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

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SavannahLion:


--- Quote from: Akuma on November 13, 2007, 04:58:47 pm ---I see, I see... Since I live in europe a TV is also an alternative because of the RGB-scart connector's compatibility.
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I hate you.  :badmood:


--- Quote --- I just have not figured out yet, how a MAK / Supergun modifies the RGB signal from 5v to 1.5 volt...

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If it works, it works, why worry about it?

MonMotha:

Scaling a signal smaller is really, really easy.  If the monitor has known input termination (with most TVs and computer monitors, this is standardized at 75 ohms), you can actually just use a single resistor in each line to form what is colloquially known to EEs as a "voltage divider".  If you want to go from 5V (common to arcades) to 0.7V (PC standard, and also the active video level for NTSC and PAL composite video), you want either a 470 (slightly dim) or a 430 (slightly too bright) in series with each video line.  You can pass composite TTL sync (as output by arcade boards) straight to a PC type monitor, and though most SCART connections expect CVBS, you can sometimes get by with just attenuating it down to 1V with a similar divider.

If you want to make things adjustable, get yourself some 500 ohm pots and put them on the line (one side at the line and the wiper hooked up to the TV side).  Then you can adjust.

Be aware that this only works when you know the monitor's input termination.  Arcade monitors tend to vary a lot from 75 to about 10k ohms.  In this case, you need a buffer amp to isolate the attenuation stage from the output, or you can do things by trial and error or look up the input termination for your monitor.

Now, making signals bigger requires a little more effort.  You need a video amp.  For most people, it's probably far easier (and possibly cheaper) to just buy one, unless you have some high bandwidth op-amps laying around.

SavannahLion:


--- Quote from: MonMotha on November 13, 2007, 07:44:39 pm ---If you want to go from 5V (common to arcades) to 0.7V (PC standard, and also the active video level for NTSC and PAL composite video),
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Hhmm... why was I thinking 1.5V for PC standard?

Akuma:


--- Quote from: SavannahLion on November 13, 2007, 05:04:59 pm ---
--- Quote from: Akuma on November 13, 2007, 04:58:47 pm --- I just have not figured out yet, how a MAK / Supergun modifies the RGB signal from 5v to 1.5 volt...

--- End quote ---

If it works, it works, why worry about it?

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Well - actually I wanted to refer to this article: http://www.engadget.com/2006/11/28/how-to-consolize-an-arcade-game/

The author says that it is easy to hook it up to a european TV but only tells in detail how to build a RGB2NTSC converter for american TVs. Furthermore, all the garage-manufactured MAKs and Superguns that are available here plug to standard TVs using RGB-Scart. I don't own one and really don't want to buy one since I love the idea of building it myself. Today I succeeded in creating an Naomi / JVS adapter for an ATX power supply, after I get the Naomi wired completely I will add some connectors to make the ATX psu compatible to jamma boards too.

I am taking ALOT of pictures and plan to write up a guide about this, since I realised that the web lacks some good (in other words detailed) home arcade guides - especially the Naomi scene! Therefore I am grateful for everyone who tells me, being an amateur, about ohms and voltages and RGB and so on. The more I spend my time on this matter, the more I learn : ) Hey kids, stay in school - learning rocks! ; )

MonMotha:

The signal that comes out of a computer into a high-impedance load is 1.5Vpp (actually 1.4Vpp, but close enough).  There is a 75 ohm back termination on it which forms a divider with the 75 ohm termination in the monitor resulting in the monitor seeing 0.7Vpp when everything is properly terminated.  This is done because the coax cable used has characteristic impedance (Zo) 75 ohms, and terminating everything in this manner prevents reflections and other transmission line end-line artifacts, which can become quite prevalent on even moderate length lines at the high resolutions PCs sometimes run (like 1600x1200).  This termination scheme is not often employed in arcades.

If you hook your PC video card straight up to a scope and display a black/white checkerboard, you'll see 1.5Vpp signal levels, but if you hook it up to a monitor and probe the signal lines then, you'll only see 0.7Vpp signal levels.  If you drive a PC monitor (or other monitor with input termination) from a low-impedance source such as directly off the output of an op-amp, there's no back termination to divide with, and the monitor will see exactly what you are driving it with.

The choice of 0.7V for active white level has to do with some things you have to do when you AC couple the video and want to restore the DC level, but is otherwise not of much concern in this application.  Note that if you have embedded sync on a PC video signal (sync on green), the total signal will be 1Vpp, with +0.7V as the full intensity level, 0V as black level, and -0.3V as sync.  NTSC-M (US) plays some tricks and defines the black level to be slightly above blanking which is again above sync.  NTSC-J (Japan) does away with this.  You can adjust the difference out by tweaking brightness.  IIRC, no PAL standard incorporates this "feature".

On monitors where you have control of brightness and contrast, you can adjust some variance back out.  Contrast is actually the "white level" adjustment (i.e. you are adjusting what voltage is considered to be "fully white"), and brightness is actually the "black level" adjustment (i.e. you are adjusting what voltage is considered to be fully black).  PC monitors often seem adjustable to handle between about 0.5Vpp and 1.25Vpp signals.  Arcade monitors are often adjustable much, much further, and TVs sometimes feature automatic gain, especially on their composite input where there are some reference voltages to lock the auto-gain to.

The signal levels of composite NTSC and PAL video are somewhat undefined, also, but people seem to have pretty well standardized on 1Vpp (since it includes sync).  It makes the math nice, since full scale is then 1, and everything is otherwise defined as a fraction of full scale.  This has to be done since, when the analog signal is recovered off the air, the TV just scales it (using an auto gain mechanism) to something usable internally, as the actual signal level received on the antenna can vary a lot.

Also, as a warning if you plan to build that RGB to NTSC converter, while it should work great [I've built them on custom boards, and while the composite output is only so-so, the s-video looks nearly as good as what you'll be feeding it], be aware that those "crystal in a can" oscillators are often not tight enough for some TVs' chroma PLLs to lock on.  I had to build my own oscillator using a parallel resonant crystal and some inverters and tune it up with a variable cap.  Also, if you're in Europe, you can set the chip to PAL mode and get something that has PAL style color modulation, but it will still have whatever timings the arcade board uses, which is normally closer to NTSC.  Many European TVs will apparently sync up just fine, though.

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