to me while I was confirming my findings.

https://www.google.com/patents/US8441382

That should give you an idea of what's happening, maybe.

I would be slightly worried about changing R3 too much. This might effect the transition times, too. I would evaluate the bandwidth before/after making that change, first and foremost.

This could also explain why some 1CHIP systems are perceived as being "not as sharp".


We want to avoid this while making changes:
Spoiler
Image
We want to keep this intact while making our changes:
Spoiler
Image
^ This is very important.
Last edited by Voultar on Mon Oct 30, 2017 8:54 pm, edited 1 time in total.
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Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by Syntax »

borti4938 wrote:I‘ve ordered a bunch of resistors. So please wait a week or two for measurements.

But I general; if you are replacing C11 anyway, you can replace R3, too. In such a way everything looks like a stock console, which is pretty neat.

However, I have to say that I‘m not a DAC expert. I also don‘t exactly know how Rset works. My calculation is based on similarities and formulars of other known video DAC-ICs. But my first few measurements (reducing R3 a bit, increasing R3 a bit) confirm my simple calculus.

Why wait on resistors when you can use a trim pot for testing?
borti4938

Re: Video probs w/ SNES RGB (Vertical Bands, etc)

Post by borti4938 »

@Voultar:
Thank you for the documents :) some literature to read over. For the upcoming days.

I measured also the rise and fall time on the small variations over R3 and they haven‘t changed (was around 194ns; at least any variations were not measureable with my small Tektronix oscilloscope). But I will keep these in mind and continue measuring them. Maybe I can use one oscilloscope from my work...

Edit: I've seen that the probes were on the wrong setup (6MHz band limit and 96pF input capacitance). So I have to redo the measurement on the rise and fall time again, anyway. Forget about the number stated above ;)

@Syntax:
I also have ordered an potentiometer ;)
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