Really weird monitor switching phenomemon

[Ummon]

I don't know if anyone can help with this, given the scarcity of the monitor and all, but what the hell. I displaying Advancemame on a Billabs pentamode (B0P). Generally, the monitor has a mode for displaying 224x256 games horizontally (monitor OSD says 18.1khz/60hz, advmame says pclock is 9.2), and one for displaying 224x228 games horizontally (19khz/60hz - advmame says pclock is 10.4).

Every now and then, something kinks or glitches, and it will start using the first mode for both resolutions. And, mysteriously, after some time, it reverts back to using one for each.

I just ran Q-bert, and it ginked it. Okay. I have no idea what will turn it back to its original behavior. Wait a minute....now no 240x256 games run horizontally will sync - except for Q-bert. I realize it could just be Advancemame being screwy. Still, anyone have any ideas what's exactly happening?

[MonMotha]

If it's outputting the wrong mode, then it's a software issue.  The monitor can only display what you feed it.  The whole "forgetting resolutions" problem is software.

However, at 19kHz, you may be right on the edge of what the digital stuff in your monitor considers a "mode boundary" as you inch from CGA into EGA territory.  The geometry settings on digital monitors are generally "per mode".  Of course, the monitor only knows certain things about your "mode", specifically the horizontal and vertical scanrates.  These scanrates are often defined as hard ranges and assigned a "mode" for which the geometry is adjusted via the OSD.  For example, 15-20kHz might be defined as "CGA", and 20-26kHz might be defined as "EGA" with separate geometry memories for each.

As oscillators drift, it may measure scanrates slightly differently and drift between two pre-programmed mode boundaries, causing two different geometry settings to be used depending on how it feels.  Solution is to not run the monitor right near such a pre-programmed "boundary".  The boundaries are usually set so that you're not likely to run right at the edge of them, but 19kHz is a little unusual.

You can sometimes determine what the likely boundaries for the geometry memories are by looking in the manual and seeing what ranges it uses for changing the S-correction outputs, but there are sometimes also other boundaries programmed in to attempt to make the OSD memory more "per game".

[Ummon]

Hm. That's interesting. I'll have to see if a different card words better, but I may be hosed if it's the monitor. I have no extensive manual, so I dunno on that. Thanks.

[Ummon]

Well, I didn't do any hardware changes, but I went and re-configured advmame with the advcfg utility and that brought back 240x256 games displayed horizontally. Didn't change the original issue, but at least I can play all games again. I also tried 'over-scanning' the image in advcfg and this made 224x256 games display horizontally go back into that other mode with different geometry I mentioned - except that the top line or two was cut off an thrown to the bottom. Hmmm.