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PostPosted: Sun Mar 10, 2019 6:12 am 
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So, for the makeshift arcade cabinet project i'm building (which has the goal of using recycled parts mostly), i had* to adapt the composite out to s-video, since the broadcasting monitor only accepts separate chroma and luma channels.

You can't use composite/s-video adapters on the market, because they're designed to combine two separate channels into one (which is the more common case i guess), not separate one channel into two.

*If your monitor has BNC inputs, you're already set. You could get a rca to bnc cable and it should work.
I didn't feel like buying a new one though, and had plenty of rca and s-video cables laying around, so i rolled my own adapter.


Anyway, here's how:
-cut a composite video and an s-video cable in half. strip the isolation a couple centimeters. strip the leads a centimeter or so each.
-connect composite signal to luma.
-also connect the same signal to a ceramic capacitor. I think the value is fairly noncritical. i picked an 1nF (102) one, simply because i had a single one laying around and had a hunch, and it worked perfect. ymmv. Edit: I just found another schematic where someone used about half the capacitance. So anything like that should work.
-connect the other end of the cap to chroma.
-connect composite ground to both ground pins of the s-video.
Image

https://frankengraphics.files.wordpress ... _video.jpg

Once confirmed, i removed the crocodile test leads, soldered and taped the connections.

Without connecting chroma, you'd likely still get *some* colour - about half the saturation or so - because the filters in your monitor have cross-secting slopes. Pardon the bad sync:
https://frankengraphics.files.wordpress ... stluma.jpg
Without the capacitor, you get full colour but also risk getting horizontal bands that are out of phase and somewhat unstable depending on screen content.
https://frankengraphics.files.wordpress ... eoboth.jpg
With the cap on the chroma channel, colour gets solid and fully saturated. The pink at the top left is just an unfortunate defect on this particular monitor and always present, but oh well.
https://frankengraphics.files.wordpress ... eodone.jpg


Note that the signal itself doesn't get any better - it's just a quick fix to get nes composite out to an s-video monitor in. It is still the same composite quality, not s-video quality.
The goal here is simply to provide some more compatibility and put some old cables to use again.

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PostPosted: Sun Mar 10, 2019 10:45 am 
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FrankenGraphics wrote:
You can't use composite/s-video adapters on the market, because they're designed to combine two separate channels into one (which is the more common case i guess), not separate one channel into two.
To the best of my knowledge, the passive s-video to composite converters are the same circuit as you have here, just with the connectors going the opposite direction.

I have this fuzzy memory of a classmate successfully using one "backwards", as you are here, for similar reasons.


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PostPosted: Sun Mar 10, 2019 11:03 am 
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Huh, i guess i was misinformed about that then and reinvented the wheel. Oh well, it works and was free. :lol: Felt good putting something from the "old cables" bin to use again, too.

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PostPosted: Sun Mar 10, 2019 11:13 am 
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Pretty darn good reasons.


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PostPosted: Wed Mar 13, 2019 5:44 pm 
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I did something similar with an Atari 800 XL. it has separate luma, but not chroma. So for S-Video I use the composite video signal instead of chroma and it definitely looks better than straight composite. I didn't use a capacitor though. I might have to try that.


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PostPosted: Wed Mar 20, 2019 9:33 pm 
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I bought an old RadioShack 5-in, 2-out powered switcher box that has S-Video and Composite in every Input and Output.

Hooking up my NES to one of the switcher's Composite Inputs and displaying it on my TV's S-Video channel looks AMAZING, much better than it does when I use the Composite signal directly from the NES to my TV. :D

I wonder why converting Composite to S-Video like this makes the picture look so much better. o.O


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PostPosted: Fri Mar 29, 2019 6:31 am 
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This definitely isn't the proper way of converting the signals as composite video (as the title says "composite") combines luma and chroma into a single lane and so the chroma bandwidth is significantly limited compared to s-video where it is separated into 2 different lanes. The theory is that composite and s-video signals both have UNCHANGED luma signal (meaning black & white pictures on both signals are 100% the same and in full resolution) while chroma (color channel) for composite is more limited than s-video chroma, so there is really no direct way to "increase" color information straight from composite. My guess why s-video looks better this way on some TVs is probably by lousy and random TV interpretation by pure luck


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PostPosted: Fri Mar 29, 2019 6:40 am 
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lidnariq wrote:
FrankenGraphics wrote:
You can't use composite/s-video adapters on the market, because they're designed to combine two separate channels into one (which is the more common case i guess), not separate one channel into two.
To the best of my knowledge, the passive s-video to composite converters are the same circuit as you have here, just with the connectors going the opposite direction.

I have this fuzzy memory of a classmate successfully using one "backwards", as you are here, for similar reasons.


Yeah, I did this in college. Most TVs have some weird signal processing on composite, and sometimes this version of s-video conversion makes this video look nicer because it's not being filtered as much.


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PostPosted: Fri Mar 29, 2019 8:26 am 
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Quote:
This definitely isn't the proper way

Another way to express this is that there is no 'proper' way, other than replacing the composite video output altogether (and if you do, you should know that you're in off-spec territory in regards to the authencity of your NES unit and that some games might not work quite as intended). The goal with the adapter is not to increase bandwidth of the signal(s) - just to provide some additional compatibility between an unmodified NES composite output and various monitors.

If the signal comes across as cleaner on the TV/monitor, it's just a welcome bonus. My hunch on why the composite in on those TVs are doing excessive filtering, is just to be on the safe side in a wider range of external environments - like bad shielding, excessive EM interference or ground noise... So i'm not surprised if the picture is a bit clearer when feeding a composite signal to your s-video input, if both are options on your tv.

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PostPosted: Fri Mar 29, 2019 10:20 am 
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If it's a TV that was built after 1985, I'm confident it's trying to be helpful¹ and do some kind of advanced filtering (vertical and/or temporal), instead of just the simple bandlimited demodulation.

¹probably actually is helpful given video camera instead of generated video.


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