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 Post subject: Universal SNES Output Amplifier
PostPosted: Sun Sep 30, 2018 1:38 am 



Joined: 01 Jul 2017
Posts: 26
I thought I'd share some notes and results from a project I've been working on. One of the most peculiar quirks of the SNES is the various different RGB output stages in the different hardware releases (NTSC, PAL, 1-CHIP/SRGB, etc.) necessitating completely different and mutually incompatible RGB cables. The challenge is that there are three different output circuits (thank you http://members.optusnet.com.au/eviltim/ ... escart.htm):

A) NTSC – the NTSC 2-CHIP SNES uses a PNP emitter follower with the output tapped between a resistor divider that attenuates the high amplitude video input and provides a parallel equivalent 75-ohm output impedance. A 220uF capacitor is necessary to remove the high DC offset as well as to preserve the DC operating point of the emitter follower.

B) PAL – the PAL 2-CHIP SNES uses an NPN emitter follower with a diode level shifter and a 30-ohm output resistor. The RGB cable must add a 75-ohm, DC coupled, resistor to ground to properly bias the emitter follower. The 75-ohm bias resistor combines with the input termination resistor of the display to form a second ~30-ohm resistor to divide the input signal amplitude in half, achieving the correct video levels.

C) 1-CHIP SNESes, modded Mini’s and modded consoles with a bypass amp use a much more conventional opamp based output with a series terminated 75-ohm resistor. This design is the only one that makes sense to me. Not sure what Nintendo was thinking about the other two…

So, what happens when you use the wrong cable? Plug a NTSC cable into a PAL console and the coupling cap blocks the DC bias path for the NPN follower, resulting in no signal. Plug a PAL cable into a NTSC console and that 75-ohm resistor to ground shunts the PNP follower’s emitter resistor network, disrupting the bias point, killing the gain and results in dim video. Similarly, the PAL cable’s 75-ohm resistor to ground forms a 0.3x voltage divider, rather than a 0.5x voltage divider, with the series termination resistor again resulting in dim video.

What we need is a circuit that properly biases the PAL consoles without affecting the gain of the other two. The solution is actually quite simple – a constant current sink rather than a resistive load accomplishes this objective nicely. A current sink, due to its high output impedance and voltage compliance, means that the gain is unity, irrespective of the various emitter/termination resistors. At the same time, we can dial the bias current low enough that it only shunts a fraction of the total current in the NTSC PNP follower, thus preserving its normal operating point.

To restore normal video levels, we can use a high input impedance buffer amplifier with a standard series 75-ohm termination resistor. Coupling the input of the buffer amplifier with a small capacitor and sync tip clamp also allows us to set the DC offset of the output signal near ground, thus completely eliminating the need for large, electrolytic coupling capacitors.

Here’s a picture of the actual board, connected to a SNES AV connector, spliced into a cheap Chinese SCART cable. The RGB lines are connected directly to the SCART pins – no coupling caps or shunt resistors. The test setup has the consoles outputting to my PVM-14L5, looping out to my scope and 75 ohm termination resistors.

Image

And here's actual measurements using 240p test suite's SMPTE test bars @ 75% intensity:

Image

Image

Image

Overall I'm pretty happy with the results! The output amp worked with all three consoles without any significant effect on the gain. And it centered the DC output offset to 150 mV above ground, which means no need for large electrolytic coupling caps. Now I just need to hunt down a PAL GC and see if it works.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Sun Sep 30, 2018 1:13 pm 



Joined: 19 Jul 2017
Posts: 859
So this ends up being a universal RGBS cable for any consoles capable of RGB over a Nintendo AV Multi-out port?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Sun Sep 30, 2018 6:13 pm 



Joined: 01 Jul 2017
Posts: 26
Yup! That's the idea -- I'm now really anxious to find a PAL GC to give it a whirl.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Wed Oct 03, 2018 3:56 pm 



Joined: 03 Jun 2018
Posts: 37
Is there a reason you didn't put the circuit in the Scart head?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Wed Oct 03, 2018 4:43 pm 


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Posts: 347
Dazdigo wrote:
Is there a reason you didn't put the circuit in the Scart head?

a good reason to put it in the console side is it allows the same design to be used if you don't want SCART as the connector

like with 4xBNC or HD-15 (VGA) as the output


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Wed Oct 03, 2018 9:27 pm 


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Joined: 11 Jul 2018
Posts: 55
Location: Alberta
Dazdigo wrote:
Is there a reason you didn't put the circuit in the Scart head?


I'm guessing the circuit boards all work little better closer to the source; not at the opposite end of non-shielded cables. Also speaking from experience, the SCART head is pretty flimsy and rattles apart pretty easily.

Interesting project... doesn't look like you could jam in the components with my product the SyncBaby (see signature); would be cool however!
_________________
8bitgasm.com || SyncBaby || SyncSlayer


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Wed Oct 03, 2018 11:01 pm 



Joined: 03 Jun 2018
Posts: 37
Well this project would be great if we could have the trusted cable companies manufacturer this. Would love to get this in a packapunch option from Retro Gaming Cables. I am just worried about issue with the injection mold templates the RGC is already using.

Would also be nice to have a sync pass through switch as well since having a sync signal stripped too many times can cause issues.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 8:34 am 



Joined: 25 Jun 2013
Posts: 23
Location: UK
Whats the problem with the injection moulding?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 1:06 pm 



Joined: 03 Jun 2018
Posts: 37
No real problem. Just that new templates might need to be made to account for pcb size.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 1:27 pm 



Joined: 25 Jun 2013
Posts: 23
Location: UK
Already on it. The case is being lengthened.

Image

I'm having some clear ABS cases made so you can see the circuit board inside.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 2:08 pm 



Joined: 20 Feb 2016
Posts: 247
A universal cable would be great. I am concerned about the amount of noise (spikes) of the universal amp scope measurements compared to the RGC cables.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 2:16 pm 



Joined: 01 Jul 2017
Posts: 26
Noise is due to the unsieleded Chinese SCART cable this was spliced into.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 2:17 pm 



Joined: 25 Jun 2013
Posts: 23
Location: UK
The cable Mike used was a generic unshielded cable so there is bound to be noise


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 3:15 pm 



Joined: 03 Jun 2018
Posts: 37
Nice can't wait to buy one.
Now I just need to get my second n64 RGB modded.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 4:12 pm 


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Joined: 09 Aug 2017
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Location: Australia
Whats stopping this from working with any other console? Just the connector?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 4:29 pm 



Joined: 19 Jul 2017
Posts: 859
Syntax wrote:
Whats stopping this from working with any other console? Just the connector?

Should work fine on PS1, which has similar differences between NTSC and PAL, needing caps and resistors respectively; although it may have limited application with the PS2 if it doesn't convert RGsB to RGBS or support up to 1080i.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 4:44 pm 



Joined: 20 Feb 2016
Posts: 247
mikechi2 wrote:
Noise is due to the unsieleded Chinese SCART cable this was spliced into.


Has this been verified with a good quality cable? I wouldn't just assume it's because of the cable.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:05 pm 



Joined: 27 Sep 2018
Posts: 93
nmalinoski wrote:
Syntax wrote:
Whats stopping this from working with any other console? Just the connector?

Should work fine on PS1, which has similar differences between NTSC and PAL, needing caps and resistors respectively; although it may have limited application with the PS2 if it doesn't convert RGsB to RGBS or support up to 1080i.

Plus isn't a SoG/RGsB sync stripper a pretty simple circuit? If the PS2 cuts the sync pin, would would it be a simple switching transistor between the two sync sources? If so, you could indeed make a universal cable for everything pre-PS4?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:16 pm 



Joined: 19 Jul 2017
Posts: 859
energizerfellow‌ wrote:
nmalinoski wrote:
Syntax wrote:
Whats stopping this from working with any other console? Just the connector?

Should work fine on PS1, which has similar differences between NTSC and PAL, needing caps and resistors respectively; although it may have limited application with the PS2 if it doesn't convert RGsB to RGBS or support up to 1080i.

Plus isn't a SoG/RGsB sync stripper a pretty simple circuit? If the PS2 cuts the sync pin, would would it be a simple switching transistor between the two sync sources? If so, you could indeed make a universal cable for everything pre-PS4?

IANAEE, so I can't speak to the simplicity of the required circuit. Would be really convenient if there was already an RGB amp chip that would automatically detect between SoG/S/HV, strip sync from green, and provide S, H, and V syncs on discrete output pins; mildly-exciting bonus if it can output SoG by jumping yet another pin, and/or if it can [optionally?] normalize the sync from the NES/SNES; but I have no idea if there's anything like that available, and I imagine that an FPGA solution would be too expensive.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:30 pm 


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Location: Australia
The only console that benefits from a SOG cleaner would be the PS2.
It would have to pass 15k RGBS aswell as clean 31k RGsB.

Not a super easy circuit as it has to detect and switch between composite video as a sync source or green as a sync source.
It would be awesome to see someone make one for the PS2 in a dedicated cable that's PS1 compatible also.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:42 pm 


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Syntax wrote:
It would have to pass 15k RGBS aswell as clean 31k RGsB.

why? SoG is basically useless - why not just output 31kHz RGBS (and then separate into HV elsewhere if needed)

or did I misunderstand you?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:45 pm 



Joined: 27 Sep 2018
Posts: 93
You also have the YPbPr situation to deal with on Playstation, which means you'll always have to have full pass-through on the RGB pins.


Last edited by energizerfellow‌ on Thu Oct 04, 2018 6:34 pm, edited 1 time in total.

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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:54 pm 



Joined: 01 Jul 2017
Posts: 26
paulb_nl wrote:
mikechi2 wrote:
Noise is due to the unsieleded Chinese SCART cable this was spliced into.


Has this been verified with a good quality cable? I wouldn't just assume it's because of the cable.


Output looks fine @ the pads. Where else could the noise be coming from?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 5:58 pm 



Joined: 01 Jul 2017
Posts: 26
All fun ideas but getting too complicated for my brain :-)

Why would you want a universal design for every console? They all have different plugs. I think a universal Nintendo AV connector makes sense, because it's a mess with the same plug but mutually incompatible circuits and incompatible/potentially dangerous sync levels.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 6:06 pm 



Joined: 27 Sep 2018
Posts: 93
mikechi2 wrote:
Why would you want a universal design for every console? They all have different plugs. I think a universal Nintendo AV connector makes sense, because it's a mess with the same plug but mutually incompatible circuits and incompatible/potentially dangerous sync levels.

Not every console, just the Nintendo and Sony multi AV, as far as I know. The pre-PS4 Playstation consoles have a near identical same-but-different situation as Nintendo, which is why people asked if the same circuit could be used with a Sony connector as well. The Playtation just has the added bonus of YPbPr/RGsB.


Last edited by energizerfellow‌ on Thu Oct 04, 2018 6:30 pm, edited 3 times in total.

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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 6:18 pm 


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Location: Australia
maxtherabbit wrote:
Syntax wrote:
It would have to pass 15k RGBS aswell as clean 31k RGsB.

why? SoG is basically useless - why not just output 31kHz RGBS (and then separate into HV elsewhere if needed)

or did I misunderstand you?


I meant "Clean the sync off green"
Was just pointing out it had to work with 2 video modes, and forgot about the third..


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 7:14 pm 


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Joined: 05 Mar 2018
Posts: 347
mikechi2 wrote:
paulb_nl wrote:
mikechi2 wrote:
Noise is due to the unsieleded Chinese SCART cable this was spliced into.


Has this been verified with a good quality cable? I wouldn't just assume it's because of the cable.


Output looks fine @ the pads. Where else could the noise be coming from?

if you look at the noise on the scope and see where it's occurring, it's pretty clearly crosstalk


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 8:40 pm 



Joined: 20 Feb 2016
Posts: 247
mikechi2 wrote:
Output looks fine @ the pads. Where else could the noise be coming from?


Good to hear, Thanks.


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 9:11 pm 



Joined: 01 Jul 2017
Posts: 26
energizerfellow‌ wrote:
mikechi2 wrote:
Why would you want a universal design for every console? They all have different plugs. I think a universal Nintendo AV connector makes sense, because it's a mess with the same plug but mutually incompatible circuits and incompatible/potentially dangerous sync levels.

Not every console, just the Nintendo and Sony multi AV, as far as I know. The pre-PS4 Playstation consoles have a near identical same-but-different situation as Nintendo, which is why people asked if the same circuit could be used with a Sony connector as well. The Playtation just has the added bonus of YPbPr/RGsB.


Thanks for explaining... I actually had a similar discussion with Rob. The regular PS1 cables with the coupling caps *should* work fine on a PS2. But I think what you'd guys really need is a sync stripper for the PS2 @ 480p and above?


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 Post subject: Re: Universal SNES Output Amplifier
PostPosted: Thu Oct 04, 2018 9:27 pm 



Joined: 27 Sep 2018
Posts: 93
mikechi2 wrote:
Thanks for explaining... I actually had a similar discussion with Rob. The regular PS1 cables with the coupling caps *should* work fine on a PS2. But I think what you'd guys really need is a sync stripper for the PS2 @ 480p and above?

With properly shielded sync-on-luma cables and modern displays/scalers I think most of the issues are solved outside of the PS1's coupling capacitors, no sync strippers needed.


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