Can Powerstrip custom timings damage my monitor?

[sean_sk]

Hi Calamity,

I'm new to custom timings in Powerstrip and wanted to get some advice on how safe it is. I'm venturing down this path to try and solve a couple of issues that I'm having with 256 line resolutions. I'll use 352x256 as an example. My setup is a standard CRT 15KHz arcade monitor in horizontal orientation, with an ArcadeVGA 3000 going through a J-Pac and Windows 7 and Groovymame 0.153.

My first problem is shown in the attached picture when running Phoenix. You'll note that the top few lines are squashed. This can be fixed by adjusting vertical hold but it will throw v-hold out for all the other resolutions. So when finding a happy medium for all resolutions this results in 256 line resolutions being squashed at the top. I found another solution by adjusting the timings in Powerstrip.

My second issue is that scrolling in Phoenix isn't smooth, it suffers from jumping. Game information is as follows: 256x208 @ 61.035156 - Switchres: 352x256 @ 58.484Hz and 16.200Hz

The original settings for 352x256 in PS are as follows:

Horizontal Geometry:

Scan Rate: 16.184 kHz
Front porch: 16 Pixels (2.197)
Sync Width: 32 Pixels (4.394)
Back Porch: 50 Pixels (6.865)

Vertical Geometry:

Refresh Rate: 59.721 Hz
Front Porch: 1 Lines (0.062)
Sync Width: 3 Lines (0.185)
Back Porch: 11 Lines (0.680)

Pixel Clock: 7.283

I found that increasing vertical back porch spread out the lines at the top and solved that issue. As i did this the refresh rate value went down. I also increased the refresh rate and got the game silky smooth. My final figures were as follows:

Horizontal Geometry:

Scan Rate: 16.784 kHz
Front porch: 4 Pixels (0.530)
Sync Width: 32 Pixels (4.237)
Back Porch: 62 Pixels (8.209)

Vertical Geometry:

Refresh Rate: 61.034 Hz
Front Porch: 0 Lines (0.000)
Sync Width: 3 Lines (0.179)
Back Porch: 16 Lines (0.953)

Pixel Clock: 7.553

This resulted in a perfectly centered image with games like Phoenix and Omega Fighter being silky smooth with no squashed lines at the top. One thing to note is that when scrolling through refresh rates, ones even lower than the one I chose, I noticed that the picture would have a slight wobble to it. Some refresh rates were worse than others. With the final results I didn't get this.

What I wanted to know is do I risk damaging my monitor with these settings? What are the limits and how do I know when I've gone too far with these settings and do damage? Am I going about it the totally wrong way? Should I leave these settings well enough alone?

I've also included my mame.ini file.

[Calamity]

This problem with 256 lines modes & ArcadeVGA is a classic one and in fact is what got me into the "modeline science" back in 2008.

The problem, as you have perfectly described, is that 256 lines modes are defined with a too short vertical back porch. Too short for a standard arcade monitor (although maybe enough for a multisync monitor such as the WG).

A standard arcade monitor needs at least 16-18 lines for the back porch, because its electronics are similar to the ones in a standard NTSC or PAL TV.

The ArcadeVGA cheats here by using a shorter back porch, in order to keep the horizontal frequency below 16.2 kHz while reaching a vertical frequency quite close to 60 Hz. But this causes problems, like visible retrace diagonals and missing lines or compression on top of the screen. This happens because the monitor doesn't have enough time to move the electron beam back to the top of the screen (this takes around 1 ms for a standard arcade monitor).

As you noticed, when using the arcade_15 preset in GM, which is limited to 16.2 kHz, you only achieve 58.48 Hz for 256 lines modes. This is because it is calculating the modeline with the proper back porch, so the vertical refresh goes down as expected.

Now what if you want to use 256 lines at 60 Hz and still get all the lines on the screen? Because you need 16-18 lines for the back porch, you must reach the 16.7 kHz horizontal frequency. This is what you get when you use the arcade_15ex preset in GM. You have done this manually by tweaking the modeline with Powerstrip.

Whether it's safe or not to use 16.7 kHz for your particular monitor, I can't give you an answer. I can tell you that I've been using this frequency all the time for my vertical games with a couple of Hantarex MTC 9100 monitors that I have since 2008 and so far so good. But that doesn't mean that you won't break your monitor by doing so.

In theory the more you move from the typical working ranges, the more you stress the circuits and increase the probability of a failure in the long run.

In the case of the MTC 9100, its service manual says it has an operational range of 15.7 +- 0.5 kHz "adjustable". This represents a "window" of 1 kHz, and adjustable means that it has an H-freq pot which you can adjust (obviously), so this 1 kHz window moves up or down. In my experience, there seems to be a maximum value for the adjustment, which is, exactly 16.7 kHz. Because of the 1 kHz window, this also imposes a minimum usable H freq value, in this case 15.7 kHz. This means that slightly lower frequecies that usually work, like 15.625 kHz will roll when using this adjustment, so you must enforce all modes to reach the minimum horizontal frequency (all this is automatically done by GM by assigning the proper frequency ranges).

[sean_sk]

Hi Calamity,

That's some very useful information. Thank you.
There's a lot to consider here. I'll be experimenting further, albeit carefully and conservatively and will post what I've learned as well as the results sometime down the track.