You don't need to know any of this to use an arcade monitor with a PC but it might help to get the best possible results!

What is the advantage of using a real arcade monitor for emulation?

Put simply, the reason is that the real games used the same monitors! Seems straightforward enough right? Maybe not, because we have to get our PC to co-operate with our plan for arcade-real pictures. But surely modern PCs can make easy work of displaying those old low-res graphics right? Well no actually... read on!...
Older game boards had very basic video circuitry which was constrained in resolution for two reasons: firstly most available early monitors were based on TV designs and so had the same frequencies, and also because memory was expensive and higher resolutions require much more RAM. There were some very clever designs around, which used the concept of "sprites" to work around the memory limit, but resolutions were still limited. The simplicity produces sharply-defined but low res graphics. For an emulator to reproduce this exactly, the beam of the monitor must have a one-to-one correlation with emulators graphics memory. We can make the VGA card behave as a 100% emulation of the game board. BUT now comes the problem. To do this the number of vertical lines and horizontal pixels that the VGA card displays on the monitor must be exactly the same as the original game board. If they are not, we have to re-sample the original memory mapping and stretch or compress it to fit the screen, or put up with an incorrectly sized picture. So the 100% emulation is lost if hardware stretching or a scan converter is used. So a game which was originally designed to run at, say, 320 X 240 must run the monitor at exactly the same resolution to give a 100% emulation. If it is run at any other resolution, just because the monitor is capable of handling it, the quality of the original game will be degraded, even if the resolution is higher.




Andy wondering if he should stick his fingers into this dead "Computer Space" monitor, actually a modified TV with VALVES (Vacuum tubes)!
The first arcade monitor... ever.










What are the limitations of arcade monitors?

With a 15Khz monitor, there is an important limit. This is vertical resolution. Assuming a vertical sync range of 50-60 Hz, a 15Khz monitor can only display approx 250-300 lines. (this figure is arrived-at by dividing Hfreq by Vfreq, and allowing about 10 "wasted" lines for vertical flyback time) This is perfect for most older games. Any game which originally used a 15Khz monitor can of course be displayed at it's native resolution providing the VGA card can be configured to do so.
Some later games use high-res monitors and have vertical resolutions of 480 or higher. For these, we can enable interlacing and still display at the native resolution. An interlaced picture scans even and odd lines separately, so we can have 480-600 lines interlaced. For the same reason, to display a Windows desktop at 640 X 480 we need to interlace. Unfortunately this does not give a good result on a Windows desktop because interlacing causes horizontal lines which are 1 pixel wide (which may Windows screens have) to flicker at 30Hz. We can run Windows 3D games pretty well though, because games hardly ever contain this kind of regular line type of graphic, and the flicker is not noticeable.
So what about games with resolutions of 301-480 lines? Well we do have a problem here with a 15Khz monitor as we cannot display them with either non-interlaced (not enough lines) or interlaced (top/bottom borders too wide). So for these few games (which originally used 25Khz medium-res monitors), we have to resort to hardware stretching, which re-samples the screen and adjusts the size. Fortunately H/W stretch and interlace degrade a high-res picture less than a low-res one.
What about games with a vertical res of 240 or less?

A game which has a native vertical resolution of 240 will look exactly the same on an arcade monitor whether the monitor is running at a vertical resolution of 240 or less than 240. The reason is this: tweaked modes of less than 240 are configured with top-bottom borders to add to the total number of lines. This is necessary to keep below the 60Hz vertical scan limit. (as number of lines goes down, the time taken to display them goes down, therefore the Vfreq goes up) So either the video mode will have borders (<240 modes) or MAME will have borders and the result is the same, and it will be arcade-real. This is why the ArcadeVGA card does not contain many <240 line modes. They are quite unnecessary. As far as horizontal res goes, there is no issue with this and monitors, they can display any horizontal resolution (note we are talking resolution here not scan rate, which is always 15Khz). The issue here is that the VGA card dot clock needs to be adjusted to provide the correct line scan rate for the required resolution. So with low frequency (15Khz) monitors, we can sometimes run into the VGA card's lower dot-clock limit. (the ArcadeVGA card has no practical dot clock lower limit).

Vertical games on a horizontal monitor.

Providing the monitor is capable of displaying the resolution (rotated) then we still have a good picture. Although the scan lines are in the wrong direction, there is still a 1 to 1 correlation with the game pixels. So we can get away with this cheat usually. If you set the resolution to be exactly that of the game, with the V and H swapped, then the game will fill the screen. Now this is not exactly what you want because it will look very strange. (great on a horizontal game though). You need to deliberately introduce side borders. Easy to do: you just run at a higher horizontal resolution. Galaga runs fine at 352x288 for example. If you calculate the aspect ratio of the original game, this is pretty much the rotated equivalent of it. Bear in mind, though, that arcade monitors are designed to display approx 240 visible lines and we are asking it to display 288 lines in this mode. This means that something has to go, and what goes are the top/bottom borders, which are normally off the screen. So the picture will be taller than a 240 line picture.

What about vertical refresh rate?

If we are going to try to emulate a game board hardware exactly, there is something else to think about. We have sorted out our resolution to exactly match the game, so what else can there be? Answer: vertical refresh rate. The issue only applies to games that scroll the graphics screen, either horizontally or vertically. Unless we can match the refresh rate of the original game, we will get a kind of "ripple" effect when scrolling, as the rate of movement of the screen does not match the screen re-write rate. The way to eliminate this is match the refresh rate exactly. Fortunately almost all games used a 60Hz rate. A few, including Mortal Kombat, used other rates such as 53 Hz, necessary because they chose a higher vertical resolution. So we need to provide for this in our choice of perfect MAME resolutions.
Unfortunately it's not possible to get a 60Hz rate on resolutions of 256 lines, which are designed for vertical games on horizontal monitors, as there are too many lines to "fit in" in the time per frame.

Conclusions

Basically, if you drive your arcade monitor at the game's native resolution, your VGA card is behaving exactly like a game board in the way it is displaying the game graphics, so the best quality will result. Actually a range of resolutions can be defined which cover all games, the res can be slightly higher than the game's native one, and a small border will result, but no loss of quality.

Why can't you run your normal PC SVGA monitor at 31Khz with these low game resolutions?

The answer is this: The vertical refresh rate is a simple calculation: Hfreq divided by lines. Therefore as the number of lines is reduced, the vertical refresh rate goes up. So for a vertical resolution of 256, the vertical refresh would be 121Hz, which the monitor (probably) cannot handle. This is why PC monitors cannot display these resolutions natively. The option of reducing the pixel clock is not there because SVGA monitors cannot display under 31Khz Horizontal. A few high-resolution arcade monitors can display the entire range 15Khz - 31Khz and these are ideal but expensive.

What about other alternatives?

TVs
We said earlier that arcade monitors were originally based on TV designs. So why can't you just use a TV and plug into the TV-Out connector on a VGA card? Well you can BUT you will not get an arcade-real picture. If you use a TV-out what you are actually doing is letting the VGA card re-sample the picture into a fixed TV resolution of 525 or 625 lines interlaced, then encoding into the NTSC or PAL colour standard, then pumping it out into the TV which decodes it back again into RGB and displays at this fixed resolution. This gives a picture that could not be further removed from an arcade game screen! There is no chance of ever displaying any game at it's native resolution. Everything runs at 525 lines and you need hardware stretching to get the game screen the right size. Definitely not advisable.
But there is one way we can use a TV very satisfactorily, which is going in via the RGB pins on a SCART connector (US readers look away at this point, SCART is a European standard!). This actually turns the TV into an arcade monitor because it by-passes all the signal-degrading PAL or NTSC decoder, We can even run native resolutions providing the TV is happy with non-interlaced screens. Beware of RGB "dongles" which attach to some VGA cards. When fitted these tell the VGA card to always use a TV-out fixed resolution so our chance of driving the monitor at the native game resolution is lost.
Scan Converter with an arcade monitor
These can give better results than a TV because there is no NTSC encoding/decoding involved. They re-sample the VGA signal and output at a fixed resolution using RGB. But we are still stuck with fixed resolutions and no way to run at the native game resolution. hardware stretching abounds!
Non 15Khz Arcade Monitors
There are three other types of arcade monitors besides 15Khz fixed frequency:
25 Khz monitors. If you have a choice, don't go for one of these. They cannot display any of the early games at native resolutions as the scan rate is too high. Only a few games used these monitors and only these few games can be displayed 100% accurately on them. Advanced MAME will be needed for these monitors.
VGA arcade monitors. Avoid these if you can. These give all the disadvantages of using a PC VGA monitor and none of the advantages. Only later games will be able to be displayed at native resolution. Almost all games will need hardware stretching.
Full-Range 15Khz - 31Khz multi-scan arcade monitors. For example the D9200. Very Nice! Expensive but can display ANY game at it's native resolution. These can be used with an ArcadeVGA card to display 15Khz and 31Khz resolutions, using a special installable utility for the card. Bear in mind that if you have one of these monitors, using it with an ordinary VGA card will only run it at 31Khz (unless you use Advanced MAME) so you will not be taking advantage of it's full potential and ability to display native arcade resolutions as well as Windows VGA resolutions.

Connecting an arcade monitor to a VGA card.
Our J-PAC or Video Amp make it easy to do this, but you maybe don't have to use these. All VGA cards (including our ArcadeVGA card) produce a RGB signal level of 1 volt. Game boards use a level of 5 volts. But Wells-Gardner and Hantarex (and many other) monitors are specc-ed to accept 1 volt or 5 volts (Hantarex have a switch, W-G have a pot). So for these monitors you can make your own cable or use our pre-stripped and labelled VGA breakout cable. You will need to know the plug pinouts for the monitor because there is no universal standard for this. The connections you need to make are: Red Green Blue Hsync Vsync GND (all 4 VGA card ground pins can be connected together) You MAY be able to combine the H and V sync into one composite sync (which is the JAMMA standard) by connecting the wires together. This works OK on the ArcadeVGA card because all video modes output negative H and V sync pulses. Some VGA cards send a mixture of negative and positive depending on which mode they are in, which prevents this method from working 100% reliably. The VGA card pinout is:



1 - Red
2 - Green
3 - Blue
4 - Monitor ID *
5 - Ground
6 - Red Ground
7 - Green Ground
8 - Blue Ground
9 - Keyway (No pin)
10 - Sync Ground
11 - Monitor ID *
12 - Monitor ID *
13 - Horizontal Sync
14 - Verical Sync
15 - Monitor ID *

Pins marked * are not required for arcade monitors.

IMPORTANT: Our ArcadeVGA card outputs 5 volts on pin 9. If this pin is routed through your cable, make sure to insulate any cut wire to prevent shorting.
Be doubly sure about grounding whenever connecting monitors. Check the continuity with an ohm-meter of the monitor chassis to power-cord ground BEFORE connecting anything else up. Also double-check the isolating transformer is still connected (see below).

Why do some arcade monitors need isolating transformers?

The reason for this is mainly historical. When the first video games came out, they used modified TVs. "Computer Space" (the first) actually used a real TV with Valves (vacuum tubes!). Then some manufacturers started to produce monitors which were basically TVs with the RF and IF stages taken out. The problem was, old TVs all had "live" chassis, in which the metal frame was connected to one side of the mains supply, and insulated from the outside world by the casing. This simplified the design of the power supply (sometimes it was just a big resistor which got very hot!). This was fine as long as nothing was ever plugged into the TV (except the aerial (antenna) which could easily be isolated using a capacitor). But when games came out, there was a need to connect a game board, and also safety issues which meant the chassis had to be no longer connected to mains, but to safety ground, hence the need for a transformer. No currently produced monitors use these. If your monitor has a transformer DO NOT WIRE IT OUT as the monitor chassis will be live, and cause major dangers not only for safety but instantly destroy anything connected to it.

Do I need to discharge my monitor?

I started my career repairing TVs and then computer CRT devices including computer vector displays. I can say that monitor techs do NOT discharge CRTs before working on a monitor UNLESS they are going to do one of the following:
Replace the CRT
Replace the flyback transformer
Separate the CRT from the main board for any reason.
It is a dangerous process! It is perfectly safe to leave the charge in the CRT as it literally cannot escape unless you dive under that CRT rubber cap or poke around with the CRT neck board removed (the focus anode voltage is in this area) The charge cannot be distributed onto the main board because it is prevented from doing so by tripler diodes sealed into the flyback molding. (otherwise the charge would quickly drain away through the EHT winding on the transformer anyway). Discharging the CRT involves putting a screwdriver across 25000 volts, which is not something to be done unless there is a very good reason. This FAQ also warns that there is a risk of scratching the glass and therefore weakening it. Handling or removing a monitor is not a good enough reason to discharge the CRT. BUT beware! Monitors have plenty of other nasty voltages around that ARE stored, in various capacitors on the main board, such as the PSU caps. These are not discharged even if you do discharge the CRT. So always leave a monitor with power off for as long as possible before handling the main board, at least 15 mins and preferably an hour or two.

What is a "Cap Kit"?

Monitors contain a lot of electrolytic capacitors, which have a dielectric inside made of a paste. This dries out over the years, especially in a hot environment. Let's face it, many arcade monitors are old and have been powered on 24 hrs a day for maybe years. So they lose their capacity over time. Rather than replace as required by troubleshooting, it is often easier just to replace all of them in one go. Somebody had the bright idea of putting together kits of all the values required. It is important to realise though that plenty of faults can develop on monitors which will not be fixed by replacing the capacitors. But it is likely that problems such as distorted or wavy pictures may be cured.


Other references:

PC2JAMMA pages. The original and one of the best references for arcade cabinet to PC connections.
Excellent generic CRT FAQ.