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How to use SCART for our hobby
Level42:
I've seen more and more questions about how to use the SCART connector that is present on all European TV's in relation to our hobby.
This posting is a try to answer all these questions for once and for all. I bet I'll fail, but it's worth the try :)
What is SCART ?
Not the official text but here goes:
SCART is a connector that was designed by the French. In a short moment of brilliance, the government in France made it mandatory for every TV set sold there to have the SCART connector. This sort of forced all big TV makers to include a SCART connector on every TV that was shipped to Europe (or built there).
SCART is NOT (I REPEAT: NOT) a definition of signals.
Instead it bundles a range of analog signals that are (were) commonly used in relation to TV's in ONE standardized connector.
SCART usually incorporates the following video signals:
1) Composite video
2) S-video
3) RGB video
I say usually because not all devices support all signals. F.I., a VHS videorecorder, by nature, does not support RGB signals. That is because the video is recorded on the tape with a signal that is simular to composite video.
Every TV however, should have at least one SCART connector that allows input of any of these signals.
(SCART also supports audio signals of course, but I leave them out of this thread, because they will be rarely used in our hobby, most people connect speakers to the PC running Mame, or in original cabs, the installed audio amplifier and speakers are used).
What is RGB video ?
RGB video is the most basic form of video signal you can have in relation to CRT driven color TV's. There are separate wires for each basic color Red Green and Blue, and one for the sync signal. Because of this, none of the signals can influence each-other. In good quality SCART cables, every signal line has it's own shield (return or ground)to further prevent cross-talk between them and other wires.
Every color TV in the world is decoding whatever signal (HF through Antenna, composite video etc.) is input to it into this basic RGB signal, before it feeds it to the CRT. So, if we use the RGB signal on the SCART connector ALL the decoding electronics in the TV are by-passed, ensuring the best possible picture quality. By doing this we use the TV exactly like a "real" arcade/open frame monitor.
There is NO quality difference in picture between a "real" arcade monitor or a TV that is used with RGB.
How about the other signals ?
Well, first there was Composite video. The one (and only) advantage of this signal is that you can transport it through a single wire (and a shield). To make this work, all the color info AND sync info is combined into one signal. The TV will have to decode this signal into all the seperate colors and sync again. This process causes a significant loss in picture quality. Composite video is NOT recommended to be used for our hobby because of this.
Then there's S-video. With S-video you have one wire for the sync, and one wire for ALL the colors. So the colors are still combined. This still requires decoding in the TV, and still introduces a reduction in quality. Outside of Europe, this is often used in our hobby, because only European TV's have SCART.
I would not recommend it, because RGB is the best option to choose, and it's present out the outputs of your PC or original game PCB.
OK,OK, I've read all the theory, I want to know how to use it !
Yeah, yeah, it can't hurt to have a little theory to understand what we're using.
OK, now, almost all PC's have a RGB - VGA output. So, it should be easy connect it to a SCART connector, right ?
Right ! Well, the physical connection is easy. However, a VGA card outputs video at much higher frequencies than a TV can handle. In our hobby, most often, we want to use 15 kHz output, as the vast majority of (classic) arcade games use(d) this frequency.
There are two main ways to get 15 kHz out of your computer:
1) Get the Ultimarc ArcadeVGA card. Basicaly, this is a normal VGA card, but it has an "adapted" BIOS that will allow the card to only run in 15 kHZ. So from the first start-up, you will see everything incl. the boot-up screens
2) Use a software tool like http://forum.arcadecontrols.com/index.php?topic=66402.0 This will let the output of a regular VGA card run in 15 kHz.
So, when we have eiter up and running OR we are using an original arcade game PCB that's running 15 kHz ( the vast majority of the classics), we are ready to wire it up !
First PC to SCART:
When we look at the picture it's simple to see that each color has it's own wire AND return.
This is also true for the sync signal (Do not confuse composite sync with composite video. Composite sync is quite simply the horizontal and vertical sync signals combined. There is NO quality difference in using separate or composite SYNC signals).
So, you think you're ready ? No.
We have to "tell" the SCART connector that we are inputting an external video signal to it and that the TV should select the AV-mode. This is a great feature of SCART, because without adjusting your TV, you can have external devices automatically select the right setting. Some TV's even come out of stand-by mode when this signal is first applied....
We can do this by supplying a Voltage between 9,5V and 12V to pin 8. Since computers have plenty of 12V outputs, and the same applies for most of the original arcade PCB's, it's easy to connect a +12V lead to pin 8.
Now, we have selected the AV-mode but we also have to "tell" the TV what kind of video signal we are supplying. This can be done by providing a voltage between 1V and 3V.
Now, that are voltages that we don't find (easily) on PC's or original arcade PCB's. However it's very easy to do it. We take a +5V supply (easy on PC connectors, red wire) and put a resistor in series with the +5V supply. This will lower the voltage to about 2V.
(Note: In the past I've uploaded a schematic on this forum in various threads from my good friend Darthnuno. This does not show the 100 Ohms resistor. On some TV's this still works. However, I've encountered a more recent TV and on this I could not get the TV to work, the cause was this missing resistor.... so stay in spec and use the resistor !!!!)
This schematic shows how to hook up both votages from a standard PC connector. On an original arcade PCB, you will need to find a +12V and +5V point yourself, which shouldn't be that hard, often there are test-points where you can pick it up from....
The included pictures are from:
http://www.idiots.org.uk/vga_rgb_scart
saint's edit -- made images local to BYOAC
grantspain:
good stuff :cheers:
Mr Wilson:
i'm using a s-video to scart at the mo its a all in one not one with a scart block can i force this to go to av1 by putting 12v on pin 8 of the scart?
Level42:
I don't think so.
This is quoted from wikipedia:
There is no switching signal to indicate S-Video. Some TVs can auto-detect the presence of the S-Video signal but more commonly the S-Video input needs to be manually selected.
....
Mr Wilson:
i've improved my soldering skills and made it now ;D i've just got to get a 100 ohms resistor tomorrow can you still use your front end if its running at 15khz?
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