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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Wed Aug 19, 2020 12:25 pm 



Joined: 27 Sep 2019
Posts: 31
Nice result! It definitely looks better than what I tested back then. Other than differences in the mod board design, the quality difference may also be due to the SHVC-CPU-01 board revision you used, since it probably has way better analog signal isolation than the GPM-02 board I have. The image looks a little under-compensated since the blur is still slightly visible in some areas though. I'm sure the sharpness test screen in the 240p test suite is a less flattering as well.

I think it's possible to improve the image further without resorting to the digital outputs, but I just have a theory and no practical implementation of said theory at the moment.


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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Thu Aug 20, 2020 4:19 am 


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Joined: 24 Jul 2016
Posts: 296
unmaker wrote:
Overall I'm very pleased with the results but things are still not perfect. There is some noise that I think is a result of overshoot or ringing which Voultar mentioned in his post. I'm wondering if this mod can be further improved upon or if this is the best that it gets without resorting to say the digital RGB mod that Opatus is working on.


TY for sharing that info! I'm going to order a couple of Borti's bypass boards and try those on the end of the chain.


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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Thu Aug 20, 2020 6:03 pm 


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Joined: 12 Jul 2007
Posts: 5754
Location: Where the fish lives, WV
God, I really hope this pans out. There are so many people that would appreciate something to make their SNES' picture look better, without having to resort to getting a 101. The RGB output on the original SNES is really bad.
We all know that, but it bears repeating. :lol:
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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Wed Aug 26, 2020 9:34 pm 



Joined: 14 Jul 2020
Posts: 15
Just finished. Not the prettiest but it works. Seems to be a slight but noticeable improvement

Image


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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Tue Sep 08, 2020 5:35 pm 



Joined: 29 Aug 2020
Posts: 1
Hi everyone! After a couple of years since I saw this mod for the first time, I finally decided to give it a try, and the result surprised me greatly. Of course it is not perfect, as there are some noticeable ringing artifacts here and there, but in overall terms, I think that the image quality is closer to a 1-chip than to a stock 2-chip. So, in opinion, the mod worth it.

For anyone interested in the details, I installed the mod on a japanese NTSC SNS-CPU-RGB-02. This board has a slightly different analog video path, as can be seen in the following image:

Image

Between the PPU2 and the video encoder, there is only one PNP common-collector stage, instead of 2. The RGB signals on the multiAV come from the video encoder, instead of coming directly from the transistor. For this reason, I had to add the missing common-collector stage, using three aditional transistors. I used the SMD version of the BC857, which seems to work well. I also used LL4148 (SMD version of 1N4148) for the diodes, and replaced 300 ohm, 200 ohm and 2 kohm resistors with 330 ohm, 220 ohm and 2k2 ohm ones, which are easier to find.

For the PCB, I made my own home-made design. Here is the board installed (forget about the green wires at the IC, I accidentaly swaped IN+ and IN- inputs on two of the three amplifiers when designing the board):

Image

I lifted the base of transistors Q1, Q2 and Q3, took the original RGB signals from the PCB pad, and reinjected the modified signals at the transistors bases. I also lifted PPU2 pin 3, to remove the diagonal chroma sub-carrier interference (this is something almost mandatory in this board revision).

Here are some comparisons before and after the mod. The console is directly connected to a Framemeister and the image is upscaled to 1080p. Sorry for the quality, I don't have any capture device, so I had to photograph the TV with the phone:

Image
Image
Image
Image

I hope someone find this information useful.


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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Tue Sep 15, 2020 10:40 am 



Joined: 27 Sep 2019
Posts: 31
Looks good! It's interesting to see different board layouts for that circuit.

I took a break from messing with this project for a month and just got back to it last week. My first adapter board didn't function properly. Graphical glitches and freezing everywhere. Not sure if I damaged the PPU2 chip or it was just bad soldering or something else, but I have more from a parts pile if needed.


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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Sun Sep 20, 2020 3:46 pm 



Joined: 23 Oct 2017
Posts: 13
Wow! This thread is awesome! Thanks to all who have worked on this or tried this!

Just one question. How does RGB with and without this mod compare to straight S-Video? I've been using S-Video on a consumer CRT and it looks pretty good to me. But when I look at RGB pics of non-modded 2 chips like mine it looks like a step down. Maybe it's just the close up effect.


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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Sun Sep 27, 2020 2:51 pm 


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Any chance of this improving any more? Or is this the final form?
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 Post subject: Re: An actual SNES mod to sharpen the pixels of 2-CHIP conso
PostPosted: Mon Sep 28, 2020 3:26 pm 



Joined: 27 Sep 2019
Posts: 31
I’ve been meaning to write up my thoughts on the Japanese mod. Now that I gained a little bit of experience from designing my own circuit, I think I understand enough about circuits to make some comments and suggestions for improvement. Still, I’m not an electrical engineer, so I could be wrong or misinformed on some details. It’s going to be technical, but hopefully someone here will understand it:

1. The first PNP buffer is driving a rather heavy load. This is mainly due to the 300Ω resistor terminated to VCC. For the AC part of the signal, the equivalent parallel resistance seen by the emitter is 279Ω, or 300Ω || (2kΩ + (2kΩ || 3MΩ)). This probably puts too much of a load on the unbuffered PPU2 signal, resulting in nonlinear distortion. Depending on the transistor grade (FQ, FR, FS), the base input resistance can be anywhere from 33.5kΩ to 156kΩ, which is quite low. For example, the THS family of video buffers have an input resistance of 800kΩ, and op amp input buffers are typically in the MΩ to GΩ range. Raising all the resistor values by one order of magnitude (3kΩ and 20kΩ) should reduce distortion and draw less current.

2. The output voltage swing of the LMH6683 is within 0.8V from the rails (VCC and GND) under a 2kΩ load. This means that the signal needs to be within 0.8V – 4.2V, assuming VCC is 5V. The circuit applies a load smaller than 400Ω at the op amp output due to the feedback and gain resistors, so that may shrink the linear region further. A single PNP buffer voltage offset is typically around 0.6V, so it isn’t enough to push the signal safely into the linear region of the op amp, resulting in more distortion at the lower end of the signal. The solution is to either add a little more DC offset or choose a different op amp that can swing well below 0.6V from the rails. Maybe it would have been better to apply the circuit after both PNP transistors? That way, the current gain is much higher, and the DC offset is about 1.2V. There are a bunch of ways to approach this.

I think replacing the op amp with a rail-to-rail, unity-gain-stable type would eliminate a lot of issues. The LMH6683 doesn’t appear to be unity gain stable, and that’s why the signal is attenuated by 6dB and amplified by 6dB. The voltage dividers seem to be the cause of unnecessary loads on the circuit. With a unity gain stable amp, the two 2kΩ resistors can be eliminated and the 300Ω one replaced with a much higher value like 10kΩ. The stock circuit already has a 10kΩ resistor between the first PNP emitter and VCC, so you will only need to tap the emitter pin to get your input. Furthermore, the 200Ω gain resistor can be removed and the 200Ω feedback resistor replaced with 100Ω to compensate. This will reduce the parts count and current load due to eliminating low-valued resistors terminated to GND or VCC.

Anyway, those are my thoughts. I’m still not quite sure how the overshoot-limiting portion of the circuit works (it starts from the 47Ω output resistor, follows though the 2 diodes and finally the 1000pF series capacitor). My own approach is a little bit different, but I don’t know if it makes that much of a difference overall. Hope this helps anyone who wants to tweak the mod further.


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