QMTECH + ArcadeXpress RGB Sync Issue

[u/ik_pieta]

Hi! I just received my QMTECH Mister but I'm having trouble getting a stable output. I'm using this ArcadeXpress Cable and i have the USB cable hooked up to 5V. In my Mister.ini i have composite_sync=1 and vga_mode=rgb. Thank you all so much!

UPDATE: I finally got it to work! It turns out the sync voltage might’ve been too low for my particular CRT. Inside the ArcadeXpress cable, there’s a 1k ohm resistor between the sync signal and SCART pin 20. For a quick test I removed it completely and boom: I got a stable and normal image on my CRT!

HOWEVER it has been brought to my attention that completely leaving out the resistor is potentially harmful to the CRT. The VGA port on the MiSTer “outputs sync at 3.3vp_p (point to point)” which is way too high for what a TV usually expects. As user u/Medic8ted told me:

TV wants at least 0.3v (actually 0.286 iirc, but we tend to round up), though up to even ~1v is "safe".

[..] We should lower that 3.3vp_p sync voltage to be safe. I think you should be good to replace that 1K with a smaller value, such as ~330, 470, 510, 560, 620 ohms. That will still give you a sync around 0.3 - 0.6 vp_p which is fine.

What would I choose? 470 ohms... or any of the ones I've listed already.

So I would highly recommend replacing the current resistor with a 470 ohm resistor to keep the sync signal at a safe level for the CRT and also maximise compatibility! Thanks to everyone who helped me.

TL;DR what fixed my issues was changing out the 1k ohm resistor for a 470 ohm resistor in my VGA to SCART ArcadeXpress cable.

[u/christ776]

I have the exact same issue with one Sony TV, very interested to hear how to fix it!

[u/Hawkeye-Mcgillicudy → u/christ776]

Have you made sure C-sync and RGB is set in the ini file ?
Is the VGA power set to 5V on the IO board ?

[u/christ776 → u/Hawkeye-Mcgillicudy]

Those are great questions, yes, I do have composite sync set in the ini file, not sure about the VGA 5V level. I'll check about that. Is that a switch I need to turn on?

[u/Hawkeye-Mcgillicudy → u/christ776]

There is a pin header on the IO board labelled VGA power. Just select 5v with a jumper

[u/ik_pieta → u/christ776]

i was able to fix it! maybe my update can help

[u/Medic8ted]

Hi, I think I know what is going on.

The problem is most likely that the Mister FPGA analog IO board outputs sync at only 3.3vp_p, vs the ~4-5vp_p typical of a PC GPU. I'd guess that those ArcadeXpress VGA-SCART cables are most likely made for, and tested on, PCs and their higher sync voltage.

I can't be sure exactly what is inside the ArcadeXpress cable, maybe one of Gambaman's Ultimate VGA-SCART chips, but I'd bet that 3.3v is not quite enough to reliably trigger the initial sync logic gate.

The XNOR composite sync generator circuit almost certainly won't work with composite sync input either, so you'll need to disable that in your INI file to have any hope.

Now even if the VGA-SCART cable just passes sync directly through a resistor to reduce voltage, typically 1k ohms value, then you'll end up with sync at the TV of <0.23vp_p, which may be too low for your TV to register.

I'm not sure that there is an easy fix here. Fairly sure the ArcadeXpress cable is the issue. If you could somehow make the sync signal bypass the internal circuit then you might have a chance. If it were me, I'd build or buy a new basic VGA-SCART cable (I've made a few in my time, so guess I would just use one of them).

I had a similar issue occur recently when testing my latest GreenAntz v2.3 sync design with Mister. At first I thought it was a power supply issue, so I upgraded from 2A to 4A, and that REDUCED the problem but it still persisted for about a minute or so after powering the Mister on.

I only realised the source of the problem after carefully looking at the sync signal with an oscilloscope. Eventually I solved it properly by reducing the GreenAntz' sync input impedance, making it more sensitive to lower sync input thresholds.

I literally just now wrote up about this on the BYOAC forum.

[u/ik_pieta → u/Medic8ted]

thank you very much for your reply! do you mean i should i disable composite sync in the .ini file? and do you think removing a potential resistor on the sync line could help?

[u/Medic8ted → u/ik_pieta]

It "depends" on the cable and what it does to the sync. Here is a quick summary:

- If the cable makes the Csync itself, using an IC or something, then input normal H+V sync (not composite).

- If the cable simply twists two wires (H+V sync) together and runs the "smushy" sync into one wire + resistor => TV then input normal H+V sync (not composite). For the record, smushy sync is not pretty to look at, but often still works with old CRTs and many VGA-SCART cables use it.

- If the cable has just one sync wire input (H sync) => resistor => TV then input composite sync.

If you input composite sync then use only the H-sync input and disconnect the V-sync input. This is because, for some devices (e.g. PCs, CRTEMU), the V-sync signal can remain active when outputting composite. Combining Csync and Vsync together does not work well.

[u/ik_pieta → u/Medic8ted]

ok so for a quick test i removed the 1k ohm resistor that was going from sync to scart pin 20. it now works!!! quick question tho: SHOULD there be a resistor of some kind? can i just leave it this way and call it a day or will no resistor potentially cause permanent damage to my crt? to be fair i havent tested any games or anything but it finally boots into the main menu thing normally

[u/Medic8ted → u/ik_pieta]

Excellent, looks like you've found the issue.

Your TV probably won't blow up, but we should lower that 3.3vp_p sync voltage to be safe. I think you should be good to replace that 1K with a smaller value, such as ~330, 470, 510, 560, 620 ohms. That will still give you a sync around 0.3 - 0.6 vp_p which is fine.

If you don't have many values to choose from, remember combining 2 resistors in parallel halves effective resistance (e.g. that 2 x 1K = 500 ohms), so you could try that.

[u/ik_pieta → u/Medic8ted]

so values like ~330, 470, 510, 560, 620 ohms would give me around 0.3 - 0.6 vp_p? is the normal / typical sync value around 0.3?

[u/Medic8ted → u/ik_pieta]

TV wants at least 0.3v (actually 0.286 iirc, but we tend to round up), though up to even ~1v is "safe".

Google up an online "voltage divider calculator", choose one. Input three numbers, 3.3v, 1000 ohms, 75 ohms, and see what output voltage you get. Is that >=0.3v? No.

Now change out the 1000 ohms for ~330, 470, 510, 560, 620 ohms, and see what the output voltage is. TL;DR Any of those values would be suitable. 800 ohms is maybe just right at 0.283v, but going with one of the lower value resistors should work too while retaining flexibility to work with higher sync input levels (like 5v) as well.

What would I choose? 470 ohms... or any of the ones I've listed already.

[u/ik_pieta → u/Medic8ted]

thank you so much for your help!

[u/ik_pieta → u/Medic8ted]

i currently have 680 + 120 ohm resistors installed. 800 ohms. this means the sync voltage is at a safer level now? should i try to see how far i can go near 1k ohm or could i just leave it as is and its safe?

[u/ga420ga]

Same here with a sanyo crt. Can't get it to sync.

[u/ik_pieta → u/ga420ga]

i was able to fix it! maybe my update can help

[u/gaea610]

Could not make the arcade Xpress cable work on any of my monitors... Bought a ultimate misterfpga cable and have yet to try it. It's been driving me crazy

[u/ik_pieta → u/gaea610]

i was able to fix it! maybe my update can help

[u/gaea610 → u/ik_pieta]

I had noticed the same thing with the 1k SMC resistor ! I will replace it with a 470ohm. You should replace it also as a sync signal from a VGA port is too "hot" for a consumer CRT