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D.I.Y. JAMMA adapter - cool idea and a question

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Fozzy The Bear:


--- Quote from: MonMotha on November 14, 2007, 07:09:35 pm ---Now forgive me here, as I'm not a SCART person (live in the USA, where SCART connectors taking RGB are unheard of), but I thought SCART RGB had an implicit DC level, meaning it needed external AC coupling, so there have to be capacitors, not resistors, in the line.  Putting resistors in the line would make it dimmer (due to the input termination).  I know that's what I've found inside all the PS2 SCART cables I've hacked up.

--- End quote ---

You may have found caps in there.... but to pull down arcade 5V to SCART 1V input signal you need resistors. The lead you are looking at is for a PS2, which already outputs its signal at the correct level for a TV Set to handle and therefore has no resistors in it. 

I'm not quite sure why you think it needs external AC coupling though. Unless you mean that it needs external AC De-Coupling. Which is actually unnecessary at the input to an RGB SCART because the signal is de-coupled internally by the TV set before being processed. It is true RGB, not composite although SCART can also accept a composite sync signal, and S-Video. Depending on the facilities available on the TV itself. Unlike in the USA, 99% of them do accept true RGB though.

Best Regards,
Julian (Fozzy The Bear)

MonMotha:

Ok, I misread you.  I thought you were implying that the signal would be to low (i.e. dim) without these parts in series on the line, not that it would be too high (i.e. bright).  This can happen due to odd DC bias showing up, and some monitors don't AC couple their inputs (didn't know if SCART called for this or not), resulting in whatever random DC bias exists from the source being passed right through.  Ah the joys of lacking a negative rail...

I know that the PS2 SCART cables I have taken apart for other uses do have 220uF caps in series with the video lines, but this may be due to a PS2 oddity (though I seem to recall reading that SCART has this all the time).  Again, we don't have that connection in the USA, so I don't get much experience with it.

Yes, if you want to take common arcade signals (3.3V to 5V) down to 1V levels expected by most consumer gear, you will need resistors in the line (see an earlier comment).  This is cheap (3 resistors, or about 10 cents), and pretty easy to do.

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Totally offtopic:

Now, when I say AC coupling, I mean effectively building a high-pass filter using a cap and the input termination.  220uF is the common value I see used because it'll pass the 50-60Hz vertical sync when used with a 75ohm input.  Perhaps there's another american/europeanism going on here and you guys call that AC de-coupling (de-coupling in the US seems always applied to caps which are designed to overcome inductance/resistance on power lines/planes so that digital parts can draw their huge current spikes).  I've never seen inserting a capacitor in a video line to pass AC but not DC called "de-coupling", but I could see how such terminology might evolve (though I would personally call that "DC de-coupling" as it "decouples", i.e. blocks, the DC and "couples", i.e. passes, the AC).

Fozzy The Bear:


--- Quote from: MonMotha on November 14, 2007, 09:04:35 pm ---Now, when I say AC coupling, I mean effectively building a high-pass filter using a cap and the input termination.  220uF is the common value I see used because it'll pass the 50-60Hz vertical sync when used with a 75ohm input.  Perhaps there's another american/europeanism going on here and you guys call that AC de-coupling (de-coupling in the US seems always applied to caps which are designed to overcome inductance/resistance on power lines/planes so that digital parts can draw their huge current spikes).  I've never seen inserting a capacitor in a video line to pass AC but not DC called "de-coupling", but I could see how such terminology might evolve (though I would personally call that "DC de-coupling" as it "decouples", i.e. blocks, the DC and "couples", i.e. passes, the AC).

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OK now I understand what you meant.... I see your point here. It probably is an American/Europeanism going on. Why we can't have a common terminology is beyond me. Especially when most of the gear we use is Japanese on both sides of the pond. LOL  :cheers:

Best Regards,
Julian (Fozzy The Bear)

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