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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Thu Dec 05, 2019 3:25 am 


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Joined: 05 Mar 2018
Posts: 1077
maxtherabbit wrote:
does this occur on all SNES if you feed the encoder dejittered sync? or is it just a 1-chip specific issue?

got curious and answered my own question - I hooked up my SHVC-CPU-01 with the encoder being fed dejittered sync from the CPLD:

on Y/C and CBVS there is a slight discoloration for the first ~10 scanlines at the top of the image

somewhat subtle under most conditions but certain images really make it stand out more


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Thu Dec 05, 2019 3:36 am 


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marqs wrote:
For 1) you need to disconnect BH7236AF video encoder pin 6 and connect that to snes_dejitter CPLD pin 11 which provides clean subcarrier. I think there was some discussion about this a few pages back. The latest firmware from nes-fix branch must also be installed on the board.

So we're thinking this should work for SNES as well? Connect CPLD pin 11 directly to the appropriate subcarrier input pin on the encoder?


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Thu Dec 05, 2019 11:44 am 



Joined: 08 Mar 2017
Posts: 1282
If you wonder about conditioning the subcarrier signal first, then yeah, this is something to look into.
Conditioning the signal will affect the final image quality somewhat.

But for just testing, a direct connection will work.
Make the wire short and don't route it along other analog video traces.
Thinking about it, maybe do add a 100 Ohm series resistor close to the subcarrier output on the CPLD (just for protection).

If it works, it may be a good idea to replicate the original subcarrier conditioning that's on the SNES board.
This is probably going to be a few attenuation resistors, maybe an AC coupling capacitor, maybe a capacitor to ground as a filter.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Thu Dec 05, 2019 3:13 pm 


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rama wrote:
If you wonder about conditioning the subcarrier signal first, then yeah, this is something to look into.
Conditioning the signal will affect the final image quality somewhat.

But for just testing, a direct connection will work.
Make the wire short and don't route it along other analog video traces.
Thinking about it, maybe do add a 100 Ohm series resistor close to the subcarrier output on the CPLD (just for protection).

If it works, it may be a good idea to replicate the original subcarrier conditioning that's on the SNES board.
This is probably going to be a few attenuation resistors, maybe an AC coupling capacitor, maybe a capacitor to ground as a filter.

That's exactly what I was thinking. I was actually thinking it would be best to inject the dejittered subcarrier at R29 as shown on this schematic. It's south of the encoder
Spoiler: show
Image


That would preserve all the filtering, and provide an easy attachment point without requiring a pin lift.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Thu Dec 05, 2019 3:52 pm 



Joined: 08 Mar 2017
Posts: 1282
Hm, no, I was more thinking of the circuit for an S-RGB (late model 1, all 1Chip SNES).
The one for the original SNES S-ENC has a lot of stuff going on that may be more than just some filtering.
I would go with a more direct connection there.

We need to consider that the CPLD probably has a different output impedance and drive strength than the orginal SNES PPU, so the original circuit should be more of a guideline.
Whatever that S-ENC circuit is, that's for the experts to tell :p

Edit:
Okay, if you're installing this to an S-ENC, then *do* use R29.
The circuit may be providing a phase shifted copy of the subcarrier to another input pin, and this may be required for the S-ENC to work.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Fri Dec 06, 2019 12:06 pm 


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Location: Finland
Also remember to try with nes-fix fw since the default image does not yet have subcarrier added on pin 11.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Sat Dec 07, 2019 12:16 am 



Joined: 28 Nov 2019
Posts: 6
Hey guys,

Thanks for your thoughts on this. I'm happy to test any of your possible solutions. I'm working with two NTSC 1-Chip systems: One full size, one Jr. I'll go ahead and program them with the NES-Fix version in anticipation. :)

I wish I could help but I am only following a little of what you are discussing.

Thanks,

Heather


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Sat Jan 04, 2020 8:46 am 



Joined: 29 Apr 2016
Posts: 192
Should the subcarrier be conditioned for NESRGB 1.x installs as well?

On my NESRGB board, I disabled the original subcarrier by removing a series resistor and capacitor to ground. The dejitter CPLD subcarrier is directly connected to the BH7236AF video encoder's pin 6.

I haven't tested this yet, as my NES needs other work before I close it back up. But is it okay like this?


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Sat Jan 04, 2020 12:23 pm 



Joined: 08 Mar 2017
Posts: 1282
https://media.digikey.com/pdf/Data%20Sh ... 240AKV.pdf
Quote:
Color subcarrier input terminal. Input the sine-wave or pulse-wave (duty 50%) according to a required output format (3.579545MHz for NTSC, 4.433619MHz for PAL). Moreover, this input must not exceed more than power supply level and less than ground level.

Sounds like a direct connection works.
I'd still add a little series resistance that helps attenuate higher frequencies and protects the input from overshoots.
I'm not good calculating good values here, but around 100 Ohm is probably doing the job.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Sat Jan 04, 2020 4:26 pm 



Joined: 04 Jan 2020
Posts: 1
Hi all,
I’m having some issues with my super Famicom and the OSSC.
It seems that the dejitter mod could solve the problem.

I have the SNS-CPU-RGB-01 board version. I haven’t seen any installation tutorial on the GitHub page for that version. There is something for the SNS-CPU-APU version.
Correct me if I’m wrong but those two pcb versions seem to be the same.
Should I follow the installation procedure of the APU for my RGB-01 revision?

Thanks!


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Sun Jan 05, 2020 12:09 am 



Joined: 29 Apr 2016
Posts: 192
Thanks. I took a picture of my NESRGB before I removed the resistor, and see that it was a 2kΩ (code 30B). Perhaps I should put this back in. Unfortunately I don't know what the capacitor value was.

Edit: I guess the question is, how similar is the dejitter subcarrier to the NESRGB's? Would the dejitter subcarrier work best with the same resistor/cap values?


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Sun Jan 05, 2020 3:24 am 



Joined: 29 Apr 2016
Posts: 192
I emailed Tim, and he answered that the capacitor to ground on the NESRGB's subcarrier line is 27 pF 5% NPO 0603.

So that's a 2kΩ series resistor and a 27 pF capacitor to ground, in case that helps anyone else.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Wed Jan 08, 2020 4:17 pm 


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Has anyone on this board tried using Tim Worthington's RGB mod plus the YPbPr transcoder board and feed that into the OSSC? Wondering if the jitter only effects RGB or is it YPbPr as well.


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 Post subject: Re: NES/SNES 240p dejitter mod
PostPosted: Wed Jan 08, 2020 9:41 pm 


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geiger9 wrote:
Has anyone on this board tried using Tim Worthington's RGB mod plus the YPbPr transcoder board and feed that into the OSSC? Wondering if the jitter only effects RGB or is it YPbPr as well.

haven't tried that, but the jitter was still a problem using a SNES with HD retrovision component cables


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