Custom built pure sync Saturn SCART- Need feedback on this!

[BuckoA51]

Okay, after asking around for what feels like forever I'm close to sourcing the bits needed to make pure sync Sega Saturn SCART cables. As a couple of us have discovered, using the Saturn's pure sync with the XRGB3 in particular results in a better picture and, most importantly, no loss of picture when the screen goes bright white during game-play.

Now, before I get the cables ordered, I heard a rumour that the sync output on the Saturn should actually be capped since it's voltage is not standard, is this true? Also, I had considered adding a switch to the design that would allow the end user to toggle between pure sync and composite video for sync, does anyone think this would be useful? It would allow the cable to be backwards compatible should, God forbid, you need to use composite but then again who does?

I eagerly await your reactions

[mesmer]

The dreamcast uses a buffer on the sync lines

maybe a good idea?

[BuckoA51]

I don't think that's needed for RGBC though is it?

[BuckoA51]

Ok, I've measured that the voltage on the Saturn's CSYNC is 1.75 volts. To make this compatible with every TV out there, I need to bring this down to 1 volt. However I've no clue on electrics as to what resistor or capacitor I need to use to achieve this, can anyone help?

[Fudoh]

You use resistors to lower the voltage. Just order a few in the range from 75 to 220ohm and give it a try. Pretty ease. Caps on the other hand are used to soften out voltage peaks.

[BuckoA51]

Ok, someone told me I needed two resistors in the circuit to lower voltage, which I didn't actually understand. I'll try 220ohm for starters then since that works nicely on the Amiga.

[Zapf]

Ok, someone told me I needed two resistors in the circuit to lower voltage, which I didn't actually understand. I'll try 220ohm for starters then since that works nicely on the Amiga.

http://en.wikipedia.org/wiki/Ohm's_law

[RGB32E]

Well... the calculation for this application goes something like this

Vs * (Rg / (Rs + Rg)) = Vo

Where
Vs - is the voltage of the input signal
Rg - is the value of the resistor to ground on input (load resistor)
Rs - is the value of the resistor in series (between the input and the resistor to ground) 75ohms is typical for video
Vo - is the resulting voltage

So, if Vs is 1.25V, Rg is 75 ohms, and Vo is 1V, Rs would be...... 18.75 ohms!

If Rg is 1k ohm, then Rs would be 250 ohms (close to the 220 ohm value mentioned earlier).

However, the resistor to ground on the input device might not be 75ohms for the sync input!

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http://ikalogic.com/beg_1_res_v_c.php





This might be clearer!
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Also, getting back to the PQ difference between CSYNC and CVIDEO.... I've only noticed a difference between the two when connecting to the XRGB-3. CSYNC gives a cleaner picture than CVIDEO. I made a Saturn RGB cable a while back with a switch and was able to compare on the fly.

[BuckoA51]

I didn't notice any difference with the sharpness of the picture when using pure sync vs composite video for sync, but there were areas of the picture that had dot crawl or colour crawl on them, for example, red text might have a crawling effect on it. What I'm hoping to do is send this cable to a few people to test to see if it does eliminate that problem or not.

[speedlolita]

Anyone know what sync the official cable uses?

[BuckoA51]

Composite video.