[Video] GroovyMAME with external V-SIZE potentiometer, solving an old dilemma

[Calamity]

Since long, I've been wanting to make this video, showing what is expectable in terms of geometry from a properly configured GroovyMAME setup.

My friend Kapsule has installed some external potentiometers on his arcade cabinet, by wiring out the geometry controls on his Hantarex MTC 9110 remote control board right below the cabinet's marquee. This allows adjusting geometry between games in a very convenient way. He's installed 3 potentiometers: V-SIZE, H-SIZE, and H-SHIFT. So theoretically, he could correct most geometry features just by playing with those three potentiometers.

However, as this video proves, for a standard arcade monitor and a properly calibrated GroovyMAME setup, only V-SIZE is actually required in order to make all games fit the screen:



Everyone knows that the vertical size (or amplitude) is the only geometry parameter that cannot be controlled by software. In other words, the amount of lines that a typical CRT can show is fixed and determined by the current V-SIZE adjustment. Due to the variety of vertical resolutions used by arcade games, which usually range from 224 to 288 lines, it's impossible to create an unique vertical size adjustment that matches all games. Having an external V-SIZE potentiometer is the only perfect solution for this old dilemma.

Notice how, on the other hand, vertical centering is perfectly resolved by GroovyMAME's modeline generator. You can see it clearly in this video, when we fiddle with the pot and both upper and lower white borders reach their respective screen boundaries exactly at the same time. Unfortunately, while this is true for arcade monitors, it doesn't work the same for TVs, and this is the main reason behind the discredit concerning automatic modeline generation.

Of course this mod involves physically modifying the monitor's pcb, so it's not advisable unless you really know what you are doing.

[rCadeGaming]

V-size pot?  Very jealous.

Next step is to use a computer controlled pot that's adjusted automatically at launch time.

[Endprodukt]

Exactly how its done on two of my cabs.

[rCadeGaming]

More jealous...

What kind of CRT are you using?

[Endprodukt]

I got a nos 25" monitor (they bought hantarex) that's identical to a polo... It is a polo with another name on it.

The vertical cab is a hantarex chassis (mtc900 if I'm not mistaken) with an intervideo tube i think.

Putting a v-size pot on a cab with an arcade monitor is really not that difficult.

[rCadeGaming]

Yeah, I'm using a TV with a digital service menu though, no adjustment pots on the chassis.  I'm working on some workarounds though...

[Calamity]

Any idea on some suitable computer controlled potentiometers that could be used? Would it be possible to connect them through USB or something?

[rCadeGaming]

You can find a lot of digital potentiometers at Digikey:

http://www.digikey.com/product-search/en/integrated-circuits-ics/data-acquisition-digital-potentiometers/2556350

They are generally controlled with serial commands or something similar, so it might be best to control them with a micro-controller that the PC can easily communicate with.  They all have a finite number of steps in terms of incrementing resistance though.

I'm planning to use an Arduino with my cab.  It's already going to be configured to control some demultiplexers for other purposes, so I'm thinking add a few extras to make my own digital potentiometer.  If you use a 16x demux to connect your signal along different points of a resistor array, that creates a 16 step pot.  If you use another 16x demux with a different magnitude of resistor array (each step being 16 times that of the other one), and put that in series with the first, you have a 256 step pot.  Do that again and you have a 1024 step pot. 

I'm already planning to use this to control the horizontal centering of console games.  256 steps should be plenty.

You could do the same with the available digital pot ICs, but then you'd have to open up multiple serial connections.

[Endprodukt]

Now this would be a step forward although adjusting the pot takes a second so it really depends on the price. Calamity, if your thinking about doing this let me know and I'd be in for beta testing. I can't code so I'd be no help with that. I can't even think of what parameters would let the interface know how far the potentiometer needs to be turned to fill the screen vertically.

[Sledge]

Need a video showing HOW to wire this up...

[Endprodukt]

If you have an arcade monitor or a TV with pots the only thing you need to do is to extend the 3 connections of the potentiometer with wire. The v pot is either Located on the chassis or on a remote pcb.

[Sledge]

If you have an arcade monitor or a TV with pots the only thing you need to do is to extend the 3 connections of the potentiometer with wire. The v pot is either Located on the chassis or on a remote pcb.

so... we desolder the pot from where it is and somehow stick it to the cab? and then solder the extension wires to it?
what if we want to leave the old pot where it is and just wire in a new one? so we can put it back how it was if we have to...

[Endprodukt]

Simply solder it back then. If it's on a remote pcb get another one for easy switching. Soldering another pot to a pot is like wiring two resistors in series. Specs would be wrong if I'm not mistaken. Don't know how it behaves in parallel. Simply extend the pot.

[rCadeGaming]

Now this would be a step forward although adjusting the pot takes a second so it really depends on the price. Calamity, if your thinking about doing this let me know and I'd be in for beta testing.

If you mean controlling the pot from the PC end, that wouldn't really be Calamity's responsibility.  It would be a feature a front-end rather than of GroovyMAME.  I'm writing a front-end, but it won't done any time soon, and will be very specific to my setup.

If you have an arcade monitor or a TV with pots the only thing you need to do is to extend the 3 connections of the potentiometer with wire. The v pot is either Located on the chassis or on a remote pcb.

Only two of the connections on the pot are likely to be used, the wiper and one end of the resistor.

Don't know how it behaves in parallel.

It would behave very differently.  Look up the rules for parallel DC circuits. 

Total resistance = 1 / [sum of (1 / the individual resistances)]

Long story short, not recommended.

[Paradroid]

so... we desolder the pot from where it is and somehow stick it to the cab? and then solder the extension wires to it?

I've got a few entries on my blog about this kind of stuff. Here are the highlights:

How to customise a SCART TV for use with GroovyUME

Blaupunkt modding mission ends in disaster!

[Sledge]

well i took the cheats way out, and just drilled a small hole through the front bezel in line with the pot...
Stick a screw driver through, and now i can turn it when i need to

[Endprodukt]

That's another option ;-)

[RSTRBLSTR]

I apologize for resurrecting an old thread, but this video sold me and now I've got questions...

does anyone know how much a resistance a potentiometer for VSIZE should have on a standard monitor?

10k? 5k? 1k? Less?  I'd like to make sure I have fine control over it.

[Paradroid]

Based on my experience with TV chassis, I highly doubt that arcade chassis have a "standard" resistance value for the V-Size potentiometer as there are so many different chassis designs out there.

To discover the value of the vertical size control on your board you could check out the Service Manual or measure it with a multimeter.

In my experience, you'll definitely achieve fine control with an externally mounted knob. One of the nice things about analog controls is that they have far more resolution than digital controls. I was able to achieve sub dot/line centering with the controls I mounted on my Blaupunkt TV.

[RSTRBLSTR]

That's a great idea. I already have a new multimeter in the mail, so this is perfect timing. I'll come back and edit this post with my results next week.

[sean_sk]

Can you see any numbers written on the original V-Size pot? If there are it will tell you what value it is.

[lettuce]

Great video....I really need to get my 2 cabs designed and built.

Games that always cause me issues were the r-type and donkey kong games, did they all calibrate fine with just the v stretch pot??

[adder]

Great video....I really need to get my 2 cabs designed and built.

Games that always cause me issues were the r-type and donkey kong games, did they all calibrate fine with just the v stretch pot??

they would be ok providing the monitor you are using can handle displaying eg. for rtype, 256 lines vertically (arcade monitors can do that ok.    crt tv's: varies from brand to brand).

[Calamity]

Games that always cause me issues were the r-type and donkey kong games, did they all calibrate fine with just the v stretch pot??

The games on the video were carefully selected to show the whole range from 224p to 288p lines.

[bandicoot]

I have a question
If my TV have a service mode were i can change V-SIZE etc is it possbile to put a POT in the Vertical Deflection to do the same thing
here-s the schematic of the TV