220uf caps?

[knohbody]

Can anyone tell me what exactly these do in relation to the rgb lines? I know capacitors hold a charge, but i don't understand what they're doing in relation to the video signal. Are they just preventing excess voltage? The Snes a/v pinouts have this to say.

According to Jim Christy, the US and NTSC SNES have a �DC offset�, which is basically extra voltage, on the RGB lines. You can filter this out with one 220uf capacitor on each of the RGB lines (+ towards console, - towards display). PAL SNES systems do not need these caps, but may require some 47 ohm resistors in their place in order to get the video contrast down to the correct level. PAL GameCubes need the caps, NTSC GameCubes don�t output RGB through this connector.

But if it's just taking up extra voltage, wouldn't a resistor do the same thing?

[ido8bit]

The capacitors block the DC while passing the video signal.  A resistor would attenuate both the DC and the video signal.

[viletim]

It's called capacitive coupling. The capacitor, along with the TV's 75 ohm termination resistor, forms a low pass filter which removes the DC bias from the video signal.

The PAL SNES' RGB output takes a different approach by getting the DC offset down to a ressonable level (less than a volt) with careful biasing of the output stage (and a diode in series with the signal!).

Unfortunately, game console manufacturers have a habit of omitting these important last few components from their RGB output circuits (I blame SEGA for starting this fucked up trend).

[knohbody]

Thanks guys, that clears it up for me.

[viletim!]

...forms a low pass filter...

should be high pass filter

[Computolio]

   Where's a good source for 220uF capacitors that are A. through-mount and B. not massively huge? Most of the capacitors of this type that I've seen in electronics stores are way too big to fit into even a SCART connector.

[NFG]

Look for 10V caps, they're much smaller (and harder to find!) than the normal 16V capacitors.

[viletim]

Computolio,
I buy Jamicon brand electro caps from a local supplier. The 220� 16V radial cap measures 6.3mm x 11mm.

Try to find a supplier that stocks a particular brand of capacitors rather that just whatever manufactrer was making the cheapest caps that week. Try Mouser if you're in the US (never used em) or  Farnell if you're not.

And like Lawrence said, 10v or even 6.3v caps will work. I've also used 100� caps without adverse effects.

[Drewman21]

Any thoughts on the miniature caps that are out there?  I see that Mouser has them in 10v and they are half the size.  I ask as i bought up a bunch of RGB system cables to mod and the enclosure i picked to do the resistors/cap connections have only a small amount of space.  Would they cause any odd problems? I don't want to have to buy new enclosures and caps again.  Sigh :unsure:
Thanks for any help in this.